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James Bandara
23/12/04, 02:15
Hi,
Many widely used Bullitien Board Services and Forum Services allow for Clients to set images such as avatars and in their signature/post.

Images work by the clients browser going to that address, like it would for a normal web page except after downloading the file, it tries to open it as an image.

Many of these services if not all have command functions like delete a thread in the form of a hyperlink.

A user could copy one of these links to delete his own thread, edit it so the querystring is for another users post, and post it up as a link or avatar.

In effect if an admin sees the image or the original user sees it, it will instantly delete the post as its on the same site no extra login is needed.

To block this I suggest you edit your service to only accept links that end in image formats for images before the querystring.

I have tested this many times on a modified version of webwiz forums, yet delete is about the only thing that works.

Stefan Paletta
23/12/04, 21:35
James Bandara wrote/schrieb/scripsit:
>To block this I suggest you edit your service to only accept links that
>end in image formats for images before the querystring.

That doesn't really help ─ the attacker can send a HTTP redirect from an
innocent-looking URL.

-Stefan
--
junior guru SP666-RIPE JID:stefanp@jabber.de.cw.net SMP@IRC

Tim Jackson
24/12/04, 01:05
On 22 Dec 2004, James Bandara wrote:

[rediscovering CSRF]

> A user could copy one of these links to delete his own thread, edit it
> so the querystring is for another users post, and post it up as a link
> or avatar.
> In effect if an admin sees the image or the original user sees it, it
> will instantly delete the post as its on the same site no extra login is
> needed.

What you've identified is part of a general class of similar problems and
was discussed quite extensively on this list back in summer 2001. Read the
thread starting here:

http://www.securityfocus.com/archive/1/191114

culminating in a comprehensive analysis by "Peter W" in which he names the
general problem "CSRF" (Cross-Site Request Forgeries):

http://www.securityfocus.com/archive/1/191390


Tim