Last week, on October 22, Apple released OS X 10.9 Mavericks, the latest version of their desktop operating system. This was a very important update, given that Safari now, in version 7, supports TLS 1.2. We are slowly moving toward a world in which TLS 1.2 is widely supported. Still, I wanted more. I was hoping that this OS X release was also going to have BEAST mitigations enabled by default. After all, the code was already a part of the previous OS X release (Mountain Lion) and the compatibility risks are minimal given that all other major browser vendors enabled the 1/n-1 split in early 2012.

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Yesterday I changed the SSL Labs rating criteria to stop penalizing sites that do not implement server-side mitigations for the BEAST attack. That means that we now consider this attack sufficiently mitigated client-side, but, there are still some things you should now.

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When it comes to the mitigation of the BEAST attack against SSL/TLS, the usual advice is to prioritize RC4 cipher suites in order to avoid CBC suites, which are vulnerable. In some cases, companies may even have to use RC4 suites exclusively. For example, I’ve recently heard of a case where a company was failed on its PCI compliance test because they had non-RC4 cipher suites enabled in their configuration.

When RC4 prioritization is discussed, people often seek assurance that RC4 is widely supported. Obviously, if you are going to disable a large number of cipher suites, you start to worry if the remaining suites are going to be sufficient for your user base. Until recently, I had no answer to those questions. Even though all major browsers support RC4 by default, there was no way to quantify the support. Further, some browsers allow users to turn off certain cipher suites, and we just didn’t know what the situation on the ground was.

About two months ago I re-enabled mod_sslhaf on the SSL Labs web server. This Apache module, a project of SSL Labs, records all cipher suites offered by clients. Today I looked at the results, curious to find out exactly how many of the SSL Labs clients supported RC4. I choose to compare the total number of unique IP addresses against the number of IP addresses that did not support TLS_RSA_WITH_RC4_128_MD5 (0x04) or TLS_RSA_WITH_RC4_128_SHA (0x05).

In about two months, SSL Labs saw 48,481 unique IP addresses. Only 45 of those — or 0.09% — did not support RC4. Looking at the user agent information, the clients not supporting RC4 seem to be largely those whose users decided to explicitly disable RC4 for one reason or another.

The usual disclaimer applies here: the sample is taken from the SSL Labs web site and may not apply to other sites. However, if the assumption that the only non RC4 clients are those where this cipher suite was disabled by their owners, you could argue that an average site will see an even smaller number of non-compatible clients.

Update (19 March 2013): This blog post advises to use RC4 to migitate the BEAST attack, but RC4 has recently been discovered to be weaker than previously known. At this point the attacks against RC4 are still not practical. The only fully safe choice at the moment is the AES-GCM suites supported only in TLS 1.2. You can find out more in this new blog post.

During the summer rumours about a new attack against SSL started circulating. Then Opera released a patch, but made no comment about what it was patching. Eventually enough information leaked out that some smart people figured what the attack was about. What remained unknown was the exact technique used in the proof of concept, and that was eventually explained in Thai’s blog post. For a comprehensive overview of related links, go to Thierry Zoller’s blog post on BEAST.

As it turns out, the attack itself was conceived years ago, deemed impractical, but it was nevertheless fixed in TLS 1.1. The new attack technique introduced a few optimizations to make it practical.

In terms of mitigation, I expect this problem will be largely addressed on the client side, despite a potential compatibility problem that may cause some TLS sites to stop working. The only reliable way to defend against BEAST is to prioritise RC4 cipher suites, as proposed by PhoneFactor.

Just as an example, here’s one way to do the above in Apache:

SSLHonorCipherOrder On
SSLCipherSuite RC4-SHA:HIGH:!ADH

Not everyone likes RC4, even though there is little to no evidence that it is insecure in the context of SSL/TLS. If your server supports TLS 1.1+ you can try the approach recommended by Steve Caligo:

SSLHonorCipherOrder On
SSLCipherSuite ECDHE-RSA-AES128-SHA256:AES128-GCM-SHA256:RC4:HIGH:!MD5:!aNULL:!EDH

The idea is that you put a few TLS 1.2 cipher suites first so that they can be picked up by TLS 1.2 clients, which are not vulnerable, followed by RC4 for TLS 1.0 clients.

Now that I’ve discussed what works as mitigation, let’s look at a few approaches that do not work:

• Supporting TLS 1.1+ server-side is a good start, but does not amount to much because very few clients support newer versions of the protocol at this time. And even with TLS 1.1+ support client-side, there’s nothing preventing the MITM to force a protocol downgrade back to TLS 1.0. (For a discussion on defense techniques against downgrade attacks, see this thread on the TLS WG mailing list).
• Enabling the empty fragment technique server-side (details for OpenSSL here) does not work either. TLS 1.0 uses two initialisation vectors (IVs), one each for client- and server-side of the communication channel. The vulnerability exploited by BEAST is on the client-side and cannot be addressed by making server-side changes to how data is sent.
• Compression is said to make the attack impossible, but, as with TLS 1.1+, the support for it client-side is inconsistent.

Update (20 Jan 2012): In testing OpenSSL 1.0.1-beta2, which came out yesterday, I realised that it will happily negotiate AES-CBC-SHA256 even on a TLSv1.0 connection. So I removed it from the recommendation, replacing it with two other TLSv1.2 cipher suites.