phpBB-IE-gif.txt

2005-10-26T00:00:00
ID PACKETSTORM:40962
Type packetstorm
Reporter alphakgen
Modified 2005-10-26T00:00:00

Description

                                        
                                            `phpBB 2.0.17 (and other BB systems as well) Cookie disclosure exploit.  
  
I sent the report to phpBB and they said that a patch will be available withing a few days and It will be integrated into 2.0.18 .  
  
Note: This works like XSS, and requires the victim to use IE (Affects all versions of IE).  
  
Special Credits to: Sven Vetsch (the original finder of "The gif bug").  
The original gif-bug article : http://www.securiteam.com/windowsntfocus/6F00B00EBY.html  
Also thanks to the experts at securiteam.com for clarifying some issues.  
  
Since what is described in the original article doesn't work, I have written this step-by-step  
article explaining how to replicate this bug successfuly.  
  
Affected: All phpBB systems allowing "Upload Avatar from URL" and most likely all other systems  
with such a feature (Other bulletin boards - but I didn't check).  
  
Well, the base for the problem lies within IE.. The core element of my Proof of Concept is the  
lately found Gif-bug in IE (Originally found by Sven Vetsch).   
  
For some reason IE renders malformed embedded content files (like gif,jpg,wav,and so on..)  
as HTML when they are accessed directly e.g. http://attacker.com/xss.gif (Not through the <img>  
tag).  
  
If we create an HTML file and rename its extention to .GIF (or other embedded content file  
extention), and upload it to an HTTP server (it dosn't work locally for some reason), when we  
will navigate to http://myserver.com/xss.gif the HTML code will be executed instead of showing  
that the image is invalid.   
  
So, if we could upload such a file to a server that allows image upload we could actually upload  
HTML code instead (Inside the image file). If the victim will be lured to navigate to this   
specially crafted image in IE, arbitary HTML code could be executed in the servers security zone,  
e.g. we could steall the users cookie, for example.  
  
However it is not that simple with systems (like phpBB) that verify the image file before it  
is uploaded to the server. If we try to upload our previosly made http://attacker.com/xss.gif  
gif file the system will complain about incorrect image size - that's because our image is invalid.  
The verification system chechs the files header. In a valid 1x1 gif file the header should be  
(in hex) : 47 49 46 38 39 61 01 00 01 00 . After the header we will insert the next HTML code:  
<HTML><HEAD><SCRIPT>alert(document.cookie);</SCRIPT></HEAD></HTML>  
So the file will look like this (in hex):  
47 49 46 38 39 61 01 00 01 00 3C 48 54 4D 4C 3E 3C 48 45 41 44 3E 3C 53 43 52 49 50 54 3E 61 6C 65   
  
72 74 28 64 6F 63 75 6D 65 6E 74 2E 63 6F 6F 6B 69 65 29 3B 3C 2F 53 43 52 49 50 54 3E 3C 2F 48 45   
  
41 44 3E 3C 2F 48 54 4D 4C 3E  
  
If we upload this file instead the old one to : http://myserver.com/xss.gif we will be able to  
upload it as a phpBB avatar. However when we access the file directly (as before) no HTML code  
is going to be executed. That is because IE sees the valid header and tries to draw the image   
instead of rendering the HTML (and fails anyway ...).  
  
However if we change the file extention from .GIF to .JPG the GIF header in the beginning will  
become meaningless to IE and the HTML code will be executed. So if we rename our image from   
xss.gif to xss.jpg when we will navigate to http://myserver.com/xss.jpg we will see an alert  
box (that should show the cookie on its current server).   
  
The phpBB avatar upload system verifies the files header - and our header is pretty much valid -  
for a GIF file, but not JPG. If we try to upload the file http://myserver.com/xss.jpg as our avatar  
it will be successfuly uploaded. Hence any one who will navigate (in IE) directly to our avatar in   
  
its new address on the phpBB forum server (the URL should look like http://phpbbforum.com/phpbb/  
  
images/avatars/2131121a2121f.jpg) will be able to see his cookie information in an alert window.  
Instead an image something like GIF89a_--. will apear, but it can be easily obfuscated with a simple  
JavaScript.  
  
As a Proof of Concept here is a ready made JPG file: (Save target as) http://planet.nana.co.il/  
  
mycoolpictures123/fake/lt2.jpg . Upload this (from its current location, or your HTTP server) as  
an avatar to phpBB (or as I believe - any Bulletin Board system). In your avatar an invalid image  
(red X) will appear, but when you navigate to it's current location (e.g. http://phpbbforum.com/  
  
phpbb/images/avatars/2131121a2121f.jpg) you will see an alert with your cookie.  
  
Using the basic idea of my PoC, the code can be manipulated to send a users cookie information to   
a CGI sniffer on a remote server. All that should be done is sending a message saying "Check out  
this image" and specifying the avatars URL.  
  
This is a major problem since 90% of the internet users use IE and lots of dynamic sites (like   
bulletin boards) allow image upload to the server.   
  
The solution could come in many ways. The best solution for the user is to use another browser  
(like FireFox) untill a vendor patch from Micrsoft is available. For bulletin board administrators  
it is highly advised to turn off the "Upload avatar from URL" option untill a patch from the vendor  
(phpBB, vBulletin, IPB, and so on...) arrives.  
  
Have a good day.  
K-Gen  
  
  
  
`