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Security Bulletin

Microsoft Security Bulletin MS99-012 - Critical

MSHTML Update Available for Internet Explorer

Published: April 21, 1999

Version: 1.0

Originally Posted: April 21, 1999

Summary

Microsoft has released an updated version of a component of Internet Explorer 4.0 and 5. The updated version eliminates three security vulnerabilities described below. It is fully supported and Microsoft recommends that affected customers download and install it, if appropriate.

Issue

MSHTML.DLL is the parsing engine for HTML in Internet Explorer. The vulnerabilities that are eliminated by the update are not related to each other except for the fact that all reside within the parsing engine.

  • The first vulnerability is a new variant of a previously-identified cross-frame security vulnerability. A particular malformed URL could be used to execute scripts in the security context of a different domain. This could allow a malicious web site operator to execute a script on the web site, and gain privileges on visiting users' machines that are normally granted only to their trusted sites.
  • The second vulnerability affects only Internet Explorer 5.0, and is a new variant of a previously-identified untrusted scripted paste vulnerability. The vulnerability would allow a script to paste a filename into the file upload intrinsic control. This should only be possible by explicit user action. Once the filename has been pasted into the control, a subsequent form submission could send the file to a remote web site. If the user has disabled the default warning that is displayed when submitting unencrypted forms, the file would be sent without any warning to the user.
  • The third vulnerability is a privacy issue involving the processing of the "IMG SRC" tag in HTML files. This tag identifies and loads image sources - image files that are to be displayed as part of a web page. The vulnerability results because the tag can be used to point to files of any type, rather than only image files, after which point the document object model methods can be used to determine information about them. A malicious web site operator could use this vulnerability to determine the size and MIME type of files on the computer of a visiting user. It would not allow files to be read or changed, and the malicious web site operator would need to know the name of each file.

While there are no reports of customers being adversely affected by any of these vulnerabilities, Microsoft is proactively releasing an updated version of MSHTML.DLL to allow customers to take appropriate action to protect themselves against it.

Affected Software Versions

  • Internet Explorer 4.0 and 5 on Windows 95, Windows 98 and Windows NT 4.0.

Vulnerability Identifiers

What Microsoft is Doing

Microsoft has released patches that fix the problem identified. The patches are available for download from the sites listed below in What Customers Should Do.

Microsoft also has sent this security bulletin to customers subscribing to the Microsoft Product Security Notification Service. See The Microsoft Product Security Notification Service for more information about this free customer service.

Microsoft has published the following Knowledge Base (KB) article on this issue:

  • Microsoft Knowledge Base (KB) article 226325, Update Available for MSHTML Security Issues in Internet Explorer https:. (Note It might take 24 hours from the original posting of this bulletin for the KB article to be visible in the Web-based Knowledge Base.)

What Customers Should Do

Microsoft highly recommends that customers evaluate the degree of risk that this vulnerability poses to their systems and determine whether to download and install the patch. The patch can be found at </https:>https:.

More Information

Please see the following references for more information related to this issue.

Obtaining Support on this Issue

If you require technical assistance with this issue, please contact Microsoft Technical Support. For information on contacting Microsoft Technical Support, please see </https:>https:.

Acknowledgements

Microsoft acknowledges Richard M. Smith, President, Phar Lap Software, Inc., for discovering the IMG SRC vulnerability, and Georgi Guninski from TechnoLogica Ltd., Bulgaria, for discovering the cross-frame and untrusted scripted paste vulnerabilities.

Revisions

  • April 21, 1999: Bulletin Created.

For additional security-related information about Microsoft products, please visit https://www.microsoft.com/technet/security

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