The HTML Help facility in Windows includes an ActiveX control that provides much of its functionality. One of the functions exposed via the control contains an unchecked buffer, which could be exploited by a Web page hosted on an attacker?s site or sent to a user as an HTML mail. An attacker who successfully exploited the vulnerability would be able to run code in the security context of the user, thereby gaining the same privileges as the user on the system.
A second vulnerability exists because of flaws associated with the handling of compiled HTML Help files that contain shortcuts. Because shortcuts allow HTML Help files to take any desired action on the system, only trusted HTML Help files should be allowed to use them. Two flaws allow this restriction to be bypassed. First, the HTML Help facility incorrectly determines the Security Zone in the case where a Web page or HTML mail delivers a CHM file to the Temporary Internet Files folder and subsequently opens it. Instead of handling the CHM file in the correct zone--the one associated with the Web page or HTML mail that delivered it--the HTML Help facility incorrectly handles it in the Local Computer Zone, thereby considering it trusted and allowing it to use shortcuts. This error is compounded by the fact that the HTML Help facility doesn?t consider what folder the content resides in. Were it to do so, it could recover from the first flaw, as content within the Temporary Internet Folder is clearly not trusted, regardless of the Security Zone it renders in.
The attack scenario for this vulnerability would be complex, and involves using an HTML mail to deliver a CHM file that contains a shortcut, then making use of the flaws to open it and allow the shortcut to execute. The shortcut would be able to perform any action the user had privileges to perform on the system.