With just a few hours until researchers unveiled an attack they say decrypts sensitive web traffic protected by the ubiquitous secure sockets layer protocol, cryptographers described a simple way website operators can insulate themselves against the exploit.
The recommendations published Friday by two-factor authentication service PhoneFactor, suggest websites use the RC4 cipher to encrypt SSL traffic instead of newer, and ironically cryptographically stronger, algorithms such as AES and DES. Google webservers are already configured to favor RC4, according to this analysis tool from security firm Qualys. A Google spokesman says the company has used those settings “for years.”
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In stark contrast, eBay’s PayPal payment service favors AES, making the site at least theoretically vulnerable to BEAST, the attack tool scheduled to be demonstrated Friday evening at the Ekoparty security conference in Buenos Aires. Short for Browser Exploit Against SSL/TLS, its creators say it targets a long-documented vulnerability in some encryption algorithms that cryptographers previously believed wasn’t practical to exploit.
Researches Thai Duong and Juliano Rizzo said they’ve refined the attack enough to decrypt SSL-protected web traffic using a piece of JavaScript that injects plaintext into the encrypted request stream. They have said they plan to prove the attack is practical by using it to recover an encrypted cookie used to access a user account on PayPal.
The chosen plaintext-recovery at the heart of BEAST attacks algorithms that use a mode known as CBC, or cipher block chaining, in which information from a previously encrypted block of data is used to encode the next block. CBC is present in both AES and DES, but not in RC4.
“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,” PhoneFactor’s Steve Dispensa wrote. “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.”
The configurations followed by Google aren’t an absolute guarantee BEAST attacks won’t work on the site, since they allow vulnerable ciphers to be used in the event the connecting browser doesn’t work with RC4. That’s an unlikely scenario, but certainly within the realm of government and government contractor employees mandated to use the Federal Information Processing Standard.
As previously reported, Google has already released a developer version of its Chrome Browser that mitigates the damage that BEAST can wreak. It remains unclear just how much of a threat the exploit poses, but the web giant isn’t taking any chances. ®
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