SSL Secure Renegotiation

Context: The Wormly SSL Tester checks to see if a web server supports secure renegotiation.

In late 2009 a vulnerability was discovered in the SSL & TLS protocols which allow a man-in-the-middle style attack to inject arbitrary data into a stream of encrypted data.

It stems from a design flaw in the protocol which fails to ensure that renegotiation of a SSL/TLS session is cryptographically tied to the underlying session. Thus an attacker could commence a secure session with the server, insert arbitrary data into the stream, then instruct the server to renegotiate and splice in a legitimate connection from that point onward.

The server would believe that the entire data stream - including the attackers’ arbitrary data - originated from the client / victim, even though it did not. Note that the attacker would not be able to decrypt any intercepted responses from the server, so attacks are limited to request forgery.

In early 2010, the IETF proposed a solution to plug this hole, and this is now supported by all major web server vendors. Ensure that you are using up to date server software and have enabled support for this feature.