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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: