A memory corruption vulnerability exists in the post-
authentication sshd process when an AES-GCM cipher
(aes128-gcm@openssh.com or aes256-gcm@openssh.com) is
selected during kex exchange.
If exploited, this vulnerability might permit code execution
with the privileges of the authenticated user and may
therefore allow bypassing restricted shell/command
configurations.
Either upgrade to 6.4 or disable AES-GCM in the server
configuration. The following sshd_config option will disable
AES-GCM while leaving other ciphers active:
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc