Vulnerability Note VU#786900

SSH host key authentication can be bypassed when DNS is used to resolve localhost

Overview

This vulnerability allows an attacker to redirect an SSH connection to an arbitary host.

I. Description

When making connections to localhost, SSH disables host key checking to provide compatibility with NFS filesystems. As a result, if the victim's machine uses a poisoned DNS server to resolve localhost, it is possible to redirect the victim's SSH session to a different host.

In most SSH clients, users are asked to confirm the acceptance of a host key the first time it is presented. If the user accepts the host key, they are asserting that the key represents the host they intended to connect to. But if an attacker exploits this vulnerability, the victim will not be asked for this confirmation because host key checking has been disabled. Therefore, even the most attentive users will not be able to detect that they have been redirected.

II. Impact

Attacker can redirect a victim's SSH connection to an arbitrary host.

III. Solution

Do not use DNS to resolve "localhost". Instead, explicitly configure all hosts to use 127.0.0.1 for localhost.

Systems Affected

Vendor	Status	Date Notified	Date Updated
OpenSSH	Not Vulnerable	29-Oct-2001
SSH Communications Security	Vulnerable	6-Feb-2001

References

http://www.ssh.com/products/ssh/cert/

Credit

The CERT/CC thanks Antti Huima, Tuomas Aura, and Janne Salmi for their analysis and Tatu Ylonen for bringing this vulnerability to our attention.

This document was written by Jeffrey P. Lanza.

Other Information

Date Public:	2001-01-18
Date First Published:	2001-01-18
Date Last Updated:	2002-03-05
CERT Advisory:
CVE-ID(s):
NVD-ID(s):
US-CERT Technical Alerts:
Metric:	0.46
Document Revision:	12

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