Throwback Thursday: Macro Viruses - Part 3 (November 1999)

2015-06-18

Igor Muttik

AVERT Labs, UK
Editor: Martijn Grooten

Abstract

Everything you ever wanted to know about macro viruses but were afraid to ask! In this part: mating, devolving, naming and prevalence.


Table of contents

(This article was first published in Virus Bulletin in November 1999.)

When different macro viruses meet on one user system they may mate. WordBasic copies macros by name. If two viruses have the same macro name one virus may copy a macro belonging to another virus. Then this cocktail may be able to travel with one (or several) macros substituted. Such mated viruses do exist and they replicate happily with the macros ‘borrowed’ from other macro viruses.

Viruses can also snatch macros from a set of legitimate macros in NORMAL.DOT. For example, there are many known macro viruses which are the result of mating between the ScanProt macro (the anti-Concept macro released by Microsoft) and this or that other macro virus.

VBA5/6 viruses also are able to mate. Apart from just two sets of macros being present in one document, they now can merge inside a single module. Most contemporary viruses live in the class module called ‘ThisDocument’ (or ‘ThisWorkbook’ for Excel 97 or Excel 2000). If two viruses using the same class module infect one DOC file they can:

  1. stop working if they use the same functions (e.g. two functions for ‘Document_Open’ in one module produce a VBA error)

  2. live happily together (e.g. one infects on ‘Document_Open’, another on ‘Document_Close’) and spread together, one attached to another

  3. produce a mixture, the behaviour of which would depend on which virus’ function is used to replicate the cocktail. Such behaviour can be very complex depending on the history – it may devolve to nonreplicating samples, lose some modules or functions, etc.

Devolving

Some viruses are badly written and can lose their own macros. For example, the original virus consists of a set: {AutoOpen, FileSave, and FileSaveAs}. If it replicates via AutoOpen the whole macro set is preserved, but if the user invokes FileSaveAs the virus fails to copy FileSave.

The resulting virus – {AutoOpen, FileSaveAs} – is called a devolved macro virus (of course, only if this reduced set is able to replicate recursively, i.e. we have a ‘viable devolved virus’) and the original virus is known as devolving. A virus can devolve more then once (losing different macros) resulting in many different variants. Such variants are distinguished by attaching a digit to the name, e.g. WM/Rapi.A and WM/Rapi.A1 (the WM/Rapi family is famous for having several devolved variants).

In some cases a devolved virus no longer works and we get a ‘non-viable devolved virus’. These do not replicate but anti-virus programs should still be able to detect and clean them as they occur as a result of a viral activity.

Naming

There is an email group called VMacro consisting of the most active anti-virus researchers in the field. They share the identification data (not the virus samples – they are sent more carefully within CARO), discussing the family relationship of macro viruses, their names and other issues related to macro viruses.

It was decided that names of macro viruses start with a platform identifier – WM (Word Macro for viruses using WordBasic), XM (Excel Macro for VBA3), APM (AmiPro Macro), A97M (Access 97 Macro), W97M (Word 97 Macro), X97M (Excel 97 Macro), PP97M (PowerPoint 97), CSC (CorelDraw Script).

Then, the family name (e.g. Wazzu) goes after the slash separator, followed by a dot and a variant suffix (which can be omitted). Variant suffixes start at .A and go through to .Z, then start again at .AA to .AZ, etc. If the virus devolves the index is attached to every variant. For viruses which infect all Office 97 applications an O97M prefix can be used or multiple prefixes can be grouped in curly brackets {W97M/X97M}. This also applies to multi-partite infectors hitting, say, DOCs and EXEs {W97M/Win95}:

  • WM/Wazzu.A

  • WM/Concept.A

  • WM/Npad.BV

  • APM/GreenStripe

  • WM/Rapi.E2

  • XM/Laroux.B

  • W97M/Appder.B

  • X97M/Laroux.JH

  • O97M/Tristate.A

  • {W97M/X97M}/Shiver.A

  • {W97M/Win95}/Coke.22231.A

  • A97M/AccessiV

  • CSC/CSV

If the virus is language-specific (e.g. it replicates only under a localized version of WinWord) the virus name can be followed by a country designator. Internet abbreviations are used, such as ‘:De’ (for Germany) or ‘:It’ (for Italy).

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