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Category:   Application (VPN)  >   OpenSSH Vendors:   OpenSSH.org
(Ubuntu Issues Fix) OpenSSH 'KbdInteractiveDevices' Lets Remote Users Bypass Security Restrictions on the Target System
SecurityTracker Alert ID:  1033287
SecurityTracker URL:  http://securitytracker.com/id/1033287
CVE Reference:   CVE-2015-5600   (Links to External Site)
Updated:  Aug 18 2015
Original Entry Date:  Aug 14 2015
Impact:   Host/resource access via network
Fix Available:  Yes  Vendor Confirmed:  Yes  Exploit Included:  Yes  

Description:   A vulnerability was reported in OpenSSH. A remote user can bypass authentication attempt limits on the target system.

A remote user can request the keyboard-interactive authentication option ('KbdInteractiveDevices') to open a large number of keyboard-interactive devices on the target server and perform a brute-force password guessing attack against the target sshd service. The number of password attempts can effectively exceed the 'MaxAuthTries' limit and are permitted to occur until the 'LoginGraceTime' limit is reached or the number of keyboard-interactive devices are used.

Servers that have keyboard-interactive authentication enabled (e.g., FreeBSD in the default configuration) are affected.

A demonstration exploit command is provided:

ssh -l[username] -oKbdInteractiveDevices=`perl -e 'print "pam," x 10000'` [target]

The original advisory is available at:

https://kingcope.wordpress.com/2015/07/16/openssh-keyboard-interactive-authentication-brute-force-vulnerability-maxauthtries-bypass/

kingcope reported this vulnerability.

Impact:   A remote user can bypass authentication attempt limits on the target system.
Solution:   Ubuntu has issued a fix.

[Editor's note: On August 18, 2015, Ubuntu issued a revised fix (USN-2710-2) because the original upstream fix contained a non-security related error that may cause random authentication failures in non-default configurations.]

The Ubuntu advisory is available at:

http://www.ubuntu.com/usn/usn-2710-2

Vendor URL:  www.ubuntu.com/usn/usn-2710-2 (Links to External Site)
Cause:   Access control error, State error
Underlying OS:  Linux (Ubuntu)
Underlying OS Comments:  12.04 LTS, 14.04 LTS, 15.04

Message History:   This archive entry is a follow-up to the message listed below.
Jul 20 2015 OpenSSH 'KbdInteractiveDevices' Lets Remote Users Bypass Security Restrictions on the Target System



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