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There is a cross-domain scripting vulnerability in the way ITS protocol handlers determine the security domain of an HTML component stored in a Compiled HTML Help (CHM) file. The HTML Help system "...uses the underlying components of Microsoft Internet Explorer to display help content. It supports HTML, ActiveX, Java, [and] scripting languages (JScript, and Microsoft Visual Basic Scripting Edition)." CHM files use the InfoTech Storage (ITS) format to store components such as HTML files, graphic files, and ActiveX objects. IE provides several protocol handlers that can access ITS files and individual CHM components: its:, ms-its:, ms-itss:, and mk:@MSITStore:. IE also has the ability to access parts of MIME Encapsulation of Aggregate HTML Documents (MHTML) using the mhtml: protocol handler.
When IE references an inaccessible or non-existent MHTML file using the ITS and mhtml: protocols, the ITS protocol handlers can access a CHM file from an alternate source. IE incorrectly treats the CHM file as if it were in the same domain as the unavailable MHTML file. Using a specially crafted URL, an attacker can cause arbitrary script in a CHM file to be executed in a different domain, violating the cross-domain security model.
