IMPORTANT INFORMATION: Distributed SSH Brute Force Attacks

Published: 2010-06-18
Last Updated: 2010-06-18 17:05:49 UTC
by Tom Liston (Version: 1)
9 comment(s)

Based on an analysis of the logs for my SSH honeypot, it appears that this latest spate of SSH brute force attacks are using keyboard-interactive authentication, rather than the standard password authentication.

2010-05-21 19:29:11+0000 trying auth password
2010-05-23 19:31:57+0000 trying auth password
2010-05-25 01:02:57+0000 trying auth password
2010-05-25 01:09:06+0000 trying auth none
2010-05-25 01:09:07+0000 trying auth password
2010-05-25 05:08:07+0000 trying auth password
2010-05-29 14:39:51+0000 trying auth password
2010-06-02 06:27:31+0000 trying auth password
2010-06-03 11:32:22+0000 trying auth none
2010-06-03 11:32:24+0000 trying auth password
2010-06-11 08:44:52+0000 trying auth password
2010-06-11 15:42:46+0000 trying auth password
2010-06-13 22:14:15+0000 trying auth password
2010-06-15 01:21:39+0000 trying auth password
2010-06-15 02:09:01+0000 trying auth password
2010-06-15 19:53:49+0000 trying auth none
2010-06-15 19:53:51+0000 trying auth password
2010-06-15 20:10:45+0000 trying auth password
2010-06-16 18:20:54+0000 trying auth keyboard-interactive
2010-06-16 18:33:35+0000 trying auth keyboard-interactive
2010-06-16 19:05:53+0000 trying auth password
2010-06-16 19:06:47+0000 trying auth keyboard-interactive
2010-06-16 19:28:54+0000 trying auth keyboard-interactive
2010-06-16 19:47:52+0000 trying auth keyboard-interactive
2010-06-16 19:57:57+0000 trying auth keyboard-interactive
2010-06-16 20:18:00+0000 trying auth keyboard-interactive
2010-06-16 20:27:40+0000 trying auth keyboard-interactive
2010-06-16 20:47:02+0000 trying auth keyboard-interactive
2010-06-16 21:27:00+0000 trying auth keyboard-interactive
2010-06-17 16:59:36+0000 trying auth password

Understand: If you have disabled password authentication in your ssh_config by uncommenting the line:

PasswordAuthentication no

that *WILL NOT* protect you against this latest round of attacks.

In order to disable keyboard-interactive logins, you must also uncomment the line:

ChallengeResponseAuthentication no

NOTE: DO NOT DO THIS unless you understand what you're doing and know that it will not break anything (I don't want a bunch of emails saying "I got in trouble because I did what Liston said...")

To test if your server is configured correctly, log in using the command line version of ssh with the "-v" option.  That will spit out a whole bunch of debugging information.  The important line is this:

debug1: Authentications that can continue: publickey,password,keyboard-interactive

If you see something like that, then you're not only vulnerable to standard password brute force attacks, but this newer keyboard-interactive attack as well.

Tom Liston
Handler - SANS Internet Storm Center
Senior Security Analyst - InGuardians, Inc.

9 comment(s)
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