Security Advisory (Reclassification) : CT21-11-2005


 
Internet Explorer JavaScript Window() Remote Code Execution
 

Affected Software: Microsoft Internet Explorer 5.5 & 6.x
Severity: Critical (Elevated from low)
Impact: Remote System Access
Solution Status: Patch now available (see below)
CVE Reference: CAN-2005-1790
Advisory Date: 21st November, 2005
   
Credits: Benjamin Tobias Franz (original bug)




1. OVERVIEW

This document serves as a reclassification advisory for the Microsoft Internet Explorer JavaScript Window() DoS vulnerability, originally reported on 31/05/2005.

Contrary to popular beliefs, the aforementioned security issue is susceptible to remote, arbitrary code execution, yielding full system access with the privileges of the underlying user.



2. TECHNICAL NARRATIVE

As well documented, the vulnerability is instigated by IE's failure to correctly initialise the JavaScript "Window()" function, when used in conjunction with a <BODY onload> event. As a result, Internet Explorer encounters an exception when trying to call a dereferenced 32bit address located in ECX, as highlighted by the following line of code:

CALL DWORD [ECX+8]

Due to the bug, ECX is inadvertently populated by the Unicode representation of a text string named "OBJECT", or more specifically 0x006F005B. As offset 0x006F005B points to an invalid (or non-existent) memory location, Internet Explorer fails to progress, and in turn the end user experiences an application crash (Denial of Service).

Therefore, as the bug does not yield control of any internal register and/or points to an offset of which we have no control, the original "low" risk classification clearly reflects the improbable scenario for remote code execution.



How improbable?


If we take a closer look at the vulnerability, we actually see that the instruction is trying to dereference an offset in the range of 0x00600000, which, coincidently, is reserved for the facilitation of all opened Window characteristics on the desktop.

These structures vary in both length and content, but in the main, take the form of window titles, buttons, and any File/edit/View menus bars attributable to a particular Window session.

Consequently, it is feasible to assume that offset 0x006F005B could be arrived at through the invocation of several large Windows structures, for example circa 12 new web browsing sessions, which would increment 0x00600000 into 0x006F005B.

If this were possible, it would just leave the problem of trying to identify a means by which custom shellcode could be inserted via one of Window Elements, and correctly aligned against the called [0x006F005B].

Accordingly, several methods were tested. By using a combination of multiple open windows (expanding the memory section), and legal techniques that allow the modification of Window elements (examples below), 3rd party code execution was eventually realised!

Examples:

1. Long HTML <TITLE>
2. Long Document file names
3. Large Alert Boxes


Unfortunately, all methods tested suffered from one major flaw - inconsistency.

The assumption that a potential victim has a clean desktop (no open apps) compounded by the fact that most window elements encompasses some form of content length restriction, result in very small opportunity for any realistic exploitation.

Except, for one particular approach......a JavaScript prompt box.

By employing a simple technique to invoke multiple occurrences of large JavaScript prompt boxes, it is possible to flood/saturate the remoteness between 0x00600000 - 0x006F005B ++ with data of our choice, yielding very reliable execution of arbitrary code.


3. PROOF OF CONCEPT

http://www.computerterrorism.com/research/ie/poc.htm

 

4. TEMPORARY SOLUTION

Until a patch is developed, users are advised to disable active scripting for non-trusted sites.

 

5. VENDOR STATUS

The original DoS vulnerability was brought to the public's attention on the 31/05/2005 by Benjamin Tobias Franz. To date, the vendor has failed to publicly acknowledge the presence of the flaw, or provide any timescales for an appropriate fix. Accordingly, as of the date of this document, this vulnerability remains UNPATCHED, affecting all users of Microsoft Internet Explorer version 5.5 and 6.x respectively.

13th December 2005 -
Microsoft release patches

Internet Explorer 5.01 SP 4 on Microsoft Windows 2000 (requires SP 4)

Internet Explorer 6 SP 1 on Microsoft Windows 2000 (requires SP 4) or on Microsoft Windows XP (requires SP 1)

Internet Explorer 6 for Microsoft Windows XP (requires SP 2)

 



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