The FreeBSD Security Team has identified a memory corruption vulnerability in OpenSSH and has issued the following security advisory: FreeBSD-SA-13:14.openssh (19/11/2013).
OpenSSH is an implementation of the SSH protocol suite, providing an encrypted and authenticated transport for a variety of services, including remote shell access.
AES-GCM (Galois/Counter Mode) is a mode of operation for AES block cipher that combines the counter mode of encryption with the Galois mode of authentication which can offer throughput rates for state of the art, high speed communication channels.
OpenSSH supports the AES-GCM algorithm as specified in RFC 5647.
II. Problem Description
A memory corruption vulnerability exists in the post-authentication sshd process when an AES-GCM cipher (email@example.com or firstname.lastname@example.org) is selected during key exchange.
If exploited, this vulnerability might permit code execution with the privileges of the authenticated user, thereby allowing a malicious user with valid credentials to bypass shell or command restrictions placed on their account.
For a workaround and solution, check out the security advisory: FreeBSD-SA-13:14.openssh