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Shareware Documentation: Virus scanning - how, why and when?

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

F-PROT is able to find practically all known viruses, by a method known as "scanning". This involves searching for a virus pattern, or a "search string", a sequence of bytes which is very unlikely to be found anywhere but in this particular virus.

The search strings are stored in a file named SIGN.DEF, which must be present in the current directory or the same directory as F-PROT.EXE. The number of search strings contained in this file is not an indication of the number of viruses F-PROT is able to detect, however - as most new viruses are created by making small changes to older viruses, the same search string can often be used to detect many different viruses.

Secure Scan or Heuristic Analysis ?

F-PROT can use two different methods when scanning for viruses. The first method ("Secure Scan") uses two different search strings for each virus. It will also search in a large block of data - usually (but not always) located either at the beginning or the end of the file. This improves the chances of detecting any virus which might have been created by modifying an older one - any change might cause a search string to be located at a different position within the virus, or it might even corrupt the string itself, but the chances of a single change invalidating both of the strings are very low.

The second method first does a "Secure Scan", and then attempts to analyse the file, using a set of rules, instead of a database of search strings. It is still only experimental, but its purpose is to detect suspicious code. It is not foolproof - it will not detect all viruses and may easily produce false alarms, so it should be used with care - not recommended for the casual user. However - unlike the other method, it is not limited to existing viruses or variants of them - it is equally effective against new viruses. For further information on this method see "What is Heuristic Analysis?"

When you select "Scan" from the initial menu, a new menu will appear, where you can select what to scan for and where to scan.

To change the setup you simply use the arrow keys to move to the option you want to change and press Enter. A window will then appear showing the available possibilities, and you select one of them.

The first option, "Method" is uses to select which search method to use, with "Secure" as the default.

The second option, "Search" is used to select on which drives and directories F-PROT should search for viruses. The possibilities are "Hard disk", "Diskette drive" and "Network", which should be self-explanatory, and finally "User-specified". The last possibility applies if you only want to scan a single directory, or perhaps just a single file. If a directory is specified, all subdirectories below it will be searched as well. The difference between selecting "Diskette drive A:" and selecting "User-specified", and entering "A:" is that in the former case it is assumed you might want to scan multiple diskettes, so after scanning each diskette a report is given and you are prompted for the next diskette. One note: If "Network" is selected, all network drives from C: to Z: will be searched, so if several drive letters have been mapped to the same physical directory, the same files might be scanned several times. The default is to search the hard disk.

The third option, "Action" is used to specify what action should be taken when a virus is found. The default operation is just to list the names of any infected files, but F-PROT can also disinfect almost all viruses. If you want disinfection, it can either be fully automatic, or F-PROT can prompt you before it attempts to disinfect any given file. Sometimes an infection cannot be removed, for example if the virus just overwrites and destroys any file it infects, or in the case of a "first-generation" sample.

A "first-generation" sample is the author's original copy of the virus, and can only exist if the file has been obtained directly or indirectly from him. Such samples are generally not found in the "real world", only in large virus collections.

In those cases the only effective disinfection is to delete the file. It is always safer to delete infected programs than to disinfect, so F-PROT offers deletion as well - any infected file will first be overwritten several times (just to make sure) and then deleted. You can select automatic deletion or have F-PROT prompt you before it deletes a file. Finally, an infected file can be renamed, and given the extension .VOM or .VXE, so it will not be executed by accident, but you will still have it around to study.

The fourth option, "Targets" is used to select the types of viruses to search for. Normally one would like to search for all known viruses, but in certain circumstances you might want to exclude boot sector viruses or program viruses. For example, if you are cleaning up after an attack by a specific boot sector virus, you might not want to search for program viruses on every single diskette. You can also instruct F-PROT to search for special user-defined search strings, but this will slow the scanning down considerably.

The fifth option, "Files" is used to select in which files F-PROT should search for viruses. Most viruses will only infect normal executable files, (.EXE, .COM and possibly .APP and .PGM files) although some may infect overlay files (.OV?) and device driver files (.SYS) as well. The default operation of F-PROT is just to scan those types of files, but it is also possible to select "All files" - this is advisable if you are cleaning up after a virus attack - just to make sure the virus is not hiding in some obscure overlay file. However, as this is quite time-consuming, it is not recommended, unless you have actually found a virus when scanning just the regular executable files. It is also possible to specify a set of file extensions - for example adding .BIN to the default list.

If any of the options are changed from their default values, F-PROT will ask if the changed values should be saved when you exit from the program. If so, a file named SETUP.F2 will be created. This does not work if the program is run from a write-protected diskette, however.

Starting the virus scan

When you have selected the correct options, you may start the scanning by selecting "Begin Scan" at the top of the menu, either by moving the cursor there, or just by pressing "B".

The small window at the bottom will display the name of the last file scanned.

The scanning can be aborted at any time simply by pressing the ESC key.

When the scanning is finished, a summary is displayed. If no viruses or suspicious programs were found, it simply says so, but otherwise a detailed listing is produced when ENTER is pressed. This listing can be saved to a disk or sent to the printer.

This report may say that a file has been packed by a program such as KVETCH, PGMPAK, SHRINK or CRUNCH and can not be scanned. This is generally not a cause for alarm, although a virus can be hidden in a program by infecting it, and then running one of those file-packing programs, which create a program which will unpack itself in memory when executed. Some virus writers use this method to distribute their viruses, but generally this only works for the first generation - second (and later) generation samples of the same virus will not be packed. F-PROT can scan inside most PKLITE, LZEXE, ICE, DIET and EXEPACK compressed files, and support for the remaining compression program will be added in the near future, if necessary. Please keep in mind that if a file is infected after compression, the virus is always detected normally. Finally, F-PROT will not scan inside self-unpacking archives, or .ZIP, .ARJ or similar files. We will add this feature in the future, but currently you have to unpack those files and scan the individual files.

A note on disinfection

When a file has been disinfected it has usually been restored to its original state before infection. In many cases the disinfected program will have 1-16 additional garbage bytes at the end. Those bytes are added by viruses, in order to make the length of the program a multiple of 16 bytes, before infection. As the number of those extra bytes cannot be determined, they cannot be removed. Normally they will not have any effect, unless the program checks its current length. In those cases it will report an incorrect length after disinfection, and will have to be restored from a backup.

Skipping the memory scan

Normally F-PROT will search the memory for viruses, and refuse to operate if any search strings are found in memory. However, a false alarm is possible, for example if an infected file has just been copied, and portions of it are in an unused disk buffer. A false positive can also happen if you have run another, incompatible anti-virus program before, that does not encrypt its search strings. Most anti-virus programs are well-behaved in this respect, and only MSAV and CPAV cause this problem regularly.

To skip the memory scan, run the program with the /NOMEM command-line switch.

Testing the scanner

The correct operation of F-PROT can be tested with a special test file. This is a dummy file which is detected by F-PROT exactly like if it were a virus. This file is known as EICAR Standard Anti-virus Test file, and it is also detected by several other anti-virus products in a similar manner. (EICAR is the European Institute of Computer Anti-virus Research).

Naturally, the file is not a virus. When executed, EICAR.COM will display the text 'EICAR-STANDARD-ANTIVIRUS-TEST-FILE' and exit.

We do not include the EICAR test file with the package to avoid alarming anyone running F-PROT or another scanner on the package, but to create the EICAR test file, use any text editor to create a file with the following single line in it:


Save the file to any name with COM extension, for example EICAR.COM. Make sure you save the file in standard MS-DOS ASCII format. The file should be 68 bytes long, but might be 70 bytes if the editor puts a CR/LF at the end. Now you can use this file to test what happens when F-PROT enconters a "real" virus.