Vulnerability Note VU#468800

Microsoft Windows VML compressed content integer underflow

Overview

Microsoft Windows VML fails to properly handle compressed content, which may allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable system.

I. Description

Microsoft IE version 5.0 and higher supports the Vector Markup Language (VML), which is a set of XML tags for drawing vector graphics. VGX.DLL provides VML support for Internet Explorer. VGX.DLL contains an integer underflow vulnerability in the handling of compressed VML content.

II. Impact

By convincing a user to view a specially crafted HTML document (e.g., a web page or an HTML email message or attachment), a remote, unauthenticated attacker may be able to execute arbitrary code with the privileges of the user. The attacker could also cause Internet Explorer (or the program using the WebBrowser control) to crash.

III. Solution

Apply an update

This issue is addressed by Microsoft Security Bulletin MS07-050. This bulletin provides an updated version of VGX.DLL.

Until the update can be applied, consider the following workarounds:

Disable VML support

Microsoft Security Bulletin MS07-050 suggests the following technique to disable VML support:

1. Click Start, click Run, type "%SystemRoot%System32 egsvr32.exe" -u "%CommonProgramFiles%Microsoft SharedVGXvgx.dll", and then click OK.
2. A dialog box appears to confirm that the un-registration process has succeeded. Click OK to close the dialog box.

Disable ActiveX

Disabling ActiveX controls in the Internet Zone (or any zone used by an attacker) appears to prevent exploitation of this and other ActiveX vulnerabilities. Instructions for disabling ActiveX in the Internet Zone can be found in the "Securing Your Web Browser" document.

Do not follow unsolicited links

In order to convince users to visit their sites, attackers often use URL encoding, IP address variations, long URLs, intentional misspellings, and other techniques to create misleading links. Do not click on unsolicited links received in email, instant messages, web forums, or internet relay chat (IRC) channels. Type URLs directly into the browser to avoid these misleading links. While these are generally good security practices, following these behaviors will not prevent exploitation of this vulnerability in all cases, particularly if a trusted site has been compromised or allows cross-site scripting.

Disable Active Scripting

Although this vulnerability does not require Active Scripting to be enabled, known exploits targeting this issue use Active Scripting to place malicious code on a vulnerable system. To block this attack vector, it is recommended that Active Scripting be disabled. For instructions on how to disable Active Scripting in Microsoft Internet Explorer, please refer to the Internet Explorer section of the Securing Your Web Browser document.

Systems Affected

Vendor	Status	Date Updated
Microsoft Corporation	Vulnerable	14-Aug-2007

References

http://www.us-cert.gov/reading_room/securing_browser/
http://www.microsoft.com/technet/security/bulletin/ms07-050.mspx
http://research.eeye.com/html/advisories/published/AD20070814a.html
http://www.iss.net/threats/273.html
http://secunia.com/advisories/26409/

Credit

Thanks to Microsoft for reporting this vulnerability, who in turn credit eEye Digital Security.

This document was written by Will Dormann.

Other Information

Date Public	08/14/2007
Date First Published	08/14/2007 04:56:23 PM
Date Last Updated	08/17/2007
CERT Advisory
CVE Name	CVE-2007-1749
Metric	21.04
Document Revision	7