Slaying BEAST: Mitigating the latest SSL/TLS Vulnerability Posted  September 23, 2011 By Steve Dispensa

Not even two years after Marsh and I described the renegotiation flaw, TLS and SSL have been hit again. The attack, dubbed BEAST by researchers, was described by Rizzo and Duong at the ekoparty security conference. It allows an adversary to decrypt parts of the encrypted data stream, potentially leading to session hijacking, information disclosure, and more. The concept behind the attack had been previously described, but it was thought at the time to be impractical.

The problem lies in the way that block ciphers are used in SSL/TLS. Block ciphers are generally operated in one of several modes that define how encrypted blocks are manipulated to ensure complete confidentiality. Cipher Block Chaining, or CBC mode, is used in SSL for all block ciphers, including AES and Triple-DES. The BEAST attack relies on a weakness in the way CBC mode is used in SSL and TLS. Non-CBC cipher suites, such as those using the RC4 stream encryption algorithm, are not vulnerable.

There have been several suggested mitigations that can be put into play from the perspective of the client, such as reorganizing the way the data is sent in the encrypted stream. Servers can protect themselves by requiring a non-CBC cipher suite. One such cipher suite is rc4-sha, which is widely supported by clients and servers.

To assist with the necessary server reconfiguration, PhoneFactor has produced a white paper describing the process. Please note that it is just a recommendation that we feel would work for most sites, but it is important for you to evaluate our suggestions in light of your actual architecture.

Download Whitepaper
Download Associated Text File

In the end, TLS is still among the most widely deployed security protocols, and the fact that it’s had a difficult couple of years is an indicator of just how many eyeballs are pointed at it. It’s still always safer to use a public, highly analyzed cryptosystem than to invent one from scratch, as attacks like BEAST show just how difficult it can be to get it right.

- Steve