Software Engineering Institute

Microsoft Windows Media Services is a software package for Windows systems that provides the ability to stream Windows Media content. Included in the Windows Media Services package is an ActiveX control called NMSA Media Description Object, which is provided by the file mdsauth.dll. This ActiveX control, which is marked "Safe for scripting" via the iObjectSafety interface, includes methods that can be used to overwrite arbitrary files with attacker-provided content.

The vulnerable control comes with Windows Media Services 4.1, including the Windows Media Administrator software. Other software may also include the vulnerable control.

By convincing a user to view a specially crafted HTML document (e.g., a web page or an HTML email message or attachment), an attacker can overwrite arbitrary files. This may allow the attacker to be able to execute arbitrary code with the privileges of the user.

Apply an update

Microsoft has addressed this issue in Microsoft Security Bulletin MS07-027. Users should review this bulletin and apply the patches it refers to.

Workarounds

Disable the NMSA Session Description Object ActiveX control in Internet Explorer

The vulnerable ActiveX control can be disabled in Internet Explorer by setting the kill bit for the following CLSID:

{D4FE6227-1288-11D0-9097-00AA004254A0}
More information about how to set the kill bit is available in Microsoft Support Document 240797. Alternatively, the following text can be saved as a .REG file and imported to set the kill bit for this control:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{D4FE6227-1288-11D0-9097-00AA004254A0}]
"Compatibility Flags"=dword:00000400
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.