OpenSSH MaxAuthTries Bypass

high Nessus Plugin ID 86122

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.

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.

See Also

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

Plugin Details

Severity: High

ID: 86122

File Name: openssh_maxauthtries_bruteforce.nasl

Version: 1.9

Type: remote

Family: Misc.

Published: 9/24/2015

Updated: 6/12/2020

Risk Information

VPR

Risk Factor: Medium

Score: 4.2

CVSS v2

Risk Factor: High

Base Score: 8.5

Temporal Score: 6.3

Vector: AV:N/AC:L/Au:N/C:P/I:N/A:C

Temporal Vector: E:U/RL:OF/RC:C

CVSS v3

Risk Factor: High

Base Score: 8.2

Temporal Score: 7.1

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H

Temporal Vector: E:U/RL:O/RC:C

Vulnerability Information

CPE: cpe:/a:openbsd:openssh

Excluded KB Items: global_settings/supplied_logins_only

Exploit Ease: No known exploits are available

Patch Publication Date: 8/11/2015

Vulnerability Publication Date: 7/16/2015

Reference Information

CVE: CVE-2015-5600

BID: 75990