On my last post I wrote about how cipher suite decision works on SSL with a practical example. Today, I would like to write about what is the impact of choosing and prioritizing the appropriate cipher suites on your SSL environment.
Back in May 2011, Juliano Rizzo and Thai Duong released a paper named Here Come The XOR Ninjas . This paper was a contribution to the security community on illustrating a concrete proof of concept against implementations of SSL 3.0 and TLS 1.0 using cipher block chaining (CBC) encryption scheme in a browser environment . The attack also become known as The BEAST (Browser Exploit Against SSL/TLS) . It has been widely publicized after it’s released since the vulnerability was known in the research community but before the paper, attacks against CBC were believed to be theoretical. As a reference, the use of predictable initialization vectors (IV) for CBC mode in a chosen-plaintext attack against SSL 3.0 and TLS 1.0 are described by Bodo Moeller here and by Gregory Bard here. In a SSL connection if the data is encrypted using CBC with chained IVs, it allows a man-in-the-middle to obtain plain text HTTP headers using blockwise-adaptive chosen-plaintext attack . In short it will allow decrypting HTTPS requests and steal information such as session cookies to be used to impersonate the user. The attack has been demonstrated by the researchers on the Ekoparty security conference using Java, a network sniffer and a popular browser .
Are you vulnerable to such attack? To quickly verify your web server SSL settings I would recommend looking at SSL Labs from Qualys and running the SSL Server test. If, for example, you use a SSL load balancer or application firewall from F5 take a look here on how to reduce the exposure to this attack. All major vendors have information about how to configure and remediate this vulnerability. On 09 July 2011 the US National Vulnerability Database published CVE-2011-3389 to address this vulnerability with a CVSS score of 4.3.
On the other hand, in order to remediate this vulnerability the focus should be on configuring the web servers to prioritize a different cipher suite, or upgrade to TLS 1.1/1.2 . Also consider that this attack has a low CVSS score and it’s not easy to pull. Therefore, unless you are exposed to someone who would have the means, motive and opportunity it might be worth to spend time on fixing other things.
If you want to read more about it, I would recommend taking a look at Thai Duong blog article he released one week after the security conference. The summary from Thierry Zoller and the below references.