A vulnerability has been discovered in versions of software development toolkits from RSA Security, which could allow an attacker to bypass SSL client authentication.

In a security notice on the issue, RSA said the vulnerability meant that hackers "might potentially gain access to data intended only for authorised users". The company has a patch and it advises customers to apply this to affected software.

Due to a bug in the SSL (Secure Socket Layer) session cacheing feature implemented in RSA BSAFE SSL-J versions 3.x, unauthorised clients may be able to impersonate authorised clients, RSA confirms.

The problem does not affect clients nor does it impact the performance of servers which do not use client authentication. But the vulnerability is noteworthy because it affects commonly used-cryptographic protection techniques.

It's been discovered that (with use of the vulnerable software libraries) if an error occurs while the handshake is being performed, the session key is, under certain conditions, stored in the cache when it should be discarded.

Once cached, this session key can be used by an attacker to cause a server to skip the full client authentication scheme, and use a much shorter sign-on procedure.

The SSL protocol provides for caching of SSL sessions between subsequent connections by the same user; this speeds up connections and lower processing overhead in most cases. The flaw would not give a user root access to a server.

The issue affects RSA BSAFE SSL-J 3.0, 3.01 and 3.1 and Cisco Internet Content Distribution Network 2.0 (because of its use of the toolkit). Users of RSA BSAFE SSL-J 1.x and 2.x are unaffected, as are RSA customers using BSAFE SSL-J 3.1.1 or 4.0 beta 2 and higher. ®

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