OpenSSH MaxAuthTries Bypass
High Nessus Plugin ID 86122
SynopsisThe SSH server running on the remote host is affected by a security bypass vulnerability that allows password brute-force attacks.
DescriptionThe remote SSH server is affected by a security bypass vulnerability due to a flaw in the keyboard-interactive authentication mechanisms.
The kbdint_next_device() function in auth2-chall.c improperly restricts the processing of keyboard-interactive devices within a single connection. A remote attacker can exploit this, via a crafted keyboard-interactive 'devices' string, to bypass the normal restriction of 6 login attempts (MaxAuthTries), resulting in the ability to conduct a brute-force attack or cause a denial of service condition.
SolutionUpgrade to OpenSSH 7.0 or later.
Alternatively, this vulnerability can be mitigated on some Linux distributions by disabling the keyboard-interactive authentication method. This can be done on Red Hat Linux by setting 'ChallengeResponseAuthentication' to 'no' in the /etc/ssh/sshd_config configuration file and restarting the sshd service.