It was found that the perl Fast CGI module did not properly clean up certain environment variables, related to a particular HTTP request, between subsequent incoming requests. Any environment variable set in the first pass through the code by processing the first request, that wasn't set in some subsequent request, has been added to the hash containing environment variables for that subsequent request. A remote attacker could use this flaw to bypass the authentication process and obtain access to resources, which would be otherwise protected by authentication.

References: [1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=607479