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Shareware Documentation: Heuristic analysis

Information provided courtesy of Frisk's shareware F-PROT documentation, version 2.25

Scanning for viruses by using search strings is not the ultimate solution to the virus problem. If using an up-to-date scanner (or better yet, two scanners from different companies), one can be fairly certain that all known viruses will be detected. The scanners may or may not detect new variants which have been created by modifying older viruses, but if a new virus is written entirely from scratch, it will probably not be detected by any existing search string.

The virus may be detected by a generic monitoring program when it activates - perhaps when trying to perform some suspicious action, such as reformatting the hard disk. It may also be detected by a checksuming program, which detects changes to files or boot sectors, after they have been infected. Nevertheless, it is preferable to try to detect the presence of the virus without actually running a virus-infected program.

The heuristic analysis is basically a small expert-system, which has a set of rules describing viruses, and attempts to apply them to the programs it analyses. It is still only in an experimental stage and is not flawless - some viruses cannot yet be detected in this way, and an occasional false alarm is to be expected.

Currently a few programs are known to cause a false positive, including:

  • Any program protected with the HyperLOCK encryptor ... not surprising, considering that it claims that "Attemping to reverse engineer this software may result in data loss".

  • Any program protected with the PROTECT encryptor.

  • A few heavily "armored" programs, that use multiple layers of anti-debugging techniques. XTG.EXE (Xtree Gold) is one example.


  • Some files belonging to Central-Point's PC-Tools.

In addition, if the heuristics report a non-executable file as suspicious, it is almost certainly a false alarm - the heuristics are only intended to analyse executable files, and will not produce meaningful results if used on a data files. You should NEVER use heuristics with "All files" selected, and doing so will produce a warning message.

Experienced users may wish to run the heuristic scan with the /PARANOID switch on - HOWEVER, THIS WILL SIGNIFICANTLY INCREASE THE RISK OF FALSE ALARMS, so use it with care.

We are not interested in receiving copies of false alarms that are only reported with /PARANOID, so don't waste your time sending them to us.