Software Engineering Institute

Microsoft COM

Microsoft COM is a technology that allows programmers to create reusable software components that can be incorporated into applications to extend their functionality. Microsoft COM includes COM+, Distributed COM (DCOM), and ActiveX Controls.

Internet Explorer

Internet Explorer will attempt to instantiate a COM object or ActiveX control that is referenced by an OBJECT tag in an HTML document.

BlnMgr Proxy COM object

The BlnMgr Proxy COM object is provided by blnmgrps.dll, which comes with various Microsoft products such as Microsoft Office.

The Problem

The BlnMgr Proxy COM object provides the IDispatch interface, but it does not implement the required methods of GetTypeInfoCount, GetTypeInfo, GetIDsOfNames, or Invoke. When Internet Explorer attempts to instantiate the BlnMgr Proxy COM object, IE will crash because the methods required by the IDispatch interface are not present. The crash occurs in a way that can be exploited to execute arbitrary code on a vulnerable system. The CLSID for this COM object is: {F27CE930-4CA3-11D1-AFF2-006097C9A284}.

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

Apply an update
Apply the appropriate update, as specified in Microsoft Security Bulletin MS05-052. This update sets the "kill bit" for certain CLSID values of COM objects known to be vulnerable, including BlnMgr Proxy.

Disable ActiveX

Disabling ActiveX controls in the Internet Zone (or any zone used by an attacker) appears to prevent exploitation of this vulnerability. With ActiveX controls disabled, COM objects will not be instantiated. Instructions for disabling ActiveX in the Internet Zone can be found in the document Securing Your Web Browser and the Malicious Web Scripts FAQ.

Note that disabling ActiveX controls in the Internet Zone will reduce the functionality of some web sites.