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- AREA:COMP.VIRUS
- From: p1@arkham.wimsey.bc.ca (Rob Slade)
- Newsgroups: comp.virus
- Subject: Viral code association
-
- FUNPIV4.CVP 911020
-
- Viral code "association"
-
- The simplest way for a viral program to avoid the detection that
- results from modifying the code of an existing program is not to
- modify the original program. This is an elementary solution,
- but would seem to have the drawback that, unless you do change
- the file in some way, the virus will never be called.
-
- There is a "solution" to this problem, and (if I may be allowed
- some enthusiasm for the concept, if not the reprehensible act) a
- rather elegant one at that.
-
- In a given situation, computers may be presented with a number
- of possible courses of action. The action taken first is
- decided by pre-programmed precedence. A number of programs may
- have very similar names, leading to potential confusion about
- which one is to be run in a given invocation. In the case of
- MS-DOS, for example, SET.COM, SET.EXE and SET.BAT are all
- "executable" files. In the normal course of events, any one
- could be invoked by giving the command "SET". If all three
- files exist, which one is to be run?
-
- The precedence of program invocation under MS-DOS is that .COM
- files are first, .EXE second and .BAT last. If three files of
- the same name do exist, this does not imply that all three will
- be run in that sequence, but rather that giving the command
- "SET" will always invoke only the SET.COM file.
-
- A certain class of viral programs; known variously as
- "companion", "spawning" or "precedence" viri; use this feature
- of the operating system. They "infect" a file with an .EXE
- extension simply by creating another file with the same name,
- but a .COM extension. Thus the .COM file is always executed in
- place of the original .EXE file. The original file remains
- unchanged, and no manner of "change detection" will tell you any
- different. (In order to further avoid detection the viral file
- will generally end with a very specific "call" to the original
- program, and the viral program has the "hidden" attribute set.
- In the Macintosh and other GUI operating systems, it is possible
- for a virus to take precendence by "overlaying" an existing icon
- with another which is either transparent or identical to the
- first.)
-
- Fortunately, companion viri are by no means perfect. For one
- thing, they are limited to those programs which are "lower" in
- the order of precedence. For another, the "hidden" attribute is
- relatively easy to overcome (particularly in MS-DOS), and an
- alphabetical listing of files will quickly turn up the anomaly
- of identical names. Of the antiviral packages tested so far, no
- change detector alerts to duplicate names, although many may
- alert the user by asking the user to "validate" a file that has
- been in use for some time. It will probably not be long,
- however, before this is a common feature.
-
- copyright Robert M. Slade, 1991 FUNPIV4.CVP 911020
-
-
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