Microsoft Internet Explorer contains a vulnerability in its handling of certain MIME headers in web pages and HTML email messages. This vulnerability may allow an attacker to execute arbitrary code on the victim's system when the victim visits a web page or views an HTML email message.
Web pages and HTML email messages usually contain HTML text, but other files may also be included. The MIME headers Content-Disposition and Content-Type provide the information needed by the HTML rendering software to determine the type of these files. In Microsoft Internet Explorer, these MIME headers are consulted when evaluating whether to process an embedded file, but they are ignored when the file is actually processed.
For example, if an executable (.exe) file is embedded with MIME headers that misrepresent it as a JPEG image file (.jpg), Internet Explorer will treat the file as a JPEG when evaluating whether it is safe to open. Once this evaluation is complete, the file will be opened according to its .exe file extension and will be executed on the local system.
By convincing a user to view a malicious web page or HTML email message, a remote attacker can cause the user to execute arbitrary code. Any such code would run with the privileges of the user who attempted to view the content.
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Microsoft has publicly acknowledged Jouko Pynnonen of Oy Online Solutions Ltd as the reporter of this vulnerability.