Sysax Multi Server 6.10 SSH DoS

6 minute read Apr 8, 2013 Comments
I was recently fuzzing a bunch of SSH servers, hoping to find some remote code execution in a non-mainstream server. I ended up finding no code execution in the several that I tried, but I did find one pre-auth denial of service in Syax Multi Server 6.10. Try this at home! The vulnerable version can be downloaded here for anyone that would like to duplicate the DoS conditions. Understanding the Key Exchange I found that during the key exchange, where the SSH client and SSH server negotiate which ciphers to use, if you messed up just a single specific byte, the server would crash.