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Home : Advisories : Serious Microsoft File Association Bug

Title: Serious Microsoft File Association Bug
Released by: Network Security Group
Date: 31st August 2000
Printable version: Click here

While working on a virus issue that we have come across, we have discovered a serious issue with Microsoft's association of file types.  Normally, when you open a file of an unknown type, it will prompt you for an application to use to open the file.  This does not prove true for Microsoft Office documents.  If you rename an Office document to an unknown extension, Windows will still use the Office application to open the file.  It seems that Windows uses the header information contained in a file to determine if it is an Office document before offering a list of applications.

Potential Risk:

Someone with malicious intent could create a macro virus embedded in an Office document, then rename the file with a .VIR extension.  Since most anti-virus software have an exclusion of .VI* this file would never be scanned by Norton.  If a user opens the file, Windows will detect that this .VIR file has MS Office header information and open it in the cooresponding application.  Given the correct circumstances, this would infect the machine and replicate to other users.

Systems Affected:

These scenarios have been tested on the following systems:

Windows NT 4 SP5 running Office 97

Windows 2000 running Office 2000

Windows 2000 SP1 running Office 2000

Windows 98 SE running Office 97

I have not tested all variations, but you can draw your own conclusions as to the extent of the problem.

Potential Solutions:

In the case of virus defense, make sure that your anti-virus software does NOT include .VI* in its exclusion list.  This is a short-term solution until a fix can be created.

Jonathan Andrews, CISSP

Network Security Group

Deloitte & Touche


**Please Note***

The opinions expressed above are my own and have no relation

to those of Deloitte & Touche.  No warranties, expressed or implied,

are given about the solutions provided.

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