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phpBB Group phpBB Arbitrary File Disclosure Vulnerability

02.22.05

BACKGROUND

phpBB is an open source bulletin board package written in the PHP web
scripting language. More information about phpBB is available from:

    http://www.phpbb.com/

DESCRIPTION

Remote exploitation of an input validation vulnerability in the phpBB
Group's phpBB2 bulletin board system allows attackers to read the
contents of arbitrary system files under the privileges of the web
server.

phpBB is an open-source web-based bulletin board system written in PHP.
The problem specifically exists due to an input validation error that
allows a remote attacker to control the arguments in a call to copy().

When a user requests to upload an avatar, the variable
'$user_avatar_upload' defaults to uploading from a remote URL and the
variable '$avatar_mode' defaults to uploading from the local computer.
The variable '$user_avatar_upload' contains either the remote URL or the
temporary server name depending on whether the source of the avatar to
upload is local or remote. In the event that both a local and remote
upload are requested simultaneously, the temporary upload location is
substituted with the remote server name. This will later be copied to
the new location. By submitting a local path rather then a URL, an
attacker is able to execute an arbitrary copy() command.

An attacker can exploit this input validation condition by selecting an
avatar from the local machine that meets the board guidelines and can
then fill the "Upload Avatar from a URL:" field with the path to an
arbitrary file (ex: /etc/passwd). When the avatar is submitted, the
destination image of the submitted avatar will contain the contents of
the requested file.

ANALYSIS

Exploitation of this vulnerability allows remote attackers to view
arbitrary system files under the privileges of the underlying web
server. An attacker must have, or be able to create an account on the
target system. Non-default settings must also be enabled for
exploitation to be possible. Upon successful exploitation an attacker
may be able to further compromise the system by gleaning system
information that would otherwise be inaccessible to the attacker.

DETECTION

iDEFENSE has confirmed the existence of this vulnerability in phpBB2
version 2.0.11. It is suspected that earlier versions are affected as
well. "Enable remote avatars" and "Enable avatar uploading" must be
enabled for the target to be vulnerable.

WORKAROUND

Disable remote avatars and remove avatar uploading. This can be done
through th phpBB administrative interface under "General Admin ->
Configuration -> Avatar Settings". Alternatively, enable the
'open_basedir' PHP security directive to lock file I/O operations to a
specific directory.

VENDOR RESPONSE

This vulnerability is addressed in phpBB version 2.0.12 available for
download at:

   http://www.phpbb.com/downloads.php

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
names CAN-2005-0259 to these issues. This is a candidate for inclusion
in the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

DISCLOSURE TIMELINE

02/09/2005       Initial vendor notification
02/11/2005       Initial vendor response
02/22/2005       Public disclosure

CREDIT

AnthraX101 (AnthraX101[at]gmail.com) is credited with this discovery.

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LEGAL NOTICES

Copyright © 2005 Verisign, 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
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct, indirect,
or consequential loss or damage arising from use of, or reliance on,
this information.