Author: Chet Simpson (secure ytunnelpro com)
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 malicious scripts.
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.
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:
- Zip format
- 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 completely uninstalled.
Until Yahoo can fix the problem the exploit can be avoided by turning off IMVironments in the Yahoo Messenger preferences.