Vulnerability Note VU#939688

Microsoft Internet Explorer HTML Help control bypasses Local Machine Zone Lockdown

The HTML Help ActiveX control (Hhctrl.ocx) is not restricted by the Local Machine Zone Lockdown. This means that the HTML Help control can be used to perform various actions, such as executing script, in the Local Machine Zone.

The HTML Help ActiveX control can be scripted to automatically open an arbitrary file as a result of certain events, such as completing a drag and drop operation. When used in combination with VU#972415, the HTML Help ActiveX control can execute script in the Local Machine Zone without requiring any user interaction beyond viewing an HTML document..

II. Impact

Install update

Install the appropriate update referenced in Microsoft Security Bulletin MS05-001. This update does not remove the vulnerability, but it provides a mitigating factor. With the update, the HTML Help control cannot be used by HTML documents that reside outside of the Local Machine Zone. This helps prevent an attacker from using the HTML Help control to execute script.

Disable Active scripting and ActiveX controls

Disabling Active scripting and ActiveX controls in the Internet Zone (or any zone used by an attacker) appears to prevent exploitation of this vulnerability. Disabling Active scripting and ActiveX controls in the Local Machine Zone will prevent widely used payload delivery techniques from functioning. Instructions for disabling Active scripting in the Internet Zone can be found in the Malicious Web Scripts FAQ. By default, the Local Machine Zone (My Computer) is not displayed in the Security tab of the Internet Options dialog box. See Microsoft Knowledge Base Article 315933 for information about accessing the security settings for this zone. Note that disabling Active scripting and ActiveX controls in the Internet Zone will reduce the functionality of some web sites. Disabling Active scripting and ActiveX controls in the Local Machine Zone may prevent several Windows components from functioning properly.

Disable "Drag and drop or copy and paste files"

Disabling the zone security preference "Drag and drop or copy and paste files" prevents drag and drop operations. One way for exploits to deliver arbitrary files to the target machine is via drag and drop. While this setting does not remove the vulnerability, it does prevent an attack vector.

Note: This preference is not honored with Windows XP and Windows Server 2003 operating systems that do not have the MS04-038 update (VU#630720). Without the patch, Windows XP and Windows Server 2003 will always allow drag and drop events to occur, regardless of the zone security setting. After the patch in MS04-038 is installed, the preference to disable drag and drop events is honored. However, in our testing, the "Prompt" option now behaves the same as "Disable" with Windows XP and Windows Server 2003. If set to "Prompt," the drag and drop events will not occur, and there will be no prompt.

Systems Affected

http://msdn.microsoft.com/security/productinfo/xpsp2/securebrowsing/locallockdown.aspx
http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/sp2brows.mspx#EHAA
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/htmlhelp/html/vsconocxov.asp
http://www.securityfocus.com/archive/1/378885
http://www.securitytracker.com/alerts/2004/Nov/1012342.html
http://xforce.iss.net/xforce/xfdb/17824

Credit

This vulnerability was publicly reported by http-equiv.

This document was written by Will Dormann.

Other Information

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