Buffer Overflow in Microsoft Internet Explorer

Systems Affected

Microsoft Windows systems running

• Internet Explorer versions 6 and later
• Other programs that host the WebBrowser ActiveX control

Overview

Microsoft Internet Explorer (IE) contains a buffer overflow vulnerability that could allow a remote attacker to execute arbitrary code with the privileges of the user running IE.

I. Description

A buffer overflow vulnerability exists in the way IE handles the SRC and NAME attributes of various elements, including FRAME and IFRAME. Because IE fails to properly check the size of the NAME and SRC attributes, a specially crafted HTML document can cause a buffer overflow in heap memory. Due to the dynamic nature of the heap, it is usually difficult for attackers to execute arbitrary code using this type of vulnerability.

However, if heap memory is prepared in a special manner, an attacker could execute arbitrary code more easily. Publicly observed exploits use scripting to prepare the heap, though this may be accomplished without scripting. Without the ability to prepare the heap, the impact is most likely limited to denial of service.

Based on limited testing and information from Microsoft, IE 5, IE 6 on Windows XP SP2, and IE 6 on Windows Server 2003 are not vulnerable.

This vulnerability is described in further detail in VU#842160.

II. Impact

By convincing a user to view a specially crafted HTML document (e.g., a web page or an HTML email message), an attacker could execute arbitrary code with the privileges of the user. The attacker could also cause IE (or any program that hosts the WebBrowser ActiveX control) to crash.

Reports indicate that this vulnerability is being exploited by malicious code propagated via email. When a user clicks on a URL in a malicious email message, IE opens and displays an HTML document that exploits the vulnerability. This malicious code may be referred to as MyDoom.{AG,AH,AI} or Bofra.

III. Solution

Install an update

Install Windows XP SP2

Microsoft Windows XP SP2 is not affected by this vulnerability. Windows XP users should consider updating to SP2.

Disable Active scripting

To help protect against attacks that use scripting to prepare the heap, disable Active scripting in any zone used to render untrusted HTML content (typically the Internet Zone and Restricted Sites Zone). Instructions for disabling Active scripting in the Internet Zone can be found in the Malicious Web Scripts FAQ.

Do not follow unsolicited links

Do not click on unsolicited URLs received in email, instant messages, web forums, or Internet relay chat (IRC) channels. While this is generally good security practice, following this behavior will not prevent exploitation of this vulnerability in all cases. For example, a trusted web site could be compromised and modified to deliver exploit script to unsuspecting clients.

Read and send email in plain text format

Outlook 2003, Outlook 2002 SP1, and Outlook 6 SP1 can be configured to view email messages in text format. Consider the security of fellow Internet users and send email in plain text format when possible. Note that reading and sending email in plain text will not necessarily prevent exploitation of this vulnerability.

Maintain updated anti-virus software

Anti-virus software with updated virus definitions may identify and prevent some exploit attempts. Variations of exploits or attack vectors may not be detected. Do not rely solely on anti-virus software to defend against this vulnerability. More information about viruses and anti-virus vendors is available on the US-CERT Computer Virus Resources page.

Appendix A. References

• Microsoft Security Bulletin MS04-040 - <http://www.microsoft.com/technet/security/bulletin/ms04-040.mspx>
• MS04-040: Cumulative Security Update for Internet Explorer (IE 6.0 SP1) - <http://support.microsoft.com/kb/889293>
• An update rollup is available for Internet Explorer 6 SP1 - <http://support.microsoft.com/kb/889669>
• US-CERT Technical Cyber Security Alert TA04-336A - <http://www.us-cert.gov/cas/techalerts/TA04-336A.html>
• Vulnerability Note VU#842160 - <http://www.kb.cert.org/vuls/id/842160>
• Security Improvements in Windows XP Service Pack 2 - <http://www.us-cert.gov/cas/alerts/SA04-243A.html>
• Malicious Web Scripts FAQ - <http://www.cert.org/tech_tips/malicious_code_FAQ.html>
• US-CERT Computer Virus Resources Page - <http://www.us-cert.gov/other_sources/viruses.html>
• About the Browser (Internet Explorer - WebBrowser) - <http://msdn.microsoft.com/workshop/browser/overview/Overview.asp>

Feedback can be directed to the authors: Will Dormann and Art Manion.

Copyright 2004 Carnegie Mellon University. Terms of use

Revision History

November 10, 2004: Initial release
November 11, 2004: Added status information about IE versions, minor changes to Windows XP SP2 workaround and SP2 reference
December 3, 2004: Removed EMBED from list of affected elements, updated systems affected, revised SP2 information, added MS04-040 and KB articles to solution