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Dec 05 2003 01:35PM
Chet Simpson (secure ytunnelpro com)
Author: Chet Simpson (secure (at) ytunnelpro (dot) com [email concealed])
Date: December 5th, 2003
Host Platforms tested: WindowsME and WindowsXP (sp1a)
Target Applications tested: Yahoo Messenger 5.5 (Build 1249)
Yahoo Messenger 5.6 (Build 1355)
Target Applications affected: ??All?? versions of Yahoo Messenger
Components Affected: ypager.exe
Prerequisites: The IMVironment feature must be enabled
Possible Dangers: Password Theft
XSS Cookie Exploits
Example included: Yes
A vulnerability found in ypager.exe allows a website to inject [malicious] html,
scripts, and possibly activex controls into a Yahoo Messenger IM window.
Yahoo Messenger installs a special URL handler to automatically launch any URL
starting with "ymsgr:". For Netscape, the YAuto.dll file is used. For Internet
Explorer the main executable (ypager.exe) is launched. The Messenger specific
URL protocol allows for automatically opening Instant Messages, Chatrooms,
and File Transfer sessions. The exploit documented here is specific to the
functionality provided by this URL protocol to initiate an Instant Messenging
session with another user. The format to initiate this session is as follows:
One of the features of this undocumented URL protocol is the ability to
specify the "IMVironment" that should be used during the IM session.
When Yahoo Messenger attempts to load an IMVironment, the name of the
IMVironment is displayed at the top of the text area in the IM window.
If the IMVironment cannot be found or an error occurs a message will be
displayed at the bottom of the same window stating that the IMVironment
cannot be loaded. Although the message at the top of the window is filtered
to prevent injection of HTML and scripts the error message is not.
By placing an IFRAME tag in place of the IMVironment name an additional
web page can be loaded in the context of Yahoo Messenger. This is extremely
dangerous as the IE HTML Control does not necessarily adhere to the current
security and privacy settings selected by the user. This allows a webpage
containing scripts to be loaded and provides an environment which to execute
There are three (3) files included with in the example archive which
demonstrate the flaw outlined in this document:
ymsgr1.html - This is the primary 'host' file containing a Yahoo
Messenger link which initiates a Yahoo Messenger
IM session. Run this first and click on the link.
ymsgr2.html - This file is loaded by Yahoo Messenger into the IM
window once it opens and the IMVironment fails to load.
work in all cases but was chosen to show the severity
of this flaw. Once loaded it will attempt to gather the
Yahoo ID and if available the encoded password stored in
the system registry.
on all systems as some anti-virus software may block it.
ymsgr2p.html - Same as ymsgr2.html but displays the Yahoo ID and encoded
password in a popup window. This will not work with
popup or ad blockers.
ymsgr3.php - This file is accessed by ymsgr2.html and is responsible
for displaying the Yahoo ID and encoded password gathered
by the included script.
Take note that the chosen script may not work on all configurations. During
testing the IFRAME injection was blocked by Y!TunnelPro and by McAfee
Anti-Virus. Norton Anti-Virus Pro 2004 and IMSecurePro did not appear to
stop the script.
A demo of this script can be seen at the following URL:
The archive containing this file and the example scripts can be found here:
http://www.ubabble.com/ymsgr.zip - Zip format
http://www.ubabble.com/ymsgr.tgz - GZipped Tarball
This exploit has an extremely nasty side effect. If the IFRAME is added to
the ymsgr URL in certain ways the IMVironment information will be saved in
such a way that Messenger will no longer log in. This requires that either
the IMVironment keys in the registry be cleaned or Yahoo Messenger to be
Until Yahoo can fix the problem the exploit can be avoided by turning off
IMVironments in the Yahoo Messenger preferences.
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