Updated OpenSSH packages are now available. These updates close an information leak caused by sshd's interaction with the PAM system.
OpenSSH is a suite of network connectivity tools that can be used to establish encrypted connections between systems on a network and can provide interactive login sessions and port forwarding, among other functions. When configured to allow password-based or challenge-response authentication, sshd (the OpenSSH server) uses PAM (Pluggable Authentication Modules) to verify the user's password. Under certain conditions, OpenSSH versions prior to 3.6.1p1 reject an invalid authentication attempt without first attempting authentication using PAM. If PAM is configured with its default failure delay, the amount of time sshd takes to reject an invalid authentication request varies widely enough that the timing variations could be used to deduce whether or not an account with a specified name existed on the server. This information could then be used to narrow the focus of an attack against some other system component. These updates contain backported fixes that cause sshd to always attempt PAM authentication when performing password and challenge-response authentication for clients.