OpenSSH MaxAuthTries Bypass

This script is Copyright (C) 2015-2017 Tenable Network Security, Inc.


Synopsis :

The SSH server running on the remote host is affected by a security
bypass vulnerability that allows password brute-force attacks.

Description :

The 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.

See also :

http://www.openssh.com/txt/release-7.0

Solution :

Upgrade 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.

Risk factor :

High / CVSS Base Score : 8.5
(CVSS2#AV:N/AC:L/Au:N/C:P/I:N/A:C)
CVSS Temporal Score : 7.0
(CVSS2#E:F/RL:OF/RC:ND)
Public Exploit Available : true

Family: Misc.

Nessus Plugin ID: 86122 ()

Bugtraq ID: 75990

CVE ID: CVE-2015-5600

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