A Practical Attack and Fixes for Current SSL/TLS Vulnerabilities

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New security flaws have put TLS, the Transport Layer Security protocol in the spotlight once again. Early in November, Marsh Ray and Steve Dispensa from PhoneFactor, Inc. published a security report detailing three new potential TLS vulnerabilities. Ray and Dispensa describe man-in-the-middle (MITM) type attacks that target the renegotiation feature of TLS that allows in-connection changes to keys and parameters. The TLS protocol specification supersedes the Secure Socket Layer (SSL) specification, although SSL is commonly used interchangeably with TLS.

The full report from PhoneFactor describing potential attacks on the renegotiation functionality in TLS is available on the PhoneFactor web site. The authors discovered the vulnerabilities in August, 2009. The Register, reports that Ray and Dispensa met with major vendors and distributors of SSL libraries in September, 2009. Developers immediately began working on fixes. PhoneFactor maintains a page on the current status of patches for major vendor’s SSL/TLS implementations.

Eric Rescorla and coauthors published a proposed change in the protocol to fix the vulnerability in an IETF draft specification titled Transport Layer Security (TLS) Renegotiation Indication Extension. Rescorla is well known in the security community for his work on secure protocols and is one of the authors of the current SSL specification. His guide to Understanding the TLS Renegotiation Attack is clearly written and informative.

Shortly after the disclosure, Anil Kurmus, showed that the vulnerabilities were not just theoretical. He illustrates a practical attack against Twitter API using the TLS renegotiation vulnerability. Twitter quickly made changes to prevent the attack from working again, but a practical attack means that vendors and and large web services are under significant pressure to implement fixes in a timely fashion. Netcraft reports that 24 of the 100 top HTTPS sites now safe from TLS renegotiation attacks.