Thai Duong and Juliano Rizzo today demoed an attack against TLS 1.0's use of cipher block chaining (CBC) in a browser environment. The authors contacted browser vendors several months ago about this and so, in order not to preempt their demo, I haven't discussed any details until now.

Contrary to several press reports, Duong and Rizzo have not found, nor do they claim, any new flaws in TLS. They have shown a concrete proof of concept for a flaw in CBC that, sadly, has a long history. Early reports of the problem date back nearly ten years ago and Bard published two papers detailing the problem.

The problem has been fixed in TLS 1.1 and a workaround for SSL 3.0 and TLS 1.0 is known, so why is this still an issue?

The workaround (prepending empty application data records) is perfectly valid according to the protocol but several buggy implementations of SSL/TLS misbehaved when it was enabled. After Duong and Rizzo notified us, we put the same workaround back into Chrome to see if the state of the Internet had improved in the years since the last attempt. Sadly it was still infeasible to implement this workaround and the change had to be reverted.

Use of TLS 1.1 would also have solved the issue but, despite it being published in 2006, common SSL/TLS libraries still don't implement it. But even if there was widespread deployment of TLS 1.1, it wouldn't have helped avoid problems like this. Due to a different, common bug in SSL 3.0 implementations (nearly 12 years after SSL 3.0 was obsoleted by TLS 1.0), browsers still have to perform SSL 3.0 downgrades to support buggy servers. So even with a TLS 1.1 capable browser and server, an attacker can trigger a downgrade to SSL 3.0 and bypass the protections of TLS 1.1.

Finally, the CBC attacks were believed to be largely theoretical but, as Duong and Rizzo have pointed out today, that's no longer the case.

Initially the authors identified HTML5 WebSockets as a viable method of exploiting the CBC weakness but, due to unrelated reasons, the WebSockets protocol was already in the process of changing in such a way that stopped it. The new WebSockets protocol shipped with Chrome 14 but several plugins have been found to offer features that might allow the attack to be performed.

Duong and Rizzo confirmed that the Java plugin can be used, but Chrome already blocks the execution of Java by default. Other plugins, if installed, can be disabled on the about:plugins page if the user wishes.

The attack is still a difficult one; the attacker has to have high-bandwidth MITM access to the victim. This is typically achieved by being on the same wireless network as the victim. None the less, it's a much less serious issue than a problem which can be exploited by having the victim merely visit a webpage. (Incidentally, we pushed out a fix to all Chrome users for such a Flash bug only a few days ago.)

Also, an attacker with MITM abilities doesn't need to implement this complex attack. SSL stripping and mixed-scripting issues are much easier to exploit and will take continued, sustained effort to address. Duong and Rizzo have highlighted this fact by choosing to attack one of the few HSTS sites.

Thanks to an idea suggested by Xuelei Fan, we have another workaround for the problem which will, hopefully, cause fewer incompatibility problems. This workaround is currently being tested on the Chrome dev and beta channels but we haven't pushed it on the stable channel yet. Since we don't really know if the fix will cause problems it's not something that we want to drop on people without testing. The benefit of a fix to Chrome's TLS stack is also limited as Chrome already uses the newer WebSockets protocol and it doesn't fix problems in plugins.

If it turns out that we've misjudged something we can quickly react, thanks to Chrome's auto-update mechanism.

It's also worth noting that Google's servers aren't vulnerable to this problem. In part due to CBC's history, Google servers have long preferred RC4, a cipher that doesn't involve CBC mode. Mention has also been made of Chrome's False Start feature but, since we don't believe that there are any vectors using Chrome's stack, that's immaterial.