HTML Help API - Privilege Escalation

["Brett Moore" <brett.moore@xxxxxxxxxxxxxxxxxxxxxxx>]

=====================================================================
= HTML Help API - Privilege Escalation
=
= Tested against:
=   HTML Help Control Version 5.2.3735.1
=
= brett.moore@xxxxxxxxxxxxxxxxxxxxxxx
= http://www.security-assessment.com
=
= Originally posted: October 24th, 2003
=====================================================================

== Background ==

Microsoft Windows allows applications to use a standard method of
displaying and handling help files. One of these methods is using 
the HTML help API.

(From MSDN)
- HTML Help API Overview
- The HTML Help application programming interface (API) enables a 
- Windows program to create a help window that displays a help topic.
- The Windows program has complete control over the type, style, and
- position of the help window. 
-
- The fundamental feature of the HTML Help API is the help window.
- Through the API commands, you can create a help window that hosts 
- a Microsoft Internet Explorer DLL (Shdocvw.dll) and displays an 
- HTML file that you specify. 
(End MSDN)

The HTML help API consists of one function that an application uses
to pass commands.

        HWND HtmlHelp(
              HWND    hwndCaller,
              LPCSTR  pszFile,
              UINT    uCommand,
              DWORD   dwData) ;

When an application loads a help file using this function it passes
the name of the file through the pszFile parameter. It appears that
this function does not drop any privileges before invoking the help
viewer.

If a SYSTEM level application uses this function to display a help file,
the HTML help viewer will be running with SYSTEM rights.

Part of the help window consists of an instance of Internet Explorer
which allows a user to browse the local drive. 

By selecting jump to URL from the window system menu, a user can enter
a path name (c:\), right-mouse-click on a file and then select open
with cmd.exe to be given a SYSTEM level command shell window.

== Example Vulnerable Programs ==

>From our testing, any application running at a higher security level
that invokes htmlhelp without dropping privileges is vulnerable.
We tested various Personal Firewall and Antivirus applications and 
found some to be vulnerable to this attack.
We found no 'default' windows applications vulnerable to this attack,
but think that it is something that application developers need to be
aware of.

== Solutions ==

The HTML help view (hh.exe) should be called externally passing the
helpfile name as a parameter.

Security rights could be dropped through the use of system() or 
CreateProcess() functions. CreateProcessAsUser() or 
ImpersonateLoggedOnUser() could be used to control the rights that
htmlhelp executes with.

If an interactive window requires SYSTEM rights, its functionality should
be limited to those functions requiring the higher level of privilege.

== Credit ==

Brett Moore from security-assessment.com

== About Security-Assessment.com ==

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research and development, and its team have previously identified a
number of vulnerabilities in public and private software vendors products.