Tag Archives: SSL

Testing for Cipher Suite Preference

It’s often important to know which SSL/TLS cipher suite is preferred by a server to alter the risk rating of a particular issue. For POODLE, if the server prefers RC4 ciphers over SSLv3 connections then it’s very unlikely that a connection will be vulnerable to POODLE. Similarly, if a server prefers block ciphers then reporting RC4 support should be appropriately adjusted. Occasionally tools conflict over which cipher suite is preferred so I thought I’d write up how to resolve this manaully in the spirit of the SSL/TLS manual cheatsheet. Continue reading


The POODLE attack announced very recently depends largely on a protocol downgrade attack (which I covered in my SSL/TLS presentation at BSides). I don’t think this aspect of TLS security was widely appreciated – but it is now! It’s a fair bet that any technical article about POODLE includes the phrase “TLS_FALLBACK_SCSV” as a remedy. This article discusses the mechanism proposed to protect us from attackers forcing TLS downgrades. NEW (16/10/14): while I was writing this I thought of a small but potential compatibility problem, which in fact could do us all a favour. I checked with the authors of the RFC and Adam Langley was kind enough to reply back so I’ve added added a new section below. Continue reading

The Small Print for OpenSSL legacy_renegotiation

The other day my attention was drawn to a switch in OpenSSL called -legacy_rengotation. I pulled up the built-in help for s_client and, sure enough, there it was. So I trawled back through the release notes and it looked to have been there since version 0.9.8m. I couldn’t believe that I hadn’t spotted this before: it looked like the perfect way to test for insecure renegotiation without the faff of having to recompile OpenSSL or use an older version. But after a bit of testing this proved to be a red herring… Continue reading