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SANS sounds alarm on Debian OpenSSL flaw

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The SANS Institute yesterday took the highly unusual step of issuing a yellow alert over a vulnerability in the cryptographic functions of Debian, the Linux distro that underpins Ubuntu.

Earlier this week Debian warned that the use of a cryptographically flawed pseudo random number generator in its implementation of OpenSSL meant that potentially predictable keys were generated. Versions of Debian's OpenSSL packages starting with 0.9.8c-1 (released in September 2006) are vulnerable.

Fears that the cryptographic key (including SSH, SSL session keys, OpenVPN and others) generated on affected systems may be weak were borne out by the discovery of scripts that allow brute forcing of vulnerable SSH keys. The SAN's Institute Internet Storm Centre warns that SSL certificates should also be regenerated because of the same cryptographic flaw.

Noted security researcher H D Moore, of Metasploit fame, was able to generate a table of 1024, 2048 and 4096-bit RSA keys in around two hours. Stronger keys would take longer to generate. Other security watchers also express concerns that certificate generated on vulnerable systems are wide open to exploit.

"Certificates build on affected systems may be weaker because they are generated using insufficient random data. One of the prerequisites for secure certificates is that there is a random seed for the generation of the keys, lack of randomness will make it more likely to predict or brute force the keys," Thomas Kristensen, CTO of security notification firm Secunia, explained.

"While this doesn't directly compromise any sites or communication channels it may make those who use these certificates more vulnerable to target attacks seeking to compromise the confidentiality of this communication or perhaps impersonate others. It is a very good idea to recreate any keys that has been generated on an affected system as soon as possible," he added.

Sysadmins are advised to generate new cryptographic keys after updating their software, as explained in Debian's advisory.

Debian said the flaw arises for a change it alone made in the OpenSSL package, suggesting that Linux distributions not derived from Debian are free from the bug. The flaw has been traced back to an attempt to silence a warning from a debugging tool.

One reader forwarded an excerpt from an internal discussion within the Debian Security Team that suggested the bug resides within OpenSSL itself and dates from May 2006. This pointed to possible flaws in OpenSSL 0.9.8b-1. However, the same source later told us, the flaw was later pinned down to OpenSSL 0.9.8c, as described in Debian's advisory. "They're still not revealing the full truth, particularly the fact the keyspace involved is only 215 possible permutations per architecture and thus very easily compromised if someone has a botnet at their disposal," our source said.

We ran the original post past cryptography guru Bruce Schneier who confirmed the gist of our source's concerns. "Any key generated using this system is far less random than you thought it was. This is, as they say, bad," he told El Reg. ®

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