What does “input validation” even mean any more?

Information Security is full of terminology.

Sometimes we even understand what we mean. I’ve yet to come across a truly awesome, yet brief, definition of “threat”, for instance.

But one that bugs me, because it shouldn’t be that hard to get right, and because I hear it from people I otherwise respect greatly, is that of “input validation”.

What is “validation”?

Fight me on this, but I think that validation is essentially a yes/no decision on a set of input, whether it’s textual, binary, or whatever other format you care to define.

Exactly what you are validating is up for debate, whether you’re looking at syntax or semantics – is it formatted correctly, versus does it actually make sense?

Syntax versus semantics

“Green ideas sleep furiously” is a famous example of a sentence that is syntactically correct – it follows a standard “Adjective noun verb adverb” pattern that is common in English – but semantically, it makes no sense: ideas can’t be green, and they can’t sleep, and nothing can sleep furiously (although my son used to sleep with his fists clenched really tight when he was a little baby).

“0 / 0” is a syntactically correct mathematical expression, but you can argue if it’s semantically correct.

“Sell 1000 shares” might be a syntactically correct instruction, but semantically, it could be you don’t have 1000 shares, or there’s a business logic limit, which says such a transaction requires extra authentication.

So there’s a difference between syntactical validation and semantic validation, but…

What’s that got to do with injection exploits?

Injection attacks occur when an input data – a string of characters – is semantically valid in the language of the enclosing code, as code itself, and not just as data. Sometimes (but not always) this means the data contains a character or character sequence that allows the data to “escape” from its data context to a code context.

Can validation stop injection exploits?

This is a question I ask, in various round-about ways, in a lot of job interviews, so it’s quite an important question.

The answer is really simple.

Yes. And no.

If you can validate your input, such that it is always syntactically and semantically correct, you can absolutely prevent injection exploits.

But this is really only possible for relatively simple sets of inputs, and where the processing is safe for that set of inputs.

How about an example?

An example – suppose I’ve got a product ordering site, and I’m selling books.

You can order an integer number of books. Strictly speaking, positive integers, and 0 makes no sense, so start at 1. You probably want to put a maximum limit on that field, perhaps restricting people to buying no more than a hundred of that book. If they’re buying more, they’ll want to go wholesale anyway.

So, your validation is really simple – “is the field an integer, and is the integer value between 1 and 100?”

What about a counter-example?

Having said “yes, and no”, I have to show you an example of the “no”, right?

OK, let’s say you’re asking for validation of names of people – what’s your validation rules?

Let’s assume you’re expecting everyone to have ‘latinised’ their name, to make it easy. All the letters are in the range a-z, or A-Z if there’s a capital letter.

Great, so there’s a rule – only match “[A-Za-z]”

Unless, you know, Leonardo da Vinci. Or di Caprio. So you need spaces.

Or Daniel Day-Lewis. So there’s also hyphens to add.

And if you have an O’Reilly, an O’Brian, or a D’Artagnan, or a N’Dour – yes, you’re going to add apostrophes.

Now your validation rule is letting in a far broader range of characters than you start out with, and there’s enough there to allow for SQL injection to happen.

Input can now be syntactically correct by your validation rule, and yet semantically equivalent to data plus SQL code.

Validation alone is insufficient to block injection attacks.

Why do people say validation is sufficient, then?

I have a working hypothesis. It goes like this.

As a neophyte in information security, you learn a trick.

That trick is validation, and it’s a great thing to share with developers.

They don’t need to be clever or worry hard about the input that comes in, they simply need to validate it.

It actually feels good to reject incorrect input, because you know you’re keeping the bad guys out, and the good guys in.

Then you find an input field where validation alone isn’t sufficient.

Something else must be done

But you’ve told everyone – and had other security folk agree with you – that validation is the way to solve injection attacks.

So you learn a new trick – a new way of protecting inputs.

And you call this ‘validation’, too

After all, it … uhh, kind of does the same thing. It stops injection attacks, so it must be validation.

What is this ‘new trick’?

This new trick is encoding, quoting, or in some way transforming the data, so the newly transformed data is safe to accept.

Every one of those apostrophes? Turn them into the sequence “'” if they’re going into HTML, or double them if they’re in a SQL string, or – and this is FAR better – use parameterised queries so you don’t have to even know how the input string is being encoded on its way into the SQL command.

Now your input can be validated – and injection attacks are stopped.

But it’s not validation any more

In fact, once you’ve encoded your inputs properly, your validation can be entirely open and empty! At least from the security standpoint, because you’ve made the string semantically entirely meaningless to the code in which it is to be embedded as data. There are no escape characters or sequences, because they, too, have been encoded or transformed into semantically safe data.

It’s encoding… or transformation.

And I happen to think it’s important to separate the two concepts of validation and encoding.

Validation is saying “yes” or “no” to the question “is this string ‘good’ data?” You can validate in a number of different ways, and with good defence in depth, you’ll validate at different locations, based on different knowledge about what is “good”. This matches very strongly with the primary dictionary definition of “validation” – it’s awesome when a technical term matches very closely with a common language term, because teaching it to others becomes easier.

Encoding doesn’t say “yes” or “no”, encoding simply takes whatever input it’s given, and makes it safe for the next layer to which the data will be handed.

Stop calling encoding “validation”

It’s not.

Padding Oracle 3–making it usable

Just a quick note, because I’ve been sick this week, but last weekend, I put a little more work into my Padding Oracle exploit tool.

You can find the new code up at https://github.com/alunmj/PaddingOracle, and because of all the refactoring, it’s going to look like a completely new batch of code. But I promise that most of it is just moving code from Program.cs into classes, and adding parsing of command-line arguments.

I don’t pretend to be the world’s greatest programmer by any stretch, so if you can tell me a better way to do what I’ve done here, do let me know, and I’ll make changes and post something about them here.

Also, please let me know if you use the tool, and how well it worked (or didn’t!) for you.

Arguments

The arguments currently supported are:

URL

The only parameter unadorned with an option letter – this is the URL for the resource the Padding Oracle code will be pounding to test guesses at the encrypted code.

-c ciphertext

Also, –cipher. This provides a .NET regular expression which matches the ciphertext in the URL.

-t encoding:b64|b64URL|hex|HEX

Also, –textencoding, –encoding. This sets the encoding that’s used to specify the ciphertext (and IV) in the URL. The default is b64

  • b64 – standard base64, URL encoded (so ‘=’ is ‘%3d’, ‘+’ is ‘%2b’, and ‘/’ is ‘%2f’)
  • b64URL – “URL safe” base64, which uses ‘!’, ‘-‘ and ‘~’ instead of the base64 characters that would be URL encoded.
  • hex – hexadecimal encoding with lower case alphabetic characters a-f.
  • HEX – hexadecimal encoding with upper case alphabetic characters A-F.

-i iv

Also, –iv. This provides a .NET regular expression which matches the IV in the URL if it’s not part of the ciphertext.

-b blocksize

Also, –blocksize. This sets the block size in bytes for the encryption algorithm. It defaults to 16, but should work for values up to 32.

-v

Also, –verbose. Verbose – output information about the packets we’re decrypting, and statistics on speed at the end.

-h

Also, –help. Outputs a brief help message

-p parallelism:-1|1|#

Also –parallelism. Dictates how much to parallelise. Specifying ‘1’ means to use one thread, which can be useful to see what’s going on. –1 means “maximum parallelisation” – as many threads as possible. Any other integer is roughly akin to saying “no more than this number of threads”, but may be overridden by other aspects of the Windows OS. The default is –1.

-e encryptiontext

Instead of decrypting, this will encrypt the provided text, and provide a URL in return that will be decrypted by the endpoint to match your provided text.

Examples

These examples are run against the WebAPI project that’s included in the PadOracle solution.

Example 1

Let’s say you’ve got an example URL like this:

http://localhost:31140/api/encrypted/submit?iv=WnfvRLbKsbYufMWXnOXy2Q%3d%3d&ciphertext=087gbLKbFeRcyPUR2tCTajMQAeVp0r50g07%2bLKh7zSyt%2fs3mHO96JYTlgCWsEjutmrexAV5HFyontkMcbNLciPr51LYPY%2f%2bfhB9TghbR9kZQ2nQBmnStr%2bhI32tPpaT6Jl9IHjOtVwI18riyRuWMLDn6sBPWMAoxQi6vKcnrFNLkuIPLe0RU63vd6Up9XlozU529v5Z8Kqdz2NPBvfYfCQ%3d%3d

This strongly suggests (because who would use “iv” and “ciphertext” to mean anything other than the initialisation vector and cipher text?) that you have an IV and a ciphertext, separate from one another. We have the IV, so let’s use it – here’s the command line I’d try:

PadOracle "http://localhost:31140/api/encrypted/submit?iv=WnfvRLbKsbYufMWXnOXy2Q%3d%3d&ciphertext=087gbLKbFeRcyPUR2tCTajMQAeVp0r50g07%2bLKh7zSyt%2fs3mHO96JYTlgCWsEjutmrexAV5HFyontkMcbNLciPr51LYPY%2f%2bfhB9TghbR9kZQ2nQBmnStr%2bhI32tPpaT6Jl9IHjOtVwI18riyRuWMLDn6sBPWMAoxQi6vKcnrFNLkuIPLe0RU63vd6Up9XlozU529v5Z8Kqdz2NPBvfYfCQ%3d%3d" -c "087gb.*%3d%3d" –i "WnfvRL.*2Q%3d%3d"

This is the result of running that command:

capture20181111175736366

Notes:

  • The IV and the Ciphertext both end in Q==, which means we have to specify the regular expressions carefully to avoid the expression being greedy enough to catch the whole query string.
  • I didn’t use the “-v” output to watch it run and to get statistics.
  • That “12345678” at the end of the decrypted string is actually there – it’s me trying to push the functionality – in this case, to have an entirely padding last block. [I should have used the letter “e” over and over – it’d be faster.]

Example 2

Same URL, but this time I want to encrypt some text.

Our command line this time is:

PadOracle "http://localhost:31140/api/encrypted/submit?iv=WnfvRLbKsbYufMWXnOXy2Q%3d%3d&ciphertext=087gbLKbFeRcyPUR2tCTajMQAeVp0r50g07%2bLKh7zSyt%2fs3mHO96JYTlgCWsEjutmrexAV5HFyontkMcbNLciPr51LYPY%2f%2bfhB9TghbR9kZQ2nQBmnStr%2bhI32tPpaT6Jl9IHjOtVwI18riyRuWMLDn6sBPWMAoxQi6vKcnrFNLkuIPLe0RU63vd6Up9XlozU529v5Z8Kqdz2NPBvfYfCQ%3d%3d" -c "087gb.*%3d%3d" –i "WnfvRL.*2Q%3d%3d" –e "Here’s some text I want to encrypt"

When we run this, it warns us it’s going to take a very long time, and boy it’s not kidding – we don’t get any benefit from the frequency table, and we can’t parallelise the work.

capture20181111215602359

And you can see it took about two hours.

Padding Oracle 2: Speeding things up

Last time, I wrote about how I’d decided to write a padding oracle exploit tool from scratch, as part of a CTF, and so that I could learn a thing or two. I promised I’d tell you how I made it faster… but first, a question.

Why build, when you can borrow?

One question I’ve had from colleagues is “why didn’t you just run PadBuster?”

It’s a great question, and in general, you should always think first about whether there’s an existing tool that will get the job done quickly and easily.

Time

Having said that, it took me longer to install PadBuster and the various language components it required than it did to open Visual Studio and write the couple of hundred lines of C# that I used to solve this challenge.

So, from a time perspective, at least, I saved time by doing it myself – and this came as something of a surprise to me.

The time it used up was my normally non-productive time, while I’m riding the bus into Seattle with spotty-to-zero network connectivity (there’s none on the bus, and my T-Mobile hot-spot is useful, but neither fast nor reliable down the I-5 corridor). This is time I generally use to tweet, or to listen to the BBC.

Interest

I just plain found it interesting to take what I thought I knew about padding oracles, and demonstrate that I had it solidly in my head.

That’s a benefit that really can’t be effectively priced.

Plus, I learned a few things doing it myself:

  • Parallelisation in C# is easier than it used to be.
  • There’s not much getting around string conversions in trying to speed up the construction of a base64-encoded URL, but then again, when executing against a crypto back-end, that’s not your bottleneck.
  • Comments and blank lines are still important, especially if you’re going to explain the code to someone else.

Performance

The other thing that comes with writing your own code is that it’s easier to adjust it for performance – you know where the bottlenecks might lie, and you can dive in and change them without as much of a worry that you’re going to kill the function of the code. Because you know at a slightly more intuitive level how it all works.

You can obviously achieve that intuitive level over time with other people’s code, but I wasn’t really going to enjoy that.

Looking at some of the chat comments directed at the PadBuster author, it’s clear that other people have tried to suggest optimisations to him, but he believes them not to be possible.

Guessing

Specifically, he doesn’t see that it’s possible to use guesses as to the plaintext’s likely contents to figure out what values should be in the ciphertext. You just plug the values 0..255 into the N-1 ciphertext block until your padding error from the N block goes away, and then that value can be XORed with the padding value to get the intermediate value from the N block. Then the intermediate value gets XORed with the original ciphertext value from the N-1 block to give the original plaintext.

Let’s see how that works in the case of the last block – where we’re expecting to see some padding anyway. Let’s say our block size is 4. Here’s what two of our ciphertext blocks might look like:

CN-1 CN
0xbe 0x48 0x45 0x30 0x71 0x4f 0xcc 0x63

Pretty random, right? Yeah, those are actually random numbers, but they’ll work to illustrate how we work here.

We iterate through values of CN-1[3] from 0..255, until we get a response that indicates no padding errors.

0x30 comes back without any padding errors. That’s convenient. So, we’ve sent “be484530714fcc63”, and we know now that we’ve got a padding byte correct. Buuut that isn’t the only right padding byte, because this is the last block, which also has a valid padding byte.

In fact, we can see that 0x30 matches the original value of the CN-1 block’s last byte, so that’s not terribly useful. Our padding count has a good chance of not being 1, and we’re trying to find the value that will set it to 1.

Keep iterating, and we get 0x32, giving us a request that doesn’t contain a padding exception. Two values. Which one made our padding byte 0x1, so we can use it to determine the intermediate value?

The only way we get two matches will be because the real plaintext ends in a padding count that isn’t 0x1. One of those values corresponds to 0x1, the other corresponds to the padding count, which could be 0x2..0x4. [Because we’re using four byte blocks as an example – a real-life example might have a 16-byte block size, so the padding count could be up to 0x10]

The clue is in the original plaintext – 0x30 MUST be the value that corresponds to the original padding count, so 0x32 MUST correspond to 0x1.

[If the original padding count was 0x1, we would only find one value that matched, and that would be the original value in CN-1]

That means the Intermediate value is 0x32 XOR 0x1 = 0x33 – which means the plaintext value is 0x3 – there’s three bytes of padding at the end of this block.

We can actually write down the values of the last three plaintext and intermediate blocks now:

CN-1 CN
0xbe 0x48 0x45 0x30 0x71 0x4f 0xcc 0x63
IN
?? 0x4b 0x46 0x33
C’N-1 PN
?? 0x4f 0x42 0x37 ?? 0x3 0x3 0x3

Wow – that’s easy! How’d we do that? Really simple. We know the last padding must be three bytes of 0x3, so we write those down. Then the intermediate bytes must be the XOR of 0x3 with the value in the CN-1 block.

[I chose in the code, instead of just “writing down” the values for each of those bytes, to check each one as I did so, to make sure that things were working. This adds one round-trip for each byte of padding, which is a relatively low cost, compared to the rest of the process.]

Now, if we want to detect the next byte, we want to change the last three bytes of CN-1, so they’ll set the PN values to 0x4, and then iterate through the target byte until we get a lack of padding errors.

So, each new value of the last few bytes of CN-1 will be C’[i] = C[i] XOR 0x3 XOR 0x4 – taking the value in the original, XORing it with the original plaintext, and then with the desired plaintext to get a new value for the ciphertext.

I’ve put those values of C’N-1 in the table above.

This trick doesn’t just stop with the padding bytes, though. I’m going to guess this is a JSON object, so it’s going to end with a ‘}’ character (close-brace), which is 0x7d.

So, C’ = C XOR 0x7d XOR 0x4 = 0xbe XOR 0x7d XOR 0x4 = 0xc7.

Let’s try that – we now send “c74f4237” – no padding error!

A successful guess for the last four bytes of PN. Now we can fill in more of the table:

CN-1 CN
0xbe 0x48 0x45 0x30 0x71 0x4f 0xcc 0x63
IN
0xba 0x4b 0x46 0x33
C’N-1 PN
0xc7 0x4f 0x42 0x37 0x7d 0x3 0x3 0x3

Awesome.

That does require me making the right guess, surely, though?

Yes, but it’s amazing how easy it is to either make completely correct guesses, or just pick a set of values that are more likely to be good guesses, and start by trying those, failing back to the “plod through the rest of the bytes” approach when you need to.

I’ve coded an English-language frequency table into my padding oracle code, because that was appropriate for the challenge I was working on.

This code is available for you to review and use at https://github.com/alunmj/PaddingOracle/blob/master/PadOracle/Program.cs

You can imagine all kinds of ways to improve your guesses – when proceeding backward through a JSON object, for instance, a ‘}’ character will be at the end; it’ll be preceded by white space, double quotes, or brackets/braces, or maybe numerics. A 0x0a character will be preceded by a 0x0d (mostly), etc.

Parallelisation

The other big performance improvement I made was to parallelise the search. You can work on one block entirely independently from another.

I chose to let the Parallel.For() function from C# decide exactly how it was going to split up work between different blocks, and the result is a whole lot faster. There are some wrinkles to manage when parallelising an algorithm, but I’m not going to get into that here. This is not a programming blog, really!

15x performance improvement

I figured I’d put that in big letters, because it’s worth calling out – the parallelisation alone obviously multiplies your performance by the number of cores you’ve got (or the number of cores the web server has, if it’s underpowered), and the predictive work on the text does the rest. Obviously, the predictive approach only works if you can separate between “likely” and “unlikely” characters – if the plaintext consists of random binary data, you’re not going to get much of a benefit. But most data is formatted, and/or is related to English/Latin text.

Bonus stage – use a decryptor to encrypt!

I haven’t published the code for this part yet, but you can use this same breach to encrypt data without knowing the key.

This is really fun and simple once you get all the previous stuff. Here goes.

Let’s encrypt a block.

Encrypting a block requires the generation of two ciphertext blocks from one plaintext block. What the second block is, actually doesn’t matter. We can literally set it to random data, or (which is important) specific data of our choosing.

The first block of the pair, acting like an IV, we can set to 0. There’s a reason for this which we’ll come to in a minute.

With these two initial blocks, we run the decrypter. This will give us a ‘plaintext’ block as output. Remember how the intermediate block is the plaintext block XORed with the first of the pair of blocks? Well, because we set that first block to all zeroes, that means the plaintext block IS the same as the intermediate block. And that intermediate block was generated by decrypting the second block of the pair. In order for that decryption to result in the plaintext we want instead, we can simply take the intermediate block, XOR it with the plaintext block we want, and then put that into the first ciphertext block. [We’re actually XORing this with the first ciphertext block, but that’s a straight copy in this case, because the first ciphertext block is zeroes.]

Now, draw the rest of the owl

Do the same thing for each of the rest of the blocks.

Sadly, there’s no parallelising this approach, and the guessing doesn’t help you either. You have to start with CN (randomly generated) and CN-1 (deduced with the approach above), then when you’ve established what CN-1 is, you can use the same approach to get CN-2, and so on back to the IV (C0). So this process is just plain slow. But it allows you to encrypt an arbitrary set of data.

Padding Oracles for a thousand, please

We did a CTF at work.

I have to say it was loads of fun – I’ve never really participated in a CTF before, so it was really good to test my hacking skills against my new colleagues.

We had one instruction from my manager – “don’t let the interns beat you”. I was determined to make sure that didn’t happen, but I was also determined to share as much knowledge and excitement for InfoSec as possible. This meant that once or twice, I may have egged an intern on in the face of the fact that they were about to discover it anyway, and it just seemed like a really good way to keep them interested.

This is not that story.

This is about me turning the corner from knowing about a security failure, to understanding how it works. Let’s see if I can help you guys understand, too.

Tales from the Crypto

That’s the title of my blog, and there’s not a whole lot of cryptography here. It’s just a play on words, which was a little more relevant when I first started the blog back in 2005. So here’s some crypto, at last.

There’s several aspects to cryptography that you have to get right as a developer:

  • Identify whether you’re doing hashing, signing, encryption, encoding, etc.
  • If you have a key, create and store it securely
  • Pick correct algorithms – modern algorithms with few known issues
  • Use the algorithms in a way that doesn’t weaken them

Having tried to teach all of these to developers in various forms, I can tell you that the first one, which should be the simplest, is still surprisingly hard for some developers to master. Harder still for managers – the number of breach notifications that talk about passwords being “encrypted” is a clear sign of this – encrypted passwords mean either your developers don’t understand and implemented the wrong thing, or your manager doesn’t understand what the developer implemented and thinks “encrypted sounds better than hashed”, and puts that down without checking that it’s still technically accurate.

Key creation (so it’s not predictable), and storage (so it can’t be found by an attacker) is one of those issues that seems to go perennially unsolved – I’m not happy with many of the solutions I’ve seen, especially for self-hosted services where you can’t just appeal to a central key vault such as is currently available in all good cloud platforms.

Picking correct algorithms is a moving target. Algorithms that were considered perfectly sound ten or twenty years ago are now much weaker, and can result in applications being much weaker if they aren’t updated to match new understanding of cryptography, and processor and memory speed and quantity improvements. You can store rainbow tables in memory now that were unthinkable on disk just a decade or two ago.

Finally, of course, if all that wasn’t enough to make cryptography sound really difficult (spoiler: it is, which is why you get someone else to do it for you), there are a number of ways in which you can mess up the way in which you use the algorithm.

Modes, block-sizes, and padding

There are a large number of parameters to set even when you’ve picked which algorithms you’re using. Key sizes, block sizes, are fairly obvious – larger is (generally) better for a particular algorithm. [There are exceptions, but it’s a good rule of thumb to start from.]

There are a number of different modes available, generally abbreviated to puzzling TLAs – ECB, CFB, OFB, CBC, GCM, CTR, and so on and so forth. It’s bewildering. Each of these modes just defines a different order in which to apply various operations to do things like propagating entropy, so that it’s not possible to infer anything about the original plaintext from the ciphertext. That’s the idea, at least. ECB, for instance, fails on this because any two blocks of plaintext that are the same will result in two blocks of ciphertext that are the same.

And if you’re encrypting using a block cipher, you have to think about what to do with the last block – which may not be a complete block. This requires that the block be filled out with “padding” to make a full block. Even if you’re just filling it out with zeroes, you’re still padding – and those zeroes are the padding. (And you have to then answer the question “what if the last block ended with a zero before you padded it?”)

There’s a number of different padding schemes to choose from, too, such as “bit padding”, where after the last bit, you set the next bit to 1, and the remaining bits in the block to 0. Or there’s padding where the last byte is set to the count of how many padding bytes there are, and the remaining bytes are set to 0 – or a set of random bytes – or the count repeated over and over. It’s this latter that is embodied as PKCS#5 or PKCS#7 padding. For the purposes of this discussion, PKCS#7 padding is a generalised version of PKCS#5 padding. PKCS#5 padding works on eight-byte blocks, and PKCS#7 padding works on any size blocks (up to 256 bytes, presumably).

So, if you have a three-byte last block, and the block size is 16 bytes, the last block is ** ** ** 0x0d 0x0d 0x0d 0x0d 0x0d 0x0d 0x0d 0x0d 0x0d 0x0d 0x0d 0x0d 0x0d (where “**” represents the last three bytes of data, and 0x0d represents the hexadecimal value for 13, the number of bytes in the padding). If your last block is full, PKCS#7 covers this by making you create an extra 16-byte block, with the value 0x10 (decimal 16) in every byte.

Tying this in to the CTF

It’s not at all unlikely that you wind up with the scenario with which we were presented in the CTF – a service that communicated with AES encryption, in CBC mode, using PKCS#7 padding. The fact that this was described as such was what tipped me off in the first place. This is the perfect setup for a Padding Oracle attack.

An Oracle is simply a device/function/machine/system/person that you send a request to, and get a response back, and which gives you some information as a result. The classical Oracles of Ancient Greece & Roman times were confusing and unhelpful at best, and that’s really something we want from any cryptographic oracle. The term “Random Oracle” refers to a hypothetical system which returns random information to every query. A good cryptographic system is one that is indistinguishable from a Random Oracle.

Sadly, CBC with PKCS#7 padding is generally very different from a Random Oracle. It is a Padding Oracle, because it will tell us when the padding is correct or incorrect. And that’s our vulnerability.

At this point, I could have done what one of my colleagues did, and download PadBuster, choosing parameters and/or modifying code, to crack the encryption.

But… I’ve been attacking this CTF somewhat … non-traditionally, using tools other than the normal ones, and so I thought I’d try and understand the algorithm’s weaknesses, and implement my own attack. I wrote it on the bus on my way into work, and was pleased to see when I got in that it worked – albeit slowly – first time.

How CBC and PKCS#5/7 is vulnerable

When decrypting each block using CBC, we say that PN = DK(CN)⊕CN-1 – which is just a symbolic way of saying that the recipient Decrypts (with key “K”) the current Ciphertext block (block N), and then XORs the result with the previous Ciphertext block (the N-1th block). Let’s also assume that we’re only decrypting those two blocks, N-1 and N, with N being the last block provided to the recipient.

In other modes, the padding check may not deliver the helpful information we’re looking for, but CBC is special. The way CBC decrypts data is to decrypt the current block of ciphertext (CN), which creates an intermediate block DK(CN). That intermediate block is combined with the previous ciphertext block, CN-1, to give the plaintext block, PN. This combining of blocks is done using the XOR (exclusive-or) operation, which has interesting properties any developer should be familiar with. Particularly, it’s important to note that XOR (represented here as “⊕”) is reversible. If X⊕Y=Z, you know also that Z⊕Y=X and Z⊕X=Y. This is one of the reasons the XOR operation is used in a lot of cryptographic algorithms.

If we want to change things in the inputs to produce a different output, we can really only change two things – the current and the previous block of Ciphertext – CN and CN-1. We should really only alter one input at a time. If we alter CN, that’s going to be decrypted, and a small change will be magnified into a big difference to the DK(CN) value – all the bytes will have changed. But if we alter CN-1, just a bit, what we wind up with is a change in the plaintext value PN which matches that change. If we alter the 23rd bit of CN-1, it will alter the 23rd bit of PN, and only that one bit. Now if we can find what we’ve changed that bit to, we can then figure out what that means we must have changed it from.

If we change the last byte of CN-1, to create C’N-1 (pronounced “C prime of N minus 1”) and cycle it through all the possible values it can take, the decryption will occur, and the recipient will reject our new plain text, P’N (“P prime of N”) because it is poorly formed – it will have a bad padding. With one (two, but I’ll come to that in a minute) notable exception. If the last byte of the plaintext decrypted is the value 0x01, it’s a single byte of padding – and it’s correct padding. For that value of the last byte of C’N-1, we know that the last byte of P’N is 1. We can rewrite PN = DK(CN)⊕CN-1 as DK(CN) = CN-1⊕PN – and then we can put the values in for the last byte: DK(CN)[15] = C’N-1[15]⊕0x01.

Let’s say, for illustration’s sake, that the value we put in that last byte of C’N-1 was 0xa5, when our padding was accepted. That means DK(CN)[15] = 0xa5 ⊕ 0x01 = 0xa4. Note the lack of any “prime” marks there – we’ve figured out what the original value of the decrypted last byte was. Note that this isn’t the same as the last byte of the plain text. No, we get that by taking this new value and XORing it with the original last byte of the previous block of ciphertext – that’s CN-1[15]. For illustration, let’s say that value is 0xc5. We calculate PN[15] = DK(CN)[15]⊕CN-1[15] = 0xa4⊕0xc5 = 0x61. That’s the lower case letter ‘a’.

OK, so we got the first piece of plaintext out – the last byte.

[Remember that I said I’d touch on another case? If CN is the original last block of ciphertext, it already contains valid padding! But not necessarily the 0x01 we’re trying to force into place.]

Let’s get the next byte!

Almost the same process is used to get the next byte, with a couple of wrinkles. First, obviously, we’re altering the second-to-last byte through all possible values. Second, and not quite so obvious, we have to tweak the last byte once as well, because we’re looking to get the sequence 0x02 0x02 (two twos) to happen at the end of P’N. The last byte of C’N-1 to achieve this is simply the last byte of C’N-1 that we used to get 0x01, XORed by 0x03 (because that’s 0x02 ⊕ 0x01). In our illustrative example, that’s 0xa6.

And the next, and the next…

Each time, you have to set the end values of the ciphertext block, so that the end of P’N will look like 0x03 0x03 0x03, 0x04 0x04 0x04 0x04, etc, all the way up to 0x10 … 0x10 (sixteen 16s).

Code, or it didn’t happen

So here’s the 200 lines or so that I wrote on the bus. I also wrote a test harness so that this would work even after the CTF finished and got shut down. You’ll find that in the same repo.

I’ve massaged the code so it’s easier to understand, or to use as an explainer for what’s going on.

I plan on expanding this in a couple of ways – first, to make it essentially command-line compatible with ‘PadBuster’, and second, to produce a graphical demo of how the cracking happens.

And in the next post, I’m going to talk a little about how I optimised this code, so that it was nearly 15x faster than PadBuster.

One Simple Thing

I’ve been a little absent from this blog for a while, mostly because I’ve been settling in to a new job where I’ve briefly changed my focus almost completely from application security to being a software developer.

The blog absence is going to change now, and I’d like to start that with a renewed effort to write something every week. In addition to whatever grabs my attention from the security news feeds I still suck up, I want to get across some of knowledge and approaches I’ve used while working as an application security guy. I’ll likely be an application security guy in my next job, whenever that is, so it’ll stand me in good stead to write what I think.

“One Simple Thing”

The phrase “One Simple Thing” underscores what I try to return to repeatedly in my work – that if you can get to the heart of what you’re working on, everything else flows easily and smoothly.

This does not mean that there’s only one thing to think about with regards to security, but that when you start asking clarifying questions about the “one simple thing” that drives – or stops – a project in the moment, it’s a great way to make tremendous progress.

The Simplest Security Thing

I’ll start by discussing the One Simple Thing I pick up by default whenever I’m given a security challenge.

What are we protecting?

This is the first question I ask on joining a new security team – often as early as the first interviews. Everyone has a different answer, and it’s a great way to find out what approaches you’re likely to encounter. The question also has several cling-on questions that it demands be asked and answered at the same time:

Why are we protecting it?

Who are we protecting it from?

Why do they want it?

Why shouldn’t they get it?

What are our resources?

These come very quickly out of the One Simple Thing of “what are we protecting?”.

Here’s some typical answers:

  • Our systems need to continue running and be accessible at all times
  • We need to keep making money
  • We need to stop [the risk of] losing money
  • Our customers trust us, and we need that to continue
  • We operate in a regulatory compliance climate, and need to keep our lawsuits down to close to zero
  • We don’t even have a product yet, and haven’t a clue what we need to secure
  • We don’t want a repeat of last week’s / month’s / year’s very public security / privacy debacle
  • We want to balance the amount we spend on security staff against the benefit it has to our bottom line – and demonstrate results

You can see from the selection of answers that not everyone has anything like the same approach, and that they don’t all line up exactly under the typical buckets of Confidentiality, Integrity and Availability.

Do you think someone can solve your security issues or set up a security team without first finding out what it is you’re protecting?

Do you think you can engage with a team on security issues without understanding what they think they’re supposed to be protecting?

Answers vary

You’ve seen from my [short] list above that there are many answers to be had between different organisations and companies.

I’d expect there to be different answers within an organisation, within a team, within a meeting room, and even depending on the time I ask the question.

“What are we protecting” on the day of the Equifax leak quickly becomes a conversation on personal data, and the damaging effect of a leak to “customers”. [I prefer to call them “data subjects”, because they aren’t always your customers.]

On the day that Yahoo gets bought by Verizon for substantially less than initially offered, the answer becomes more about company value, and even perhaps executive stability.

Next time you’re confused by a security problem, step back and ask yourself – and others – “What are we protecting?” and see how much it clarifies your understanding.

My contribution to Movember

Whether it’s “No-shave November” or “Movember”, there’s a lot of attention given this time of year to men’s health in general, and cancer in particular.

I don’t take part in either of these events, partly because I don’t like the way a beard / moustache feels, but mostly because I already spend my November extremely aware of men’s cancer issues.

So, let me tell you, on this International Men’s Day, how my right testicle tried to kill me. Warning – rude words ahead, including the dreaded “c-word”.

Here we go gathering nuts in May

A little over fifteen years ago, I was living a fantastic life.

A wife, a six-year-old son, a house in a nice suburb of Austin, working from home on my own projects, and making enough money with those projects to justify doing so.

As anyone who’s ever watched any “funny home video” shows on TV will tell you, the purpose of a six year old is to throw things at your crotch, or to swing things at your crotch, or to hit you in your crotch, or to head-butt you in your crotch.

OK, so that’s maybe not his sole purpose, but that year it seemed like this was happening more often than usual. It wasn’t, of course, but it was noticeable that I was … feeling the impact a little more keenly than usual.

It takes balls to go see a doctor

I checked, my wife checked, and we concurred – something was definitely not as it had been. I mean, everyone knows that a man’s testicles aren’t the same size and shape on each side, and I’d been blessed with a particularly disparate pair from my teenage years.

But this was something new – swelling that just increased gradually, and a firmness that was inappropriately placed.

It was time to see the doctor.

Even knowing this, and reading about how badly – and how quickly – testicular diseases can impact men, it was extraordinarily difficult to face the task of picking up the phone, calling to speak to a doctor’s [female] receptionist, and tell them exactly why I wanted to come and see the doctor. Nonetheless, I girded my loins as much as I could, swallowed hard, and made the call.

The key is to remind yourself that this is probably the fifth call that receptionist has received this week on the same topic, and that she wouldn’t be working in a doctor’s office if she weren’t ready to hear medical terms briefly describing anatomical parts. I’m surprised how quickly I came to this conclusion, given how many decades it took me to learn that when a doctor asks “so, how are you doing today?”, they actually want to hear the details, rather than “oh, fine, thanks, and you?”

“Good luck”

The doctor’s visit was quick and clinical, just what you’d hope for. A flashlight applied to the nether regions, in much the same way you might check a hen’s egg for occupants, a little uncomfortable palpation, and a quick inspection of nearby things while you have your underpants down.

“You’ve got a hydrocele,” he said, doing that thing with the rubber gloves where you snap them off, startling an already nervous patient. “A short surgery should fix that.”

Relief. Nothing quite as horrifying or scary as I had expected.

“I’ll set you up with a urologist, and we’ll get that taken care of in the next couple of weeks. Good luck.”

I’d never had a doctor wish me “good luck” before, and it quite chilled me.

You’re never ready for the C-word

I visited the urologist, got set up for surgery, and discussed plans with my wife.

It was always in the back of my head that this could be something more than merely having a little extra fluid to drain.

So we talked about the C-word. I think of it that way, because on all the forms since, this is the one word the medical establishment goes out of its way to avoid writing in full. There are long words, foreign words, culturally taboo words, and all of them are written in full on some or other medical forms. There are abbreviations, but no word more than this one results in hardened medical professionals ceding to decency and refusing to name it in full:

Cancer

You kind of guessed that was going to be the result, right?

We kind of did, too, and had discussed the idea that if there was any cancerous signs, that quite frankly I preferred being a living eunuch, if that was necessary, to being a dead, but otherwise intact, cancerous corpse. It seems such an obvious decision to make, but it’s still a very hard one to bring to bear.

And my wife did so on her own.

Because the only way to tell if the testicle looked cancerous was while I was under general anaesthetic in the operating room.

And sure enough, the doctor came out mid-surgery, while I’m away with the fairies, to talk to my wife about the situation at hand. I can only imagine how that conversation went, so I shan’t try to replay it here. I can only express how truly grateful I am that my wife gave consent to do what we had already discussed – to remove that cancerous nasty thing and send it to a lab for study.

So I woke up to a woman looking unutterably upset at the prospect that she’d had to make life-altering medical decisions, for which I have always been truly grateful. There literally isn’t a day that goes by that I wish she’d made any other choice.

And yet even to this day, it still bothers her – that’s how upsetting it is to be on the outside of this disease.

It wasn’t much fun on the inside, either, to be honest, and that’s my story which I can tell.

Time to hulk up!

This was all in the week before Thanksgiving, 2002, a year when the first movie  featuring an all-CGI Incredible Hulk was being advertised on the TV.

Poor Bruce Banner, strapped to a table, unable to move, while gamma rays coursed through his body under the control of a malfunctioning computer, turning him into the hangriest super-anti-hero ever.

After a trip to San Antonio, during which I felt every pothole on the I-35 from Austin, to have Thanksgiving dinner with my inlaws, we returned home and started observational and preventive treatment as follow up for good ole “testicular C”.

First, the tattoos. I have five tattoos now, each one a single dot, in the shape of a cross.

For targetting.

I wasn’t exactly strapped to a table, but I was unable to move, while gamma rays coursed through my body, laser cross-hairs ensuring that the focused radiation hit only the right parts of my intestines. They call it radiotherapy, and when you go to an oncologist / radiologist to get radiotherapy in Austin in 2002, you sit in a waiting room surrounded by inspirational photos of Lance Armstrong. Whatever you feel about his drug use while winning the Tour de France competing against others who almost certainly used most of the same drugs themselves, he continues to be inspirational to many cancer survivors like myself, simply for having survived enough to be able to ride a bike.

Testicular cancer doesn’t travel across, it goes up – so the process is, remove the testicle, fry the intestines lightly, and monitor the chest with ongoing X-rays just to make sure. Removing the testicle is called an “orchiectomy” – true story, the orchid plant is named after testicles, because that’s what the plant’s bulbs allegedly look like. This is why testicular cancer awareness pins are orchid-coloured.

Insurance companies make me sick

One of the side effects you think of with any cancer treatment is serious nausea, and this is definitely the case with radiotherapy. It makes you feel uncomfortably unwell. American medical care being run by insurance companies, I was given leave to have fifteen anti-nausea pills. For 25 days of treatment. During which I’d need multiple pills per day.

The only thing to do – snack on saltine crackers, and where possible actually cook some meals at least for my son. Bland food was really pretty much all I could manage. To this day, he quite rightly refuses to eat chicken and rice.

Because my wife had to return to work, and was travelling as a result, I drove myself to appointments, and that’s probably my biggest mistake in all of this – the American Cancer Society offers free rides to patients attending hospital and doctor appointments, and has many other services besides. Take advantage of them, I donate to them specifically for you to use their services.

Cat scans and pregnancy tests

After that, every six months to a year, I’d get a CT scan of my abdomen, and a blood test every month. CT scans are not the most comfortable of procedures, particularly with the iodine contrast dyes.

Once in a while, the person administering the blood test would question whether the test was really for me. On my doctor’s advice, I would ask them to re-check the form. It turns out that I was basically being given a monthly pregnancy test, to ensure the cancer wasn’t coming back.

More surgeries, and more insurance

Still more surgeries were in my future over the next year – apparently, skin likes to stick to skin in unusual situations and in uncomfortable ways.

The insurance company raised our rates – presumably in line with regular price rises, but to the point where it was difficult to afford. After all, even back before the ACA, it wasn’t right to raise insurance rates just because someone got sick. However, what WAS legal back then was the ability of other insurance providers to call the cancer a pre-existing condition, and to use that as reason to either refuse to sell me a policy, or to jack up the rates. Personal insurance policies are expensive to begin with, but when you can’t shop around (or threaten to do so), you’re really out of luck.

And that’s why I took the Microsoft job, and jacked in my personal business for the most part. Because American health insurance kills the American dream more often than it deserves to.

And now, the lesson

So, the final lesson – and there always is one – is that if you are a man, aged between twenty and thirty-five, or you know someone who fits, or will fit, that description, know that it’s important to check your health – actually touch and feel your body, particularly your ‘man parts’ – on a regular basis. When things change in a way that isn’t expected, it’s really important to give your doctor a call. That week. Perhaps even that day that you notice it. The person who takes your call has heard it all before – and if you aren’t comfortable talking to them, you can actually ask to speak to a nurse, a physician’s assistant, and even specifically to a man, if that’s what you need to feel comfortable to cover this.

Your doctor will tell you if it’s important, or something not to worry about. They’ll give you advice on what to watch for in future, and wish you good luck if you need it.

Above all, don’t literally die of embarrassment.

Lessons from my first official bounty

Sometimes, it’s just my job to find vulnerabilities, and while that’s kind of fun, it’s also a little unexciting compared to the thrill of finding bugs in other people’s software and getting an actual “thank you”, whether monetarily or just a brief word.

About a year ago, I found a minor Cross-Site Scripting (XSS) flaw in a major company’s web page, and while it wasn’t a huge issue, I decided to report it, as I had a few years back with a similar issue in the same web site. I was pleased to find that the company was offering a bounty programme, and simply emailing them would submit my issue.

Lesson 1: The WAF… it does nothing!

The first thing to notice, as with all XSS issues, is that there were protections in place that had to be got around. In this case, some special characters or sequences were being blocked. But not all. And it’s really telling that there are still many websites which have not implemented widespread input validation / output encoding as their XSS protection. So, while the WAF slowed me down even when I knew the flaw existed, it only added about 20 minutes to the exploit time. So, my example had to use “confirm()” instead of “alert()” or “prompt()”. But really, if I was an attacker, my exploit wouldn’t have any of those functions, and would probably include an encoded script that wouldn’t be detected by the WAF either. WAFs are great for preventing specific attacks, but aren’t a strong protection against an adversary with a little intelligence and understanding.

Lesson 2: A quick response is always welcome

My email resulted in an answer that same day, less than an hour after my initial report. A simple “thank you”, and “we’re forwarding this to our developers” goes a long way to keeping a security researcher from idly playing with the thought of publishing their findings and moving on to the next game.

Lesson 3: The WAF… still does nothing!

In under a week, I found that the original demo exploit was being blocked by the WAF – but if I replaced “onclick” with “oclick”, “onmouseover” with “omouseover”, and “confirm” with “cofirm”, I found the blocking didn’t get in the way. Granted, since those aren’t real event handlers or JavaScript functions, I can’t use those in a real exploit, but it does mean that once again, all the WAF does is block the original example of the attack, and it took only a few minutes again to come up with another exploit string.

Lesson 4: Keep communicating

If they’d told me “hey, we’re putting in a WAF rule while we work on fixing the actual bug”, I wouldn’t have been so eager to grump back at them and say they hadn’t fixed the issue by applying their WAF and by the way, here’s another URL to exploit it. But they did at least respond to my grump and reassure me that, yes, they were still going to fix the application.

Lesson 5: Keep communicating

I heard nothing after that, until in February of this year, over six months later, I replied to the original thread and asked if the report qualified for a bounty, since I noticed that they had actually fixed the vulnerability.

Lesson 6: Keep communicating

No response. Thinking of writing this up as an example of how security researchers still get shafted by businesses – bear in mind that my approach is not to seek out bounties for reward, but that I really think it’s common courtesy to thank researchers for reporting to you rather than pwning your website and/or your customers.

Lesson 7: Use a broker

About a month later, while looking into other things, I found that the company exists on HackerOne, where they run a bug bounty. This renewed my interest in seeing this fixed. So I reported the email exchange from earlier, noted that the bug was fixed, and asked if it constituted a rewardable finding. Again, a simple “thanks for the report, but this doesn’t really rise to the level of a bounty” is something I’ve been comfortable with from many companies (though it is nice when you do get something, even if it’s just a keychain or a t-shirt, or a bag full of stickers).

Lesson 8: Keep communicating

3/14: I got a reply the next day, indicating that “we are investigating”.

3/28: Then nothing for two weeks, so I posted another response asking where things were going.

4/3: Then a week later, a response. “We’re looking into this and will be in touch soon with an update.”

4/18: Me: Ping?

5/7: Me: Hey, how are we doing?

5/16: Anything happening?

Lesson 9: Deliver

5/18: Finally, over two months after my report to the company through HackerOne, and ten months after my original email to the first bug bounty address, it’s addressed.

5/19: The severity of the bug report is lowered (quite rightly, the questionnaire they used pushed me to a priority of “high”, which was by no means warranted). A very welcome bounty, and a bonus for my patience – unexpected but welcome, are issued.

Lesson 10: Learn

The cheapest way to learn things is from someone else’s mistakes. So I decided to share with my readers the things I picked up from this experience.

Here are a few other lessons I’ve picked up from bug bounties I’ve observed:

Lesson 11: Don’t start until you’re ready

If you start a bug bounty, consider how ready you might be. Are you already fixing all the security bugs you can find for yourself? Are you at least fixing those bugs faster than you can find more? Do your developers actually know how to fix a security bug, or how to verify a vulnerability report? Do you know how to expand on an exploit, and find occurrences of the same class of bug? [If you don’t, someone will milk your bounty programme by continually filing variations on the same basic flaw]

Lesson 12: Bug Bounty programmes may be the most expensive way to find vulnerabilities

How many security vulnerabilities do you think you have? Multiply that by an order of magnitude or two. Now multiply that by the average bounty you expect to offer. Add the cost of the personnel who are going to handle incoming bugs, and the cost of the projects they could otherwise be engaged in. Add the cost of the developers, whose work will be interrupted to fix security bugs, and add the cost of the features that didn’t get shipped on time before they were fixed. Sure, some of that is just a normal cost of doing business, when a security report could come at you out of the blue and interrupt development until it’s fixed, but starting a bug bounty paints a huge target on you.

Hiring a penetration tester, or renting a tool to scan for programming flaws, has a fixed cost – you can simply tell them how much you’re willing to pay, and they’ll work for that long. A bug bounty may result in multiple orders of magnitude more findings than you expected. Are you going to pay them all? What happens when your bounty programme runs out of money?

Finding bugs internally using bug bashes, software scanning tools or dedicated development staff, has a fixed cost, which is probably still smaller than the amount of money you’re considering on putting into that bounty programme.

12.1: They may also be the least expensive way to find vulnerabilities

That’s not to say bug bounties are always going to be uneconomical. At some point, in theory at least, your development staff will be sufficiently good at resolving and preventing security vulnerabilities that are discovered internally, that they will be running short of security bugs to fix. They still exist, of course, but they’re more complex and harder to find. This is where it becomes economical to lure a bunch of suckers – excuse me, security researchers – to pound against your brick walls until one of them, either stronger or smarter than the others, finds the open window nobody saw, and reports it to you. And you give them a few hundred bucks – or a few thousand, if it’s a really good find – for the time that they and their friends spent hammering away in futility until that one successful exploit.

At that point, your bug bounty programme is actually the least expensive tool in your arsenal.

Security Questions are Bullshit

 

I’m pretty much unhappy with the use of “Security Questions” – things like “what’s your mother’s maiden name”, or “what was your first pet”. These questions are sometimes used to strengthen an existing authentication control (e.g. “you’ve entered your password on a device that wasn’t recognised, from a country you normally don’t visit – please answer a security question”), but far more often they are used as a means to recover an account after the password has been lost, stolen or changed.

 

I’ve been asked a few times, given that these are pretty widely used, to explain objectively why I have such little disregard for them as a security measure. Here’s the Too Long; Didn’t Read summary:

  1. Most questions are equivalent to a low-entropy password
  2. Answers to many of these questions can be found online in public documents
  3. Answers can be squeezed out of you, by fair means or foul
  4. The same questions – and answers – are shared at multiple sites
  5. Questions (and answers) don’t get any kind of regulatory protection
  6. Because the best answers are factual and unchanging, you cannot change them if they are exposed

Let’s take them one by one:

Questions are like a low-entropy password

What’s your favourite colour? Blue, or Green. At the outside, red, yellow, orange or purple. That covers most people’s choices, in less than 3 bits of entropy.

What’s your favourite NBA team? There’s 29 of those – 30, if you count the 76ers. That’s 6 bits of entropy.

Obviously, there are questions that broaden this, but are relatively easy to guess with a small number of tries – particularly when you can use the next fact about Security Questions.

The Answers are available online

What’s your mother’s maiden name? It’s a matter of public record.

What school did you go to? If we know where you grew up, it’s easy to guess this, since there were probably only a handful of schools you could possibly have attended.

Who was your first boyfriend/girlfriend? Many people go on about this at length in Facebook posts, I’m told. Or there’s this fact:

You’ll tell people the answers

What’s your porn name? What’s your Star Wars name? What’s your Harry Potter name?

All these stupid quizzes, and they get you to identify something about yourself – the street you grew up on, the first initial of your secret crush, how old you were when you first heard saxophones.

And, of course, because of the next fact, all I really have to do is convince you that you want a free account at my site.

Answers are shared everywhere

Every site that you visit asks you variants of the same security questions – which means that you’ll have told multiple sites the same answers.

You’ve been told over and over not to share your password across multiple sites – but here you are, sharing the security answers that will reset your password, and doing so across multiple sites that should not be connected.

And do you think those answers (and the questions they refer back to) are kept securely by these various sites? No, because:

Questions & Answers are not protected like passwords

There’s regulatory protection, under regimes such as PCI, etc, telling providers how to protect your passwords.

There is no such advice for protecting security questions (which are usually public) and the answers to them, which are at least presumed to be stored in a back-end database, but are occasionally sent to the client for comparison against the answers! That’s truly a bad security measure, because of course you’re telling the attacker.

Even assuming the security answers are stored in a database, they’re generally stored in plain text, so that they can be accessed by phone support staff to verify your answers when you call up crying that you’ve forgotten your password. [Awesome pen-testing trick]

And because the answers are shared everywhere, all it takes is a breach at one provider to make the security questions and answers they hold have no security value at all any more.

If they’re exposed, you can’t change them

There’s an old joke in security circles, “my password got hacked, and now I have to rename my dog”. It’s really funny, because there are so many of these security answers which are matters of historical fact – while you can choose different questions, you can’t generally choose a different answer to the same question.

Well, obviously, you can, but then you’ve lost the point of a security question and answer – because now you have to remember what random lie you used to answer that particular question on that particular site.

And for all you clever guys out there…

Yes, I know you can lie, you can put in random letters or phrases, and the system may take them (“Your place of birth cannot contain spaces” – so, Las Vegas, New York, Lake Windermere are all unusable). But then you’ve just created another password to remember – and the point of these security questions is to let you log on once you’ve forgotten your password.

So, you’ve forgotten your password, but to get it back, you have to remember a different password, one that you never used. There’s not much point there.

In summary

Security questions and answers, when used for password recovery / reset, are complete rubbish.

Security questions are low-entropy, predictable and discoverable password substitutes that are shared across multiple sites, are under- or un-protected, and (like fingerprints) really can’t be changed if they become exposed. This makes them totally unsuited to being used as password equivalents in account recovery / password reset schemes.

If you have to implement an account recovery scheme, find something better to use. In an enterprise, as I’ve said before, your best bet is to use something that the enterprise does well – the management hierarchy. Every time you forget your password, you have to get your manager, or someone at the next level up from them, to reset your password for you, or to vouch for you to tech support. That way, someone who knows you, and can affect your behaviour in a positive way, will know that you keep forgetting your password and could do with some assistance. In a social network, require the

Password hints are bullshit, too

Also, password hints are bullshit. Many of the Adobe breach’s “password hints” were actually just the password in plain-text. And, because Adobe didn’t salt their password hashes, you could sort the list of password hashes, and pick whichever of the password hints was either the password itself, or an easy clue for the password. So, even if you didn’t use the password hint yourself, or chose a really cryptic clue, some other idiot came up with the same password, and gave a “Daily Express Quick Crossword” quality clue.

How to do password reset

First, a quick recap:

Credentials include a Claim and a Proof (possibly many).

The Claim is what states one or more facts about your identity.

A Username is one example of a Claim. So is Group Membership, Age, Eye Colour, Operating System, Installed Software, etc…

The Proof is what allows someone to reliably trust the Claim is true.

A Password is one example of a Proof. So is a Signature, a Passport, etc…

Claims are generally public, or at least non-secret, and if not unique, are at least specific (e.g. membership of the group “Brown eyes” isn’t open to people with blue eyes).

Proofs are generally secret, and may be shared, but such sharing should not be discoverable except by brute force. (Which is why we salt passwords).

Now, the topic – password resets

Password resets can occur for a number of reasons – you’ve forgotten your password, or the password change functionality is more cumbersome than the password reset, or the owner of the account has changed (is that allowable?) – but the basic principle is that an account needs a new password, and there needs to be a way to achieve that without knowledge of the existing password.

Let’s talk as if it’s a forgotten password.

So we have a Claim – we want to assert that we possess an identity – but we have to prove this without using the primary Proof.

Which means we have to know of a secondary Proof. There are common ways to do this – alternate ID, issued by someone you trust (like a government authority, etc). It’s important in the days of parody accounts, or simply shared names (is that Bob Smith, his son, Bob Smith, or his unrelated neighbour, Bob Smith?) that you have associated this alternate ID with the account using the primary Proof, or as a part of the process of setting up the account with the primary Proof. Otherwise, you’re open to account takeover by people who share the same name as their target.

And you can legally change your name.

What’s the most common alternate ID / secondary Proof?

E-mail.

Pretty much every public web site relies on the use of email for password reset, and uses that email address to provide a secondary Proof.

It’s not enough to know the email address – that’s unique and public, and so it matches the properties of a Claim, not a Proof, of identity.

We have to prove that we own the email address.

It’s not enough to send email FROM the email address – email is known to be easily forged, and so there’s no actual proof embodied in being able to send an email.

That leaves the server with the prospect of sending something TO the email address, and the recipient having proved that they received it.

You could send a code-word, and then have the recipient give you the code-word back. A shared secret, if you like.

And if you want to do that without adding another page to the already-too-large security area of the site, you look for the first place that allows you to provide your Claim and Proof, and you find the logon page.

By reusing the logon page, you’re going to say that code-word is a new password.

[This is not to say that email is the only, or even the best, way to reset passwords. In an enterprise, you have more reliable proofs of identity than an email provider outside of your control. You know people who should be able to tell you with some surety that a particular person is who they claim to be. Another common secondary identification is the use of Security Questions. See my upcoming article, “Security Questions are Bullshit” for why this is a bad idea.]

So it’s your new password, yes?

Well, yes and no. No, actually. Pretty much definitely no, it’s not your new password.

Let’s imagine what can go wrong. If I don’t know your password, but I can guess your username (because it’s not secret), I can claim to be you wanting to reset your password. That not only creates opportunity for me to fill your mailbox with code-words, but it also prevents you from logging on while the code-words are your new password. A self-inflicted denial of service.

So your old password should continue working, and if you never use the code-word, because you’re busy ignoring and deleting the emails that come in, it should keep working for you.

I’ve frequently encountered situations in my own life where I’ve forgotten my password, gone through the reset process, and it’s only while typing in the new password, and being told what restrictions there are on characters allowed in the new password, that I remember what my password was, and I go back to using that one.

In a very real sense, the code-word sent to you is NOT your new password, it’s a code-word that indicates you’ve gone the password reset route, and should be given the opportunity to set a new password.

Try not to think of it as your “temporary password”, it’s a special flag in the logon process, just like a “duress password”. It doesn’t replace your actual password.

It’s a shared secret, so keep it short-lived

Shared secrets are fantastic, useful, and often necessary – TLS uses them to encrypt data, after the initial certificate exchange.

But the trouble with shared secrets is, you can’t really trust that the other party is going to keep them secret very long. So you have to expire them pretty quickly.

The same is true of your password reset code-word.

In most cases, a user will forget their password, click the reset link, wait for an email, and then immediately follow the password reset process.

Users are slow, in computing terms, and email systems aren’t always directly linked and always-connected. But I see no reason why the most usual automated password reset process should allow the code-word to continue working after an hour.

[If the process requires a manual step, you have to count that in, especially if the manual step is something like “contact a manager for approval”, because managers aren’t generally 24/7 workers, the code-word is going to need to last much longer. But start your discussion with an hour as the base-point, and make people fight for why it’ll take longer to follow the password reset process.]

It’s not a URL

You can absolutely supply a URL in the email that will take the user to the right page to enter the code-word. But you can’t carry the code-word in the URL.

Why? Check out these presentations from this year’s Black Hat and DefCon, showing the use of a malicious WPAD server on a local – or remote – network whose purpose is to trap and save URLs, EVEN HTTPS URLs, and their query strings.

Every URL you send in an email is an HTTP or HTTPS GET, meaning all the parameters are in the URL or in the query string portion of the URL.

This means the code-word can be sniffed and usurped if it’s in the URL. And the username is already assumed to be known, since it’s a non-secret. [Just because it’s assumed to be known, don’t give the attacker an even break – your message should simply say “you requested a password reset on an account at our website” – a valid request will come from someone who knows which account at your website they chose to request.]

So, don’t put the code-word in the URL that you send in the email.

Log it, and watch for repeats, errors, other bad signs

DON’T LOG THE PASSWORD

I have to say that, because otherwise people do that, as obviously wrong as it may seem.

But log the fact that you’ve changed a password for that user, along with when you did it, and what information you have about where the user reset their password from.

Multiple users resetting their password from the same IP address – that’s a bad sign.

The same user resetting their password multiple times – that’s a bad sign.

Multiple expired code-words – that’s a bad sign.

Some of the bad things being signaled include failures in your own design – for instance, multiple expired code-words could mean that your password reset function has stopped working and needs checking. You have code to measure how many abandoned shopping carts you have, so include code that measures how many abandoned password reset attempts you have.

In summary, here’s how to do email password reset properly

  1. Consider whether email is an appropriately reliable secondary proof of identification.
    1. A phone call from the user, to their manager, followed by the manager resetting the user’s password is trustworthy, and self-correcting. A little slow and awkward, but stronger.
    2. Other forms of identification are almost all stronger and more resistant to forgery and attack than email.
    3. Bear in mind that, by trusting to email, you’re trusting that someone who just forgot their password can remember their other password, and hasn’t shared it, or used an easily-guessed password.
  2. When an account is created, associate it with an email address, and allow & encourage the validated account holder to keep that updated.
  3. Allow anyone to begin a password reset process – but don’t allow them to repeatedly do so, for fear of allowing DoS attacks on your users.
  4. When someone has begun a password reset process, send the user a code-word in email. This should be a random sequence, of the same sort of entropy as your password requirements.
    1. Encoding a time-stamp in the code-word is useful.
    2. Do not put the code-word in a URL in the message.
    3. Do not specify the username in the message, not even in the URL.
  5. Do not change the user’s password for them yet. This code-word is NOT their new password.
  6. Wait for up to an hour (see item 4.1. – the time stamp) during which you’ll accept the code-word. Be very cautious about extending this period.
  7. The code-word, when entered at the password prompt for the associated user, is an indication to change the password, and that’s ALL it can be used for.
  8. Allow the user to cancel out of the password change, up to the point where they have entered a new password and its verification code and hit OK.

And here’s how you’re going to screw it up (don’t do these!)

  • You’ll use email for password resets on a high-security account. Email is relatively low-security.
  • You’ll use email when there’s something more reliable and/or easier to use, because “it’s what everyone does”.
  • You’ll use email to an external party with whom you don’t have a business relationship, as the foundation on which you build your corporate identity tower of cards.
  • You won’t have a trail of email addresses associated with the user, so you don’t have reason to trust you’re emailing the right user, and you won’t be able to recover when an email address is hacked/stolen.
  • You’ll think the code-word is a new password, leading to:
    • Not expiring the code-word.
    • Sending passwords in plain-text email (“because we already do it for password reset”).
    • Invalidating the user’s current password, even though they didn’t want it reset.
  • You’ll include the credentials (username and/or password) in a URL in the email message, so it can be picked up by malicious proxies.
  • You’ll fail to notice that someone’s DoSing some or all of your users by repeatedly initiating a password reset.
  • You’ll fail to notice that your password reset process is failing, leading to accounts being dropped as people can no longer get in.
  • You won’t include enough randomness in your code-word, so an attacker can reset two accounts at once – their own, and a victim’s, and use the same code-word to unlock both.
  • You’ll rely on security questions
  • You’ll use a poor-quality generation on your code-words, leading to easy predictability, reuse, etc.
  • You won’t think about other ways that your design will fail outside of this list here.

Let the corrections begin!

Did I miss something, or get something wrong? Let me know by posting a comment!

Final parts of the Git move – VSTS

I’ve posted before how I’d like to get my source code out of the version control system I used to use, because it was no longer supported by the manufacturer, and into something else.

I chose git, in large part because it uses an open format, and as such isn’t going to suffer the same problem I had with ComponentSoftware’s CS-RCS.

Now that I’ve figured out how to use Bash on Ubuntu on Windows to convert from CS-RCS to git, using the rcs-fast-export.rb script, I’m also looking to protect my source control investment by storing it somewhere off-site.

This has a couple of good benefits – one is that I’ll have access to it when I’m away from my home machine, another is that I’ll be able to handle catastrophic outages, and a third is that I’ll be able to share more easily with co-conspirators.

I’m going to use Visual Studio Team Services (VSTS), formerly known as Visual Studio Online, previous to that, as Team Foundation Services Online. You can install VSTS on your own server, or you can use the online tool at <yourdomain>.visualstudio.com. If your team is smaller than five people, you can do this for free, just like you can use Visual Studio 2015 Community Edition for free. This is a great way in which Microsoft supports hobbyist developers, open source projects, college students, etc.

Where do we start?

After my last post on the topic, you have used git and rcs-fast-export.rb to create a Git repository.

You may even have done a “git checkout” command to get the source code into a place where you can work on it. That’s not necessary for our synchronisation to VSTS, because we’re going to sync the entire repository. This will work whether you are using the Bash shell or the regular Command Prompt, as long as you have git installed and in your PATH.

If you’ve actually made any changes, be sure to add and commit them to the local Git repository. We don’t want to lose those!

I’m also going to assume you have a VSTS account. First, visit the home page.

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Under “Recent Projects & Teams”, click “New”.

Give it a name and a description – I suggest leaving the other settings at their default of “Agile” and “Git” unless you have reason to change. The setting of “Git” in particular is required if you’re following along, because that’s how we’re going to synchronise next.

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When you click “Create project”, it’ll think for a while…

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And then you’ll have the ability to continue on. Not sure my team’s actually “going to love this”, considering it’s just me!

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Yes, it’s not just your eyes, the whole dialog moved down the screen, so you can’t hover over the button waiting to hit it.

Click “Navigate to project”, and you’ll discover that there’s a lot waiting for you. Fortunately a quick popup gives you the two most likely choices you’ll have for any new project.

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As my team-mates will attest, I don’t do Kanban very well, so we’ll ignore that side of things, I’m mostly using this just to track my source code. So, hit “Add Code”, and you get this:

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Some interesting options here

Don’t choose any yet Smile

“Clone to your computer” – an odd choice of the direction to use, since this is an empty source directory. But, since it has a “Clone in Visual Studio” button, this may be an easy way to go if you already have a Visual Studio project working with Git that you want to tie into this. There is a problem with this, however, in that if you’re working with multiple versions of Visual Studio, note that any attempt from VSTS to open Visual Studio will only open the most recently installed version of Visual Studio. I found no way to make Visual Studio 2013 automatically open from the web for Visual Studio 2013 projects, although the Visual Studio Version Selector will make the right choice if you double click the SLN file.

“Push an existing repository from command line” – this is what I used. A simple press of the “Copy to clipboard” button gives me the right commands to feed to my command shell. You should run these commands from somewhere in your workspace, I would suggest from the root of the workspace, so you can check to see that you have a .git folder to import before you run the commands.

BUT – I would strongly recommend not dismissing this screen while you run these commands, you can’t come back to it later, and you’ll want to add a .gitignore file.

The other options are:

“Import a repository” – this is if you’re already hosting your git repository on some other web site (like Github, etc), and want to make a copy here. This isn’t a place for uploading a fast-import file, sadly, or we could shortcut the git process locally. (Hey, Microsoft, you missed a trick!)

“Initialize with a README or gitignore” – a useful couple of things to do. A README.md file is associated with git projects, and instructs newcomers to the project about it – how to build it, what it’s for, where to find documentation, etc, etc – and you can add this at any time. The .gitignore file tells git what file names and extensions to not bother with putting into. Object files, executables, temporary files, machine generated code, PCH & PDB files, etc, etc. You can see the list is long, and there’s no way to add a .gitignore file with a single button click after you’ve left this page. You can steal one from an empty project, by simply copying it – but the button press is easier.

What I’ve found

I’ve found it useful to run the “git remote” and “git push” commands from the command-line (and I choose to run them from the Bash window, because I’m already there after running the RCS export), and then add the .gitignore. So, I copy the commands and send them to the shell window, before I press the “Add a .gitignore” button, choose “Visual Studio” as my gitignore type, and then select “Initialize”:

First, let’s start with a recap of using the rcs-fast-export command to bring the code over from the old RCS to a new Git repository:

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Commands in that window:

  • cd workspace/
  • mkdir Juggler
  • cd Juggler
  • ../rcs-fast-export.rb -A ../AuthorsFile /mnt/c/RCS/c/stress/Juggler > Juggler.gx
  • git init
  • git fast-import < Juggler.gx

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Commands:

  • git reset

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No commands – we’ve imported and are ready to sync up to the VSTS server.

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Commands (copied from the “Add Code” window):

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But that’s not quite all…

Your solution still has lines in it dictating what version control you’re using. So you want to unbind that.

[If you don’t unbind existing version control, you won’t be able to use the built-in version control features in Visual Studio, and you’ll keep getting warnings from your old version control software. When you uninstall your old version control software, Visual Studio will refuse to load your projects. So, unbinding your old version control is really important!]

I like to do that in a different directory from the original, for two reasons:

  1. I don’t want to overwrite or delete the working workspace I’ve been working in until the new workspace works. So I still have the old directory to work from if I need to, while I’m moving to the new place.
  2. I want to make sure that a developer (even if it’s just me six months from now, after I’ve wiped everything in a freak electromagnet accident) can connect to this version control source, and build everything.

So, now it’s Command Prompt window time…

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Yes, you could do that from Visual Studio, but it’s just as easy from the command line. Note that I didn’t actually enter credentials here – they’re cached by Windows.

Commands entered in that window:

  • md workspace/Juggler
  • cd workspace/Juggler
  • git clone https://<yourdomain>.visualstudio.com/DefaultCollection/_git/Juggler .
  • Juggler2.sln

Your version control system may complain when opening this project that it’s not in the place it remembers being in… I know mine does. Tell it that’s OK.

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[Yes, I’ve changed projects, from Juggler to EFSExt. I suddenly realised that Juggler is for Visual Studio 2010, which is old, and not installed on this system.]

Now that we’ve opened the solution in Visual Studio, it’s time to unbind the old source control. This is done by visiting the File => Source Control => Change Source Control menu option:

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You’ll get a dialog that lists every project in this solution. You need to select every project that has a check-mark in the “Connected” column, and click the “Unbind” button.

Luckily, in this case, they’re already selected for me, and I just have to click “Unbind”:

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You are warned:

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Note that this unbinding happens in the local copy of the SLN and VCPROJ, etc files – it’s not actually going to make any changes to your version control. [But you made a backup anyway, because you’re cautious, right?]

Click “Unbind” and the dialog changes:

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Click OK, and we’re nearly there…

Finally, we have to sync this up to the Git server. And to do that, we have to change the Source Control option (which was set when we first loaded the project) to Git.

This is under Tools => Options => Source Control. Select the “Microsoft Git Provider” (or in Visual Studio 2015, simply “Git”):

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Press “OK”. You’ll be warned if your solution is still bound in some part to a previous version control system. This can happen in particular if you have a project which didn’t load, but which is part of this solution. I’m not addressing here what you have to do for that, because it involves editing your project files by hand, or removing projects from the solution. You should decide for yourself which of those steps carries the least risk of losing something important. Remember that you still have your files and their history in at least THREE version control systems at this point – your old version control, the VSTS system, and the local Git repository. So even if you screw this up, there’s little real risk.

Now that you have Git selected as your solution provider, you’ll see that the “Changes” option is now available in the Team Explorer window:

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Save all the files (but I don’t have any open!) by pressing Ctrl-Shift-S, or selecting File => Save All.

If you skip this step, there will be no changes to commit, and you will be confused.

Select “Changes”, and you’ll see that the SLN files and VCPROJ files have been changed. You can preview these changes, but they basically are to do with removing the old version control from the projects and solution.

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It wants a commit message. This should be short and explanatory. I like “Removed references to old version control from solution”. Once you’ve entered a commit message, the Commit button is available. Click it.

It now prompts you to Sync to the server.

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So click the highlighted word, “Sync”, to see all the unsynced commits – you should only have one at this point, but as you can imagine, if you make several commits before syncing, these can pile up.

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Press the “Sync” button to send the commit up to the server. This is also how you should usually get changes others have made to the code on the server. Note that “others” could simply mean “you, from a different computer or repository”.

Check on the server that the history on the branch now mentions this commit, so that you know your syncing works well.

And you’re done

Sure, it seems like a long-winded process, but most of what I’ve included here is pictures of me doing stuff, and the stuff I’m doing is only done once, when you create the repository and populate it from another. Once it’s in VSTS, I recommend building your solution, to make sure it still builds. Run whatever tests you have to make sure that you didn’t break the build. Make sure that you still have valid history on all your files, especially binary files. If you don’t have valid history on any files in particular, check the original version control, to see if you ever did have. I found that my old CS-RCS implementation was storing .bmp files as text, so the current version was always fine, but the history was corrupted. That’s history I can’t retrieve, even with the new source control.

Now, what about those temporary repositories? Git makes things really easy – the Git repository is in a directory off the root of the workspace, called “.git”. It’s hidden, but if you want to delete the repository, just delete the “.git” folder and its contents. You can delete any temporary workspaces the same way, of course.

I did spend a little time automating the conversion of multiple repositories to Git, but that was rather ad-hoc and wobbly, so I’m not posting it here. I’d love to think that some of the rest of this could be automated, but I have only a few projects, so it was good to do by hand.

Final Statement

No programmer should be running an unsupported, unpatched, unupdated old version control system. That’s risky, not just from a security perspective, but from the perspective that it may screw up your files, as you vary the sort of projects you build.

No programmer should be required to drop their history when moving to a new version control system. There is always a way to move your history. Maybe that way is to hire a grunt developer to fetch versions dated at random/significant dates throughout history out of the old version control system, and check them in to the new version control system. Maybe you can write automation around that. Or maybe you’ll be lucky and find that someone else has already done the automation work for you.

Hopefully I’ve inspired you to take the plunge of moving to a new version control system, and you’ve successfully managed to bring all your precious code history with you. By using Visual Studio Team Services, you’ve also got a place to track features and bugs, and collaborate with other members of a development team, if that’s what you choose to do. Because you’ve chosen Git, you can separate the code and history at any time from the issue tracking systems, should you choose to do so.

Let me know how (if?) it worked for you!