Bulletproof TLS Newsletter #2
POODLE attack on SSL 3
16 October 2014
This issue was distributed to 7,999 email subscribers.

After days of persistent rumours, a serious SSL 3 vulnerability was announced late on October 14th. The so-called POODLE attack exploits the CBC padding as implemented in SSL 3 to retrieve fragments of encrypted information (e.g., cookies). The attack scenario is complex: the attacker must execute an active network attack that is a combination of network-level manipulation and malicious JavaScript executing in the victim's browser. But, if those two preconditions are met, the attack itself is relatively straightforward. If you're interested in the details, the original announcement comes with a short paper, but I also recommend Adam Langley's blog post.

We haven't seen a proof of concept yet, but given that the attack is not overly complex, it's reasonable to expect that someone will write it sooner or later.

The good news is that this problem affects only SSL 3, but the bad news is that it's still possible to downgrade modern browsers from whatever best version they support all the way down to the vulnerable protocol. (I discuss this in the "Protocol Downgrade Attacks" section in Chapter 6 of Bulletproof SSL and TLS.)

There are two things you should do: 1) disable SSL 3 in your browser and 2) if at all possible disable SSL 3 in your servers. Please refer to my blog post for a longer discussion. Scott Helme has a detailed blog post with step-by-step instructions for popular browsers and servers.

On the SSL Labs side, we released three new features yesterday to address POODLE: 1) warnings for vulnerable servers, 2) detection of the support for TLS_FALLBACK_SCSV (you'll read more about it in my blog or, better, in my book) and 3) adding detection of SSL 3 support to the client test.