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Re: [SECURITY] [DSA 2826-1] denyhosts security update
From: Helmut Grohne <helmut () subdivi de>
Date: Sun, 22 Dec 2013 19:51:29 +0100

On Sun, Dec 22, 2013 at 07:26:15PM +0100, Yves-Alexis Perez wrote:
Helmut Grohne discovered that denyhosts, a tool preventing SSH
brute-force attacks, could be used to perform remote denial of service
against the SSH daemon. Incorrectly specified regular expressions used
to detect brute force attacks in authentication logs could be exploited
by a malicious user to forge crafted login names in order to make
denyhosts ban arbitrary IP addresses.

A bit of background on this issue:

I discovered the issue on the 19th of December ant contacted:
 * Debian security team
 * Maintainer of the Debian package: Kyle Willmon
 * Upstream: Phil Schwartz

Example exploit:

ssh -l 'Invalid user root from'

This causes a log line of the form

sshd[123]: input_userauth_request: invalid user Invalid user root from [preauth]

and results in both IP addresses being blocked.

CVE-2013-6890 was assigned from the Debian pool.

The proposed solution is to tighten up the regular expressions for
matching log file entries. Specifically including the $ pattern to match
the end of log lines. For your convenience I attach the final patch.

The Debian security advisory is the initial public disclosure.

I am not aware of any upstream response to this issue and the last
denyhosts release is from 2008.


Attachment: 13_CVE-2013-6890.patch

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