Cross Site Scripting Vulnerability in phpBB2's [IMG] tag and remote avatar

[Martijn Boerwinkel <xim () xs4all nl>]

phpBB2 Cross Site Scripting Vulnerability 
-------------------------------------------- 

Affected Program: phpBB2 version 2.0.0 
  (possibly earlier versions too, but not tested) 
Vendor: http://www.phpbb.com 
Vendor Status: informed on 24/04/2002, fixed issued on 20/05/2002 
Discovery Date: 24/04/2002 
Release Date: 26/05/2002 
Vulnerability Class: Cross Site Scripting 


Severity 
-------- 
Malicious users can steal other user's and admin's cookies, allowing
them to impersonate other users on the board and access to the
administration panel. 


Problem 
------- 
The problem is very similar to SQL injection. 
phpBB2 uses a user provided string (through the [IMG] tag) 
in the following HTML tag: 

<img src="$user_provided" border="0" /> 

While there is a check to force the string to begin with "http://"; it
doesn't disallow ". That means a malicious user can escape the src="" in
the HTML tag and insert his own html code. 
This same problem also exists in the remote avatar part of the user
profile. 


Example 
------- 
Enter the following anywhere in a message: 

[img]http://a.a/a"onerror="javascript:alert(document.cookie)[/img] 

When reading that message it should popup an alert box with your
cookies. 


Solutions 
--------- 
* Upgrade to 2.0.1 

-- 
XiM 
(#icerealm on irc.icerealm.net)

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