TLS Renegotiation attack – Microsoft workaround/patch

Hidden by the smoke and noise of thirteen (13! count them!) security bulletins, with updates for 26 vulnerabilities and a further 4 third-party ActiveX Killbits (software that other companies have asked Microsoft to kill because of security flaws), we find the following, a mere security advisory:

Microsoft Security Advisory (977377): Vulnerability in TLS/SSL Could Allow Spoofing

It’s been a long time coming, this workaround – which disables TLS / SSL renegotiation in Windows, not just IIS.

Disabling renegotiation in IIS is pretty easy – you simply disable client certificates or mutual authentication on the web server. This patch gives you the ability to disable renegotiation system-wide, even in the case where the renegotiation you’re disabling is on the client side. I can’t imagine for the moment why you might need that, but when deploying fixes for symmetrical behaviour, it’s best to control it using switches that work in either direction.

The long-term fix is yet to arrive – and that’s the creation and implementation of a new renegotiation method that takes into account the traffic that has gone on before.

To my mind, even this is a bit of a concession to bad design of HTTPS, in that HTTPS causes a “TOC/TOU” (Time-of-check/Time-of-use) vulnerability, by not recognising that correct use of TLS/SSL requires authentication and then resource request, rather than the other way around. But that’s a debate that has enough clever adherents on both sides to render any argument futile.

Suffice it to say that this can be fixed most easily by tightening up renegotiation at the TLS layer, and so that’s where it will be fixed.

Should I apply this patch to my servers?

I’ll fall back to my standard answer to all questions: it depends.

If your servers do not use client auth / mutual auth, you don’t need this patch. Your server simply isn’t going to accept a renegotiation request.

If your servers do use client authentication / mutual authentication, you can either apply this patch, or you can set the earlier available SSLAlwaysNegoClientCert setting to require client authentication to occur on initial connection to the web server.

One or other of these methods – the patch, or the SSLAlwaysNegoClientCert setting – will work for your application, unless your application strictly requires renegotiation in order to perform client auth. In that case, go change your application, and point them to documentation of the attack, so that they can see the extent of the problem.

Be sure to read the accompanying KB article to find out not only how to turn on or off the feature to disable renegotiation, but also to see which apps are, or may be, affected adversely by this change – to date, DirectAccess, Exchange ActiveSync, IIS and IE.

How is Microsoft’s response?

Speed

I would have to say that on the speed front, I would have liked to see Microsoft make this change far quicker. Disabling TLS/SSL renegotiation should not be a huge amount of code, and while it has some repercussions, and will impact some applications, as long as the change did not cause instability, there may be some institutions who would want to disable renegotiation lock, stock and barrel in a hurry out of a heightened sense of fear.

I’m usually the first to defend Microsoft’s perceived slowness to patch, on the basis that they do a really good job of testing the fixes, but for this, I have to wonder if Microsoft wasn’t a little over-cautious.

Accuracy

While I have no quibbles with the bulletin, there are a couple of statements in the MSRC blog entry that I would have to disagree with:

IIS 6, IIS 7, IIS 7.5 not affected in default configuration

Customers using Internet Information Services (IIS) 6, 7 or 7.5 are not affected in their default configuration. These versions of IIS do not support client-initiated renegotiation, and will also not perform a server-initiated renegotiation. If there is no renegotiation, the vulnerability does not exist. The only situation in which these versions of the IIS web server are affected is when the server is configured for certificate-based mutual authentication, which is not a common setting.

Well, of course – in the default setting on most Windows systems, IIS is not installed, so it’s not vulnerable.

That’s clearly not what they meant.

Did they mean “the default configuration with IIS installed and turned on, with a certificate installed”?

Clearly, but that’s hardly “the default configuration”. It may not even be the most commonly used configuration for IIS, as many sites escape without needing to use certificates.

Sadly, if I add “and mutual authentication enabled”, we’re only one checkbox away from the “default configuration” to which this article refers, and we’re suddenly into vulnerable territory.

In other words, if you require client / mutual authentication, then the default configuration of IIS that will achieve that is vulnerable, and you have to make a decided change to non-default configuration (the SSLAlwaysNegoClientCert setting), in order to remain non-vulnerable without the 977377 patch.

The other concern I have is over the language in the section “Likelihood of the vulnerability being exploited in general case”, which discusses only the original CSRF-like behaviour exploited under the initial reports of this problem.

There are other ways to exploit this, some of which require a little asinine behaviour on the part of the administrator, and others of which are quite surprisingly efficient. I was particularly struck by the ability to redirect a client, and make it appear that the server is the one doing the redirection.

I think that Eric and Maarten understate the likelihood of exploit – and they do not sufficiently emphasise that the chief reason this won’t be exploited is that it requires a MITM (Man-in-the-middle) attack to have already successfully taken place without being noticed. That’s not trivial or common – although there are numerous viruses and bots that achieve it in a number of ways.

Clarity

It’s a little unclear on first reading the advisory whether this affects just IIS or all TLS/SSL users on the affected system. I’ve asked if this can be addressed, and I’m hoping to see the advisory change in the coming days.

Summary

I’ve rambled on for long enough – the point here is that if you’re worried about SSL / TLS client certificate renegotiation issues that I’ve reported about in posts 1, 2 and 3 of my series, by all means download and try this patch.

Be warned that it may kill behaviour your application relies upon – if that is the case, then sorry, you’ll have to wait until TLS is fixed, and then drag your server and your clients up to date with that fix.

The release of this advisory is by no means the end of the story for this vulnerability – there will eventually be a supported and tested protocol fix, which will probably also be a mere advisory, followed by updates and eventually a gradual move to switch to the new TLS versions that will support this change.

This isn’t a world-busting change, but it should demonstrate adequately that changes to encryption protocols are not something that can happen overnight – or even in a few short months.

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