SMF upload XSS vulnerability

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Advisory : SMF upload XSS vulnerability
Release Date : December 4th, 2006
Application : Simple Machines Forum
Version : SMF 1.1 Final (and earlier versions)
Platform : PHP
Vendor URL : http://www.simplemachines.org
Authors : Jessica Hope ( [email protected] )
: rotwang ( [email protected] )

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Overview

Due to various failures in sanitising user input, it is possible to
construct XSS attacks using files masquerading as images.

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Discussion

A often ignored XSS hazard sprouts from the Internet Explorer's habit
to "guess"
the type of displayed data, when mime-type and header do not match.
This is especially dangerous in software allowing image uploads; the accepted
counter-measure is to use getimagesize to guarantee that the correct mime-type
is chosen.

SMF's implementation of this check is faulty, as it can be overridden by simply
setting the parameter "image". In that case, the file will be
delivered with the
type "image/gif", regardless of the file's content or name.
Even an uploaded text file is able to carry an XSS vector.
Neither the upload function, nor the delivery code actually act upon the file's
content. The admin function "Check attachment's extension" has no
impact on that
behaviour.

Vulnerable code in Display.php

Line 1045
if (filesize($filename) != 0)
{
$size = @getimagesize($filename);
if (!empty($size) && $size[2] > 0 && $size[2] < 4)
header('Content-Type: image/' . ($size[2] != 1 ? ($size[2] != 2 ?
'png' : 'jpeg') : 'gif'));
// Errr, it's an image… what kind? A… gif? Yeah that's it,
gif! Like JIF, the peanut butter.
elseif (isset($_REQUEST['image']))
header('Content-Type: image/gif');
}

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Solution

It is possible to work around the issue like so:

$size = @getimagesize($filename);
if (!empty($size) && $size[2] > 0 && $size[2] < 4) {
header('Content-Type: image/' . ($size[2] != 1 ? ($size[2] != 2 ?
'png' : 'jpeg') : 'gif'));
}
// Errr, it's not an image… what kind? Ah, let's play it safe
else {
header('Content-Disposition: attachment; filename="' . $real_filename . '"');
header('Content-Type: application/octet-stream');

            }

Moreover, the upload function should check the actual filtype. Files
with invalid
extensions should not be accepted as uploads.
The avatar function already implements such checks; they should be applied for
the attachment function as well.

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History:

Having dealt with SMF in the past, I know that they do not take security
seriously (have a look at my earlier IP spoofing SMF report). Thus until
I believe that SMF have shaped up in terms of security, any issues that I
come across I'll be posting a full disclosure immediately.
Incidently, the IP spoofing still exsists in the latest SMF too. They never
learn.

04th December 2006: Full disclosure

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Credit

This issue is to be credited to Jessica Hope ( [email protected] ),
and rotwang ( [email protected] )