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Advisory 17/2005: phpBB Multiple Vulnerabilities
Oct 31 2005 12:16AM
Stefan Esser (sesser hardened-php net)
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-= Security Advisory =-
Advisory: phpBB Multiple Vulnerabilities
Release Date: 2005/10/31
Last Modified: 2005/10/31
Author: Stefan Esser [sesser (at) hardened-php (dot) net [email concealed]]
Application: phpBB <= 2.0.17
Severity: Multiple vulnerabilities allow XSS, SQL injection
and remote code execution
Vendor Status: Vendor has released an updated version
Quote from www.phpbb.com:
"phpBB is a high powered, fully scalable, and highly customizable
Open Source bulletin board package. phpBB has a user-friendly
interface, simple and straightforward administration panel, and
helpful FAQ. Based on the powerful PHP server language and your
choice of MySQL, MS-SQL, PostgreSQL or Access/ODBC database
servers, phpBB is the ideal free community solution for all
Because of our research into register_globals deregistration
codes, the implementation within phpBB was audited and several
weaknesses were found, that allowed to completely bypass the
protection on PHP5 servers.
After these weaknesses were found and disclosed to the vendor
nearly 80 days ago, several problems with unitialised variables
were discovered that allow XSS, SQL injection and even remote
execution of arbitrary PHP code, when phpBB is used with
register_globals turned on.
While register_globals=off is the recommended setting, most web-
hosters, even those that actually run PHP5, still have it
enabled because it is their customers wish.
To get rid of possible security problems caused by not properly
initialised variables phpBB comes with the following piece of
code, that is intended to deregister global variables, which were
created because of the register_globals directive. Unfortunately
there are atleast 3 ways to bypass the protection.
// PHP4+ path
$not_unset = array('HTTP_GET_VARS', 'HTTP_POST_VARS',
'HTTP_POST_FILES', 'phpEx', 'phpbb_root_path');
// Not only will array_merge give a warning if a parameter
// is not an array, it will actually fail. So we check if
// HTTP_SESSION_VARS has been initialised.
$HTTP_SESSION_VARS = array();
// Merge all into one extremely huge array; unset
// this later
$input = array_merge($HTTP_GET_VARS, $HTTP_POST_VARS,
while (list($var,) = @each($input))
if (!in_array($var, $not_unset))
 In PHP5 <= 5.0.5 it is possible to register f.e. the global
variable $foobar by supplying a GET/POST/COOKIE variable
with the name 'foobar' but also by supplying a GPC variable
called 'GLOBALS[foobar]'. If the variable is supplied in
that way, the code above will not try to unset $foobar, but
$GLOBALS, which completely bypasses the protection.
 When the session extension is not started by a call to
session_start(), PHP does not know about the variables
$_SESSION or $HTTP_SESSION_VARS, which means, it is possible
to fill them with any value if register_globals is turned on.
Combined with the fact (that was even documented in the phpBB
code), that array_merge() will fail in PHP5, when at least
one of the parameters is not an array, it is possible for an
attacker to simply set HTTP_SESSION_VARS to a string and let
the complete protection fail, because $input ends up empty.
 When register_long_array is turned off PHP does not know
anymore about all the HTTP_* variables. This means they can
be filled with anything that is completely unrelated to the
existing global variables. It is obvious that the protection
cannot work, when this configuration is choosen.
Additonally to the 3 possible ways to bypass the globals
deregistration code, several not properly initalised variables
were disclosed to the vendor, that can even lead to remote code
Not properly initialised variables
 Within usercp_register.php the variable 'error_msg' is not
properly initialised and can therefore be used to inject
arbitrary HTML code
 Within login.php the variable 'forward_page' is not properly
initialised and can be used to inject arbitrary HTML code
 Within search.php the variable 'list_cat' is not properly
initialised and can be used to inject arbitrary HTML
 Within usercp_register.php the variable 'signature_bbcode_uid'
is not properly initialised and can be used for SQL injection
of arbitrary 'field=xxx' statements into queries operating
on the user table, when magic_quotes_gpc is turned off.
 The same variable  can be used to inject f.e. the 'e'
modifier into the first parameter of a preg_replace()
statement, which means, that the second parameter is
evaluated as PHP code. Because the second parameter is
entirely filled with the user supplied signature, it is
possible to execute any PHP code. This can be exploited,
no matter if magic_quotes_gpc is turned on or off, just
2 different code paths need to be triggered.
Proof of Concept:
The Hardened-PHP project is not going to release exploits for any
of these vulnerabilities to the public.
14. August 2005 - First contact with vendor through their
30. October 2005 - Vendor released new version.
31. October 2005 - Public disclosure.
It is strongly recommended to upgrade to the new phpBB release
or to switch to another bulletin board application.
You can get the new phpBB release at:
pub 1024D/0A864AA1 2004-04-17 Hardened-PHP Signature Key
Key fingerprint = 066F A6D0 E57E 9936 9082 7E52 4439 14CC 0A86 4AA1
Copyright 2005 Stefan Esser. All rights reserved.
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