Software Engineering Institute

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.

This behavior results in a vulnerability that allows attackers to bypass the security measures that typically screen out executable code. This code would be executed with the privileges the user who views the web page or email message.

Users who view a malicious web site or HTML email message may be able to prevent the execution of the attacker's code by using the download progress dialog box to cancel the download. However, depending on the size of the embedded file and the speed of the network connection, users may not have time to cancel the file download.

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.

Apply a patch from your vendor

Microsoft has released a cumulative patch for Internet Explorer that corrects this vulnerability and several others. For more information about the patch and the vulnerabilities, please see Microsoft Security Bulletin MS01-058:

http://www.microsoft.com/technet/security/bulletin/MS01-058.asp

Disable file downloads in untrusted security zones

As a workaround, you can prevent malicious files from being downloaded by disabling all file downloads in untrusted security zones. Note that this decision will impact browser functionality.