My take on the SSL MitM Attacks – part 2 – clarifications

Since the last post I made on the topic of SSL renegotiation attacks, I’ve had a few questions in email. Let’s see how well I can answer them:

Q. Some stories talk about SSL, others about TLS, what’s the difference?

A. For trademark reasons, when SSL became an open standard, it had to change its name from SSL to TLS. TLS 1.0 is essentially SSL 3.1 – it even claims to be version “3.1” in its communication. I’ll just call it SSL from here on out to remind you that it’s a problem with SSL and TLS both.

Q. All the press coverage seems to be talking about HTTPS – is this limited to HTTPS?

A. No, this isn’t an HTTPS-only attack, although it is true that most people’s exposure to SSL is through HTTPS. There are many other protocols that use SSL to protect their connections and traffic, and they each may be vulnerable in their own special ways.

Q. I’ve seen some posts saying that SSH and SFTP are not vulnerable – how did they manage that?

A. Simply by being “not SSL”. SFTP is a protocol on top of SSH, and SSH is not related to SSL. That’s why it’s not affected by this issue. Of course, if there’s a vulnerability discovered in SSH, it’ll affect SSH and SFTP, but won’t affect SSL or SSL-based protocols such as HTTPS and FTPS.

Q. Is it OK to disable SSL renegotiation to fix this bug?

A. Obviously, if SSL didn’t need renegotiation at all, it wouldn’t be there. So, in some respects, if you disable SSL renegotiation, you may be killing functionality. There are a few reasons that you might be using SSL renegotiation:

  1. Because that’s how client authentication works – while you can do client authentication without renegotiation, most HTTPS implementations use renegotiation to request the client certificate. Disabling renegotiation will generally prevent most clients from authenticating with client authentication.
  2. After 10 hours, renegotiation is required, so as to refresh the session key. Do you have SSL connections lasting 10 hours? You probably should be looking at some disconnect/reconnect scenario instead.
  3. Because you can’t disable SSL renegotiation in all cases. In OpenSSL, you can only disable renegotiation if you download and install the new version, and in other SSL implementations, there is no way to disable renegotiation outside of modifying the application.

Q. Since this attack requires the attacker to become a man-in-the-middle, doesn’t that make it fundamentally difficult, esoteric, or close to impossible?

A. If becoming a man-in-the-middle (MitM) was impossible or difficult, there would be little-to-no need for SSL in the first place. SSL is designed specifically to protect against MitM attacks by authenticating and encrypting the channel. If a MitM can alter traffic and make it seem as if everything’s secure between client and server over SSL, then there’s a failure in SSL’s basic goal of protecting against men-in-the-middle.

Once you assume that an attacker can intercept, read, and modify (but not decrypt) the SSL traffic, this attack is actually relatively easy. There are demonstration programs available already to show how to exploit it.

I was asked earlier today how someone could become a man-in-the-middle, and off the top of my head I came up with six ways that are either recently or frequently used to do just that.

Q. Am I safe at a coffee shop using the wifi?

A. No, not really – over wifi is the easiest way for an attacker to insert himself into your stream.

When using a public wifi spot, always connect as soon as possible to a secured VPN. Ironically, of course, most VPNs are SSL-based, these days, and so you’re relying on SSL to protect you against possible attacks that might lead to SSL issues. This is not nearly as daft as it sounds.

Q. Is this really the most important vulnerability we face right now?

A. No, it just happens to be one that I understood quickly and can blather on about. I think it’s under-discussed, and I don’t think we’ve seen the last entertaining use of it. I’d like to make sure developers of SSL-dependent applications are at least thinking about what attacks can be performed against them using this step, and how they can prevent these attacks. I know I’m working to do something with WFTPD Pro.

Q. Isn’t the solution to avoid executing commands outside the encrypted tunnel?

A. Very nearly, yes. The answer is to avoid executing commands sent across two encrypted sessions, and to deal harshly with those connections who try to send part of their content in one session and the rest in a differently negotiated session.

In testing WFTPD Pro out against FTPS clients, I found that some would send two encrypted packets for each command – one containing the command itself, the other containing the carriage return and linefeed. This is bad in itself, but if the two packets straddle either side of a renegotiation, disconnect the client. That should prevent the HTTPS Request-Splitting using renegotiation.

One key behaviour HTTPS has is that when you request a protected resource, it will ask for authentication and then hand you the resource. What it should probably be doing is to ask for authentication and then wait for you to re-request the resource. That action alone would have prevented the client-certificate attacks discussed so far.

Q. What is the proposed solution?

A. The proposed solution, as I understand it, is for client and server to state in their renegotiation handshake what the last negotiated session state was. That way, an interloper cannot hand off a previously negotiated session to the victim client without the client noticing.

Note that, because this is implemented as a TLS handshake extension, it cannot be implemented in SSLv3. Those of you who just got done with mandating SSLv2 removal throughout your organisations, prepare for the future requirement that SSLv3 be similarly disabled.

Q. Can we apply the solution today?

A. It’s not been ratified as a standard yet, and there needs to be some discussion to avoid rushing into a solution that might, in retrospect, turn out to be no better – or perhaps worse – than the problem it’s trying to solve.

Even when the solution is made available, consider that PCI auditors are still working hard to persuade their customers to stop using SSLv2, which was deprecated over twelve years ago. I keep thinking that this is rather akin to debating whether we should disable the Latin language portion of our web pages.

However, it does demonstrate that users and server operators alike do not like to change their existing systems. No doubt IDS and IPS vendors will step up and provide modules that can disconnect unwarranted renegotiations.

Update: Read Part 3 for a discussion of the possible threats to FTPS.

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