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Retroviruses - How viruses fight back

Mikko Hyppönen
Virus Bulletin Conference, VDAT

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"The GoldBug virus has extensive anti-anti-virus routines. It can install itself while several resident anti-virus monitors are running. It will prohibit most popular anti- virus programs from running, and will also by-pass several integrity checking programs" - from the original source code of the GoldBug virus


This paper will discuss the methods viruses use or might use in the future to attack anti-virus programs. Attacks of this kind are becoming more common, as virus writers seem to be constantly looking for ways to make their viruses more efficient and vigorous. This paper also suggests how to make anti-virus products more resistant to such attacks. The scope of this paper is limited to PC-compatible machines.


There is a constant battle going on between computer virus authors and virus fighters. virus writers are looking for ways to create more complicated, more difficult-to-analyse and more inconspicuous viruses. At the same time, anti-virus people are building methods to address these threats.

It's not surprising that virus authors have realised that anti-virus tools are one of their creations' worst enemies. The logical step for them has been to make ...their viruses fight back, either directly or indirectly..

Several viruses explicitly target anti-virus programs. The attack routines may be generic or targeted against a specific program. Many virus authors obviously consider an attack to be the best defence, when the objective is to keep the virus alive in order to spread it as widely as possible.

There is a battle going on in computer systems world-wide - it's survival of the fittest, one might say. Hopefully, this paper will provide some ideas how to make anti-virus applications fitter than viruses.

A virus that fights back

For the purposes of this paper, a retrovirus is defined as follows:

Retrovirus is a computer virus that specifically tries to by-pass or hinder the operation of an anti-virus program or programs. The attack may be specific to a known product or a generic one.

Retroviruses are sometimes known as anti-anti-viruses. Anti-anti-viruses should not be confused with anti-virus-viruses, which are viruses that will disable or disinfect other viruses. To avoid confusion, the term retrovirus will be used here.

The creation of a virus which incorporates retro-routines is not necessarily a difficult task. In most cases, virus writers have access to the anti-virus programs they want to by-pass. All they need to do is experiment by trial and error until they find a way to attack the anti-virus program in a way the anti-virus developer has not foreseen.

Some virus authors have gone all the way and disassembled the offending anti-virus programs in order to find the most effective way to attack them. They often look for methods to attack a product in a way that would be most difficult to circumvent in future versions of the product.

As the virus authors are pretty efficiently connected to each other via different types of electronic networks, information on how to attack specific products spreads quickly.

It should be noted that virus writers typically have access to only those anti-virus products that are available as freeware or shareware. Some virus exchange BBS systems are known to make pirated copies of commercial products available, but the shareware products seem to be targeted most often.

It can be expected that more retroviruses, using more advanced retro-routines, will be seen in the future.

Rules of the game

Viruses using retro-routines started to show up during late 1980's - before that, there was no point in creating retroviruses, as anti-virus products weren't widely used. As the popularity of anti-virus programs has grown, so has the number of viruses that attempt to subvert them in some way.

Several approaches are possible, including:

The basic principle is that the virus must somehow hinder the operation of an anti-virus program in such a way that the virus itself benefits from it.

Methods like encryption, stealth, polymorphic routines, code armouring, anti-debugging tricks and confusion code can also be considered attacks against anti-virus programs. However, they are often generic in type and therefore outside the scope of this paper.

Attacks against non-resident scanners

Non-resident scanners are probably the most commonly used anti-viral products. They are also the favourite target of real-world retroviruses.

There are several different ways a scanner can be attacked against.

Deletion and replacement

A virus can locate the anti-virus program and delete it. A more sophisticated attack would be a modification or a patch that would alter the operation of the scanner in a way that would be beneficial to the virus. A virus could locate the search strings used by the scanner and overwrite them, making the scanner unable to find any virus, but still appear to be functional.

A virus can replace the scanner program with a Trojan horse which could trigger a damage routine when run or just simply display an error message and abort. Such an error message would also make the scanning product look bad in the eyes of the users, especially if the error message would be something like 'only 620kB of free DOS memory, unable to run' or 'BRUN30 GW-Basic run-time library not found, aborting'.

If the virus stays resident in memory, it can do similar attacks when it sees that an anti-virus program is executed. It can also by-pass a self-check routine of an anti-virus program by patching it only after the application has finished the check on its own code.

Modification of parameters

There is at least one known case of a virus that modifies the command-line parameters when it sees a specific anti-virus program to be started (see below). This technique allows the virus to modify the operation of the scanner to its advantage without patching the actual program code.

A similar attack in which the virus modifies the configuration file of an anti-virus program might also be possible - these files are often left unencrypted and are not checked for such modifications.

Altering the output

If the visual interface of the anti-virus program isn't complex (ie. command-line driven), it might be feasible for a retro-virus to mimic the operation of the program. This way, the user might not notice anything strange.

A variation of the theme would be that the virus would patch the texts displayed by the product. If the text string 'Virus found!' were to be changed to 'All clear!', a typical user wouldn't probably doubt anything.

In many installations, anti-virus programs are run automatically and the alarms are set off depending on the exit codes (errorlevels) returned by a program. A successful attack on such a system might consist of a retrovirus that would always set the return-code of an anti-virus program to zero.

False false alarms

Scanners are prone to false alarms ie. detecting a virus in a clean file. Viruses can use this as one way to attack. If a virus incorporates code sections from popular applications, it is quite possible that an anti-virus vendor without a proper false-positive testing routine might include a search string that would cause a large amount of false positives.

One way to implement this kind of an attack would be to include an encryption routine to a virus, but borrow the decryption code from some known application - the encryption would limit the traditional search strings to only strings that would cause false positives, and this in itself would cause problems for some scanning products.

Problems with packed files

Several scanners are able to scan inside compressed executables that have been packed with some of the most popular EXE-packers. Some scanners do not scan packed files at all, but only flag them as packed so the user is aware of them. This provides one way a virus could cause problems for a scanner. If a virus used a section of fake code that would make an infected program look like it had been packed, it could by-pass the scanning by such a product completely. The virus could also replicate in packed form, making it even more difficult for some scanners to detect.

A similar attack might be possible against products that actually unpack the programs and scan underneath the packing. In order to uncompress the program, the scanner fetches program info from the unpacking code. If this code contained irrational values, it could cause some scanners to crash or run out of memory.

One man's data is another man's code

Almost all scanners default to scanning only the executable files instead of all files. File type is usually determined by the extension (ie. COM, EXE, SYS).

Since a virus can control the system in any way it wants, one way to by-pass a scanner would be to change the file extensions of all infected files to non-executable ones, for example from EXE to XEX. While the virus is resident in memory, it can use stealth techniques to hide this change - but it will also make sure that all executables copied to floppies have the valid extension, to ensure that the virus gets a chance to spread. The advantage of such a method is that even if the machine is booted up from a clean diskette and all executables are scanned with a scanner that can detect the virus, it will only be found in the initial carrier file.

Exploitation of technical limits

A virus writer could analyse in detail how a scanner actually does the scanning and develop infection methods that cause detection problems for a specific scanner. The virus doesn't have to be difficult to find - it is enough that it is very slow to search for.

The Command Bomber virus is an example of this: it inserts its code in the middle of the host file and builds a complicated series of branching commands to transfer the flow of the program code to the actual code. The detection of such virus would force some scanners to scan the whole file from the beginning to the end - which would be enough to make them unusably slow.

Attacks against resident scanners and behaviour blockers

Resident anti-virus programs are vulnerable to special attacks. Since DOS does not provide any kind of memory protection, a program can modify the memory space of another program. This makes it possible for a virus to locate and patch or disable a resident scanner or a behaviour blocker.

Unloading the protection

Some anti-virus TSRs can be unloaded from memory (actually, they will have to be unloadable if the product is wanted to be Novell-certified). If such mechanisms exist, they can also be called by a virus. Viruses use this method quite successfully with some products for which it is known to work.

Through the back door

Practically every TSR scanner has a back door, which is used by the non-resident scanner of the same package. This back door either turns off the checking done by the TSR or provides an alternative access method to the file system. If such a back door did not exist, the TSR part would clash with the normal scanner, as the TSR would notice an infection when the non-resident part would open an infected file for scanning.

A virus can use such back doors for its own benefit, either disabling the resident part or by using the clean path to file system provided by the TSR.

Yet another way for a virus to attack a resident scanner is to observe the display routines, and trap the alarm messages displayed by the TSR. If the user never sees the alarm messages of the TSR, the protection is not doing its job.

Attacks against disinfectors

A retrovirus can attack programs that try to disinfect boot sectors and files. The purpose of such an attack might be to cause the disinfector to damage the host files while disinfecting. If a disinfection program does not do an exact identification on a virus before disinfecting it, any virus that contains a known search string for another virus can cause such damage during the disinfection process.

Cleaning the clean

There even exists a virus called Mirror, which is the exact opposite of a stealth-virus: when Mirror is resident in memory, it makes all programs look like they have been infected by it. This can be potentially dangerous when disinfection is attempted, but this technique poses no danger if the disinfection is done in a proper way, ie. after a clean boot.

Complicating the recovery

The recovery process of an infected machine can be severely complicated if the virus denies access to the hard drive. Several MBR-viruses (for example, members of the Monkey family) do this by modifying the partition data in such a way that no logical DOS drives can be found when the machine is booted from a clean floppy. A recovery attempt done by overwriting the MBR code with the FDISK /MBR or a similar command will not return access to the hard drive.

The ExeBug virus family uses another way to make it difficult to boot up an infected machine from a clean diskette. The virus modifies the BIOS Setup information to indicate that the machine does not have A: drive at all. Such machine will always boot up from the hard drive. Once the booting has started and the virus code is executed, the virus will check if there is a diskette in drive A:. If so, it will continue the booting from there. In most cases the user is unable to notice this, and thinks that the machine has been booted clean when the virus is already resident.

Yet another way to complicate the recovery process is to set the BIOS boot-up password on with a random password during an activation routine. The method of doing this is documented on most new BIOS brands.

Some integrity checkers are capable of performing a generic disinfection. This means that they try to restore the original file according to the information the checker has previously saved (typically length, checksum, first and last bytes). Such generic routines won't work if a virus makes extensive changes to the program files, for example by encrypting the host file during infection.

Attacking heuristic cleaners

Viruses use a different kind of an attack against heuristic disinfection programs. A heuristic cleaner works by loading the infected file to memory and emulating the program code. It uses a combination of disassembly, emulation and sometimes execution to trace the flow of the virus and to emulate what the virus is normally doing. When the virus restores the original first instructions of the host file and jumps back to the original entry point, the cleaner stops the emulation. The repaired start of the program is copied back to the program file on disk, and the part of the program that was 'executed' will be removed. [Veldman]

The inherent risk of heuristic cleaning is that if the cleaner tries to emulate everything, the virus may assume control inside the emulated environment and finally escape from it - after which it can propagate further or trigger a destructive retaliation routine. There are documented cases of at least one virus doing this, see below.

Attacks against integrity checkers

The operation of integrity checking programs varies between vendors but they almost always rely upon some form of a database which contains details of objects (typically files and boot sectors) to be checked.

Deleting the database

Several viruses have attacked integrity checkers by locating the integrity database and deleting it. In some cases, the result of deleting the database files is that the integrity checker will blindly assume that the original checksums have not been calculated yet, and proceeds to initialise the database without informing the user that something might be amiss. This was exactly the case with the Peach virus.

Peach attacked an integrity checker which worked by creating a checksum file, containing checksums of all executable programs. Peach attacked by deleting this file. After the database was deleted and the checker was executed again, it recreated the file, calculating new checksums from the infected files and failing to report any changes in the system.

It should be noted that the Peach virus will not be successful against newer versions of this integrity checker, as the name of the checksum file has been changed in newer versions of the product. Similar types of attack still seem to be possible, though.

Even if a checksumming package did report to the user that the database has been deleted without approval, it would be difficult to find the affected files if no recent backup of the database exists.

Making checked unchecked

A similar attack works also against programs that do not store the integrity data in a separate database, but add it to the end of the executable files themselves. Since there is no info about which files have been checksummed, a virus can just remove the validation data without any side effects - and the checker will not complain that the file has changed.

Several generic attack methods against integrity checkers are discussed in length in.

Real world retroviruses

When we look at viruses that attack specific anti-virus products directly, we notice that they mostly seem to target McAfee Associate's ViruScan (SCAN.EXE), Microsoft Anti-virus from MS-DOS 6 (MSAV.EXE), Central Point Antivirus (CPAV.EXE) and the resident parts of these applications (VSHIELD and VSAFE). This is not surprising, as these are some of the most popular anti-virus products, and thus good targets for retroviruses.

Here are some examples of known viruses that incorporate retro-routines:

CPW virus family:

tries to delete programs called TOOLKIT, GUARD, CHKVIRUS, SCAN, CLEAN, CPAV and VSAFE

deletes CHKLIST.CPS files created by CPAV


deletes CHKLIST.CPS files

removes the validation information added by SCAN and CPAV


uninstalls VSAFE from CPAV or MSAV

contains a segment of nested loops to confuse F-PROT's heuristic scanning




deletes or stops the execution of programs called SCAN, CLEAN, NETSCAN, CPAV, MSAV, TNTAV - and deletes the contents of CMOS memory at the same time

specifically by-passes the TBAV boot-sector check

deletes CHKLIST.* files, by-passing CPAV and MSAV


disables TBDriver from TBAV by patching it in memory

when TBScan is executed, adds the command-line parameter 'co', which will allow the stealth routines of the virus to operate

patches text strings inside TBScan's code to make the operation of the program look like it has been started without the 'co' switch

Lockjaw virus family:

deletes F-PROT, SCAN, IM, CPAV

uninstalls VSAFE

MtE.Groove and MtE.Encroacher:

tries to delete files belonging to the following products: Central Point Anti-Virus, Certus Novi, Fifth Generation Systems Untouchable, Norton Anti-Virus, Dr. Solomon's Antivirus Toolkit and VDS Virus Secure.


overwrites the first 256 sectors of first hard disk whenever SCAN is run


deletes CHKLIST.CPS files


tries to delete files belonging to CPAV, NAV, Untouchable, Dr. Solomon's Antivirus Toolkit and Integrity Master

will not infect if FluShot is installed


tries to remove the validation codes added by SCAN

guards its own are-you-there interrupt call to make it difficult to detect the virus in memory with it [CM-Base]


deletes files that have validation codes added by SCAN

does not infect EXE-files which have the letters SC or V in their names


hooks INT 13h via a VSAFE back-door

modifies its own memory allocation when F-PROT is executed [VB2]


tries to escape and go resident during the cleaning process of TBClean

Is there a real problem with retroviruses?

Do retroviruses pose a realistic threat to current anti- virus products? The most popular anti-virus tool nowadays is a stand-alone scanner, which by itself is almost always helpless against any new virus. Are there any special risks in a virus that, in addition to being a new one, also specifically tries to by-pass a product?

Dangers of optimised virus analysis systems

If a retrovirus exploits a specific flaw or the back door of a product, it cannot be considered a very special case, as the detection of a new virus requires usually an update to the product anyway. At the same time, it is possible to upgrade the product so that the attack method used by the virus can be circumvented or made obsolete.

The main problem in this case is whether the anti-virus vendor notices what the virus is trying to do. Today, when several new viruses are found every day, there is a limited time in which to analyse any single virus. Virus analysis systems are automated as much as possible, and a virus typically only gets a cursory look - which is usually enough to add detection, identification and disinfection. Such ana- lysis will not reveal any special features the virus may contain. This also explains why there are no anti-virus products which can provide detailed information about each and every virus.

If a retrovirus is run through a standard analysis system, and the product is tested by running it against a sample that is not resident in memory, the retro-features of a virus may not become known until they are observed directly in the real world - after which the virus will certainly get more attention, but this might already be a bit too late. The virus may also start its attack behaviours only after a certain latency time.

Opening the door to other viruses

It should also be noted that a virus which disables an anti- virus product in some way may also make the system vulnerable to other viruses, which the product might otherwise have handled fine.

In many cases this is the only benefit a retrovirus gains from unloading a resident scanner. The scanner can't be unloaded before it is resident. If the virus is known to the scanning engine, a resident scanner will not let the virus run. If the virus is unknown to the scanner, it can operate even when the scanner is resident. The case is different with behaviour blockers, as they are not trying to find known viruses.

There is very little a product can do against an attack which consists of deleting or replacing the program file itself - if the virus gets control before the anti-virus, the virus makes the rules.

How should an anti-virus product protect itself?

It is obvious that viruses can utilise a variety of tricks against anti-virus products. However, anti-virus programs can fight back just as efficiently.

Making the program difficult to locate

First of all, the anti-virus program itself should be renameable by the user. This alone would make it a lot harder for a virus to locate its enemy. Unfortunately, many anti-virus products refuse to run if they find that their program files have been renamed.

As the virus can try to locate the anti-virus program by its contents as well as by name, the structure or contents of the program file should change with each update.

The best way to make sure that no retrovirus is making its tricks is the old, well-known recipe: boot from a clean diskette and run a fresh copy of the anti-virus program from diskette.


Since many attack routines work by modifying an anti-virus program, it is imperative that all anti-virus programs make thorough checks on their own code. A cursory check against modifications that would result from an infection is not enough: if the code is not protected internally against patching, the integrity of the whole program code should be checked during start-up.

It is not enough to ensure that the program code has not been changed. As demonstrated earlier in this paper, it is enough for a retrovirus to modify the texts or configuration info belonging to the application.

Even though the size of an anti-virus application probably changes during every update, a clever retro-virus can still locate the code it wants to patch by using a search string. This can be overcame by encrypting the application. The protection will be even better if the encryption method or key is changed with every update. Another, easier way to achieve the same results is to provide the executable in packed form, as the packing algorithm will invalidate search strings between different versions of the same program.

Resident security

Since it is often much easier to patch a program in memory rather than on disk, an anti-virus application should make checksum checks on its memory image to ensure that no unwanted changes have taken place. This is especially important with resident anti-virus utilities.

The communication channels to a resident part of an anti- virus program should be carefully thought out. If the TSR needs to have an uninstallation routine, it should be implemented so that other programs will find it difficult to request the uninstallation without the user noticing it.

Prohibiting disassembly

It can be expected that determined virus writers will try to disassemble anti-virus products in order to find out what makes them tick. Thus, some anti-debug and armouring code to protect the application might be a good idea - although nothing will stop a dedicated cracker.

At least three different scanners are known to have been analysed by crackers, up to the point of extracting all search strings of the program. Such attack can be harmful in several ways: the virus writers get to see exactly what they will have to change in a virus to make a new, undetectable variant, and well-chosen search strings are also closely guarded trade secrets.

Popular, easy-to-get programs are the most probable targets for attack routines. This makes commercial products theoretically more safe than shareware or freeware products.


Retroviruses are nothing new - the first ones were found in the late 1980's. There are several attack methods that will certainly be used in future viruses - and some of these can be quite efficient. Therefore, extreme care should be taken by producers of anti-virus software to avoid the possible pitfalls.

It's time to make sure your anti-virus product is not vulnerable to an attack it could avoid.

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