MS IE HTML Help Shortcut Vulnerability

The window.showHelp() method in Internet Explorer allows IE to open HTML help files (.chm). These files can contain shortcuts to executables which will be run at the privilege level of the current user. The .chm files cannot be opened via http, but a remote .chm file can be viewed if it is on any server with Microsoft Networking (including Samba) installed.

Therefore, an attacker can set up a webpage with a link pointing to a .chm file on any NetBios-enabled host. That .chm file can in turn be made to execute any program that is on the victim's machine or on any NetBios-enabled server.

It is also possible to get the .chm file onto the target host by sending it encapsulated in a .elm file. IE and Outlook Express will save .elm files in a known location (eg: the system TEMP directory in IE). These files can contain attachments, such as a .chm file.

Update (November 20, 2000): An attacker is able to discover the location of a Temporary Internet Files folder. Any cached .chm file can be executed by window.showHelp() once the path to a Temporary Internet Files folder is known.


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