Advisory 03/2007: Multiple Browsers Cross Domain Charset Inheritance Vulnerability

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                    Hardened-PHP Project
                    www.hardened-php.net

                  -= Security  Advisory =-


 Advisory: Multiple Browsers Cross Domain Charset Inheritance Vulnerability

Release Date: 2007/02/23
Last Modified: 2007/02/23
Author: Stefan Esser [[email protected]]

Application: Firefox <= 2.0.0.1, Internet Explorer 7, Opera 9
Not affected: Internet Explorer 6, Opera 8
Severity: Web-pages without a defined charset will be rendered
with the charset of the parent page when put into an
(i)frame. This might allow bypassing XSS filters
with for example UTF-7 payload
Risk: Low
Vendor Status: Only Mozilla reacted and released Firefox 2.0.0.2 which fixes this issue
References: http://www.hardened-php.net/advisory_032007.142.html

Overview:

While testing Firefox it was discovered that pages not specifying
a charset in a HTTP Content-Type header or from within a HTML META
tag, inherit the charset of the parent page when they are rendered
within an (i)frame, even when both pages are on different domains.

This opens up Firefox to all the UTF-7 XSS vulnerabilities that were
reported in the past (google.com, mediawiki, …) and are usually
attributed to only affect Internet Explorer due to its charset
autodetection. All an attacker needs to get it working is put the
XSS attack into an iframe on a site using UTF-7.

After the initial contact with the Mozilla team Internet Explorer 7
was released which unlike Internet Explorer is also vulnerable to
the charset inheritance issue. Hinted by the Mozilla developers it
was also discovered that Opera 9 unlike Opera 8 also introduced
this vulnerability.

Unfortunately neither Microsoft nor Opera were interested in the
vulnerability. Opera did not react at all on our bug report and
Microsoft just sent a nonsense mail to us, claiming that we had
disclosed this already to the public and that they like getting
advance notice. We never heard back from them after that initial
email. Not really surprising because it is a similar behaviour we
previously encountered when dealing with them.

Proof of Concept:

The Hardened-PHP Project is not going to release a proof of concept
exploit for this vulnerability.

Disclosure Timeline:

11. October 2006 - Notified [email protected]
12. February 2007 - Firefox 2.0.0.2 released
13. February 2007 - Public Disclosure

Recommendation:

We strongly recommend to upgrade to Firefox 2.0.0.2 which also
fixes several other security vulnerabilities not reported by us
and therefore not covered by this advisory.

http://mozilla.org/

GPG-Key:

http://www.hardened-php.net/hardened-php-signature-key.asc

pub 1024D/0A864AA1 2004-04-17 Hardened-PHP Signature Key
Key fingerprint = 066F A6D0 E57E 9936 9082 7E52 4439 14CC 0A86 4AA1

Copyright 2007 Stefan Esser. All rights reserved.

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