SquirrelMail G/PGP Plugin gpg_recv_key() Command Injection Vulnerability

iDefense Security Advisory 07.11.07
http://labs.idefense.com/intelligence/vulnerabilities/ 
Jul 11, 2007

I. BACKGROUND

The SquirrelMail G/PGP Encrpytion Plugin is a general purpose
encryption, decryption, and digital signature plug-in for SquirrelMail
that implements the OpenPGP standard using GPG. More information is
available at the following URL.

http://www.squirrelmail.org/plugin_view.php?id=153 

II. DESCRIPTION

Remote exploitation of a command injection vulnerability in the G/PGP
Encrpytion Plugin for The SquirrelMail Project Team's SquirrelMail
webmail package allows attackers to execute arbitrary commands with the
privileges of the underlying web server.

The problem specifically exists within the function gpg_recv_key()
defined in gpg_key_functions.php. A call is made to exec() with
unfiltered user-supplied data as demonstrated in the following piece of
code:

    $command = "$path_to_gpg --batch --no-tty --homedir $gpg_key_dir \
             --keyserver hkp://$keyserver --recv-key $searchkeyid 2>&1";
    [...]
    exec($command, $output, $returnval);

The aforementioned '$keyserver' variable is supplied in the POST data to
the gpg_options.php script. The attacker must have a valid authenticated
session to exploit this vulnerability.

III. ANALYSIS

Exploitation of the described vulnerability allows authenticated remote
attackers to execute arbitrary commands with the privileges of the
underlying web server.

This vulnerability could be exploited by webmail users to gain shell
access on the target server and potentially further compromise the
system with local privilege escalation vulnerabilities.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in the latest
version of the G/PGP Encryption Plugin for SquirrelMail, version 2.1.
Furthermore, this vulnerability has been confirmed to exist as early as
version 2.0. Other versions may be affected.

V. WORKAROUND

Disable the G/PGP Plugin if it is not required. Alternatively, add the
following line above the initialization of the '$command' variable just
prior to the call to exec():

    $keyserver = escapeshellarg($keyserver);

Please note that this is an unofficial source patch, but should be
sufficient as a workaround until an official patch is released from the
vendor.

VI. VENDOR RESPONSE

The maintainers of the SquirrelMail G/PGP plug-in have not responded to
repeated inquires regarding this vulnerability. As such, it remains
unpatched, even in the most current release made on July 7th, 2007.

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2005-1924 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org/), which standardizes names for 
security problems.

VIII. DISCLOSURE TIMELINE

10/27/2005  Initial vendor notification
10/27/2005  Initial vendor response
03/02/2006  Second vendor notification
02/16/2007  Third vendor notification
07/11/2007  Public disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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X. LEGAL NOTICES

Copyright =A9 2007 iDefense, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
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