IE 5.x/Outlook allows executing arbitrary programs using .chm files and temporary internet files folder

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Georgi Guninski security advisory #28, 2000

IE 5.x/Outlook allows executing arbitrary programs using .chm files and temporary internet files folder

Systems affected:
IE 5.x/Outlook/Outlook Express - probably other versions, have not tested

Risk: High
Date: 20 November 2000

Legal Notice:
This Advisory is Copyright (c) 2000 Georgi Guninski. You may distribute it unmodified. You may not modify it and distribute it or distribute parts of it without the author's written permission.

The opinions expressed in this advisory and program are my own and not of any company.
The usual standard disclaimer applies, especially the fact that Georgi Guninski
is not liable for any damages caused by direct or  indirect use of the information or functionality provided by this advisory or program.
Georgi Guninski, bears no responsibility for content or misuse of this advisory or program or any derivatives thereof.


There is a security vulnerability in IE 5.x/Outlook/Outlook Express which allows executing arbitratrary programs using .chm files and revealing the location of temporary internet files folder. This may lead to taking full control over user's computer.

I reported a similar vulnerability regarding .chm files sometime ago and Microsoft fixed it
by allowing .chm files to run programs only if the .chm was loaded from the local file system.
But it is possible to find the temporary internet files folder - there are several folders with
random names.
The following HTML code:
<OBJECT DATA="http://SOMEHOST.COM/chmtemp.html" TYPE="text/html" WIDTH=200 HEIGHT=200>
where SOMEHOST.COM is a web server or alias that is different from the web server from which
the HTML page is loaded may reveal one of the temporary internet files folders thru document.URL.
Once a temporary internet files folder name is known it is possible to cache a .chm in any
temporary internet files folder and then use window.showHelp() to execute it.
There are other ways to execute programs once a temporary internet files folder is known and
document is cached in it but showHelp() seems to be the simplest.
If the demonstration does not work wait a minute and reload the page or increase the number of 
"chm*.chm" files in <IMG> and showHelp() or increase the time to wait if it is insufficient 
to download the chm files.
The code is:
<IMG SRC="chm1.chm" WIDTH=1 HEIGHT=1>
<IMG SRC="chm2.chm" WIDTH=1 HEIGHT=1>
<IMG SRC="chm3.chm" WIDTH=1 HEIGHT=1>
<IMG SRC="chm4.chm" WIDTH=1 HEIGHT=1>
<IMG SRC="chm5.chm" WIDTH=1 HEIGHT=1>
<IMG SRC="chm6.chm" WIDTH=1 HEIGHT=1>
<IMG SRC="chm7.chm" WIDTH=1 HEIGHT=1>
<IMG SRC="chm8.chm" WIDTH=1 HEIGHT=1>
<IMG SRC="chm9.chm" WIDTH=1 HEIGHT=1>
<IMG SRC="chm10.chm" WIDTH=1 HEIGHT=1>
The object below must be loaded from a server with name different from the parent document - 
it may be the same server but use the IP address or another alias.
If this does not work try increasing the number of "chm*.chm" in IMG and showHelp.
<OBJECT DATA="" TYPE="text/html" WIDTH=200 HEIGHT=200>

function g()
alert("One of your temp files directory is: "+path);  
setTimeout("g()",5000); // if you are on a slow internet connection you must increase the delay

Demonstration is available at:

Vendor status:
Microsoft was contacted on 15 November 2000.


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