XSS vulnerability in phpMyID

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Subject: XSS vulnerability in phpMyID
Credits: Raphael Geissert <[email protected]>
Release date: 2008-10-27
Affects: v0.9 [23-Jul-2008]

Resources:
* Homepage: http://siege.org/projects/phpMyID/
* Demo: http://phpmyid.com

Background:
phpMyID is a single user OpenID identity provider implemented in PHP.

Problem description:
The MyID.php script does not sanitize the input it is supposed to be given
by the site where the user wants to be authenticated. When the return_to
address does not have the same "root" as trust_root it aborts, opening a
hole for XSS attacks.

Impact:
A user can be tricked and redirected to its vulnerable identity provider,
place where the specially crafted data exploits the security hole.

Example exploit:
MyID.php?openid_mode=checkid_immediate&openid_return_to=bar
&openid_trust_root=%3Cscript%3Ewindow.alert%28%29%3B%3C%2Fscript%3E
&openid_identity=foo
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=A51u
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