Tavis Ormandy and Jonathan Underwood have discovered a vulnerability and a weakness in DenyHosts, which can be exploited by malicious people to bypass certain security restrictions or cause a DoS (Denial of Service).

1) An error exists in the parsing of log files before adding an entry in /etc/hosts.deny. This can be exploited to add arbitrary IP addresses to /etc/hosts.deny resulting in a DoS for that IP.

The vulnerability is confirmed in version 2.5 and 2.6. Other versions may also be affected.

2) A weakness exists in the parsing log files containing login attempts blocked by the "AllowUsers" OpenSSH directive. This can be exploited to bypass the logging capability of DenyHosts by trying to login using a user not specifically permited by "AllowUsers".

The weakness is confirmed in version 2.6. Other versions may also be affected.

Solution
Edit the regular expressions to ensure that log files are parsed properly.
Further details available in Customer Area

Provided and/or discovered by
1) Tavis Ormandy
2) Jonathan Underwood

Changelog
Further details available in Customer Area

Original Advisory
1) http://bugs.gentoo.org/show_bug.cgi?id=157163
http://www.ossec.net/en/attacking-loganalysis.html
2) https://bugzilla.redhat.com/show_bug.cgi?id=237449

Deep Links
Links available in Customer Area