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 <http://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.gifwe 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
Source : seclists.org/lists/fulldisclosure/2005/Oct/0494.html
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