Virus.MSExcel.Paix

Parent class: VirWare

Viruses and worms are malicious programs that self-replicate on computers or via computer networks without the user being aware; each subsequent copy of such malicious programs is also able to self-replicate. Malicious programs which spread via networks or infect remote machines when commanded to do so by the “owner” (e.g. Backdoors) or programs that create multiple copies that are unable to self-replicate are not part of the Viruses and Worms subclass. The main characteristic used to determine whether or not a program is classified as a separate behaviour within the Viruses and Worms subclass is how the program propagates (i.e. how the malicious program spreads copies of itself via local or network resources.) Most known worms are spread as files sent as email attachments, via a link to a web or FTP resource, via a link sent in an ICQ or IRC message, via P2P file sharing networks etc. Some worms spread as network packets; these directly penetrate the computer memory, and the worm code is then activated. Worms use the following techniques to penetrate remote computers and launch copies of themselves: social engineering (for example, an email message suggesting the user opens an attached file), exploiting network configuration errors (such as copying to a fully accessible disk), and exploiting loopholes in operating system and application security. Viruses can be divided in accordance with the method used to infect a computer:

• file viruses
• boot sector viruses
• macro viruses
• script viruses

Any program within this subclass can have additional Trojan functions. It should also be noted that many worms use more than one method in order to spread copies via networks.

Class: Virus

Viruses replicate on the resources of the local machine. Unlike worms, viruses do not use network services to propagate or penetrate other computers. A copy of a virus will reach remote computers only if the infected object is, for some reason unrelated to the virus function, activated on another computer. For example: when infecting accessible disks, a virus penetrates a file located on a network resource a virus copies itself to a removable storage device or infects a file on a removable device a user sends an email with an infected attachment.

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Platform: MSExcel

Microsoft Excel (MS Excel) is a popular spreadsheet program and part of the Microsoft Office productivity suite. Excel files have .xls or .xlsx extensions.

Description

Technical Details

This virus replicates itself by the same manner as other Excel viruses do. It hooks system events (window activating - OnWindows) and copies its code to each sheet that is activated. On first start (on first opening an infected sheet) the virus installs itself into the system: it registers its host file as Add-In with the XLSHEET.XLA name in the current or in the C:WINDOWS directory. On such request Excel automatically creates new copy of infected document (with XLSHEET.XLA name) and on each next Excel start it will load and activate this Add-In, i.e. virus code. As a result after creating infected Add-In the virus is active all the time Excel is run and infects all files that are opened or created.

The virus is of French origin (see routines names below) but is able to infect and replicate under any local Excel version starting from 4.

The virus has five routines: auto_ouvrir, activation_feuille, protect, !!!GO, auto_fermer. All of them (except !!!GO) call infection routine. Depending on the system random counter (with probability 2%) the virus activate the trigger subroutine (that is places in !!!GO routine). The trigger routine hides all opened tables and Excel elements (buttons, menus, status bar) and replaces the "Microsoft Excel" text at the top of the Excel window with the text: "Enfin la paix ..."

It is not possible to detect and disinfect the virus by using standard methods (entering Tools/Macro and looking for macros) because the virus sets VeryHidden attribute for its macros. Such attribute cannot be disabled by using Excel menus. To find and look at virus code that's necessary to write special routine on Excel Basic (macro routine).

As a result a user has no tools to detect this virus on its computer, and all known anti-virus programs are not able to detect it now. The virus can be found only by its traces:

- in Tools/Add-Ins menu there is reference for the XLSHEET file
- infected files contain the text strings:
Enfin la paix ...
!!!GO

Partial protection can be achieved by creating in the C:WINDOWS directory the Read-Only dummy file with the XLSHEET.XLA name. After that the virus will be not able to install its Add-In in the C:WINDOWS directory. If it creates this Add-In in other directories, you should also create the same dummy XLSHEET.XLA file in these directories.

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