+++ This bug was initially created as a clone of Bug #164661 +++

From Bugzilla Helper:
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7.7) Gecko/20050414

Description of problem:
The OpenSSH server included with RHEL 3.0 (apparently all updates) is vulnerable
to CAN-2003-0386. Apparently the fix for this vulnerability has never been
backported into Red Hat's openssh-server-3.6.1p2 RPMs.

An attacker can bypass IP-based AllowUser restrictions in the
/etc/ssh/sshd_config file if the attacker controls reverse DNS for his system by
providing a reverse DNS hostname for his machine which is the IP address allowed
by the AllowUser restriction.

This vulnerability can be mitigated by turning on VerifyReverseMapping, but this
is turned off by default.

Version-Release number of selected component (if applicable):
openssh-server-3.6.1p2-*

How reproducible:
Always

Steps to Reproduce:
1. Add an AllowUser restriction to /etc/ssh/sshd_config. 

E.g.:
AllowUser *@192.168.0.20

2. Restart sshd.

E.g.:
% service sshd restart

3. Set up reverse DNS to return the allowed IP address for a different machine.    

E.g.:
% host 192.168.0.40
40.0.168.192.in-addr.arpa domain name pointer 192.168.0.20.

4. From the remote host which should be disallowed, ssh to the vulnerable host.

Actual Results:  Login is allowed via ssh.

Expected Results:  The user should receive a "permission denied" message and the
login should be disallowed.

Additional info:

-- Additional comment from mjc on 2005-08-01 10:58 EST --
Note: At the time of announcement the official response was:  "If you depend on
IP or DNS based access control, make sure VerifyReverseMapping is turned on in
your sshd_config file."

http://marc.theaimsgroup.com/?l=openssh-unix-dev&m=105492565028235