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9.5.4 Antivirus and Anti-Antivirus Techniques

Viruses try to hide and users try to find them, which leads to a cat-and-mouse game. Let us now look at some of the issues here. To avoid showing up in directory listings, a companion virus, source code virus, or other file that should not be there can turn on the HIDDEN bit in Windows or use a file name beginning with the . character in UNIX. More sophisticated is to modify Windows' explorer or UNIX' ls to refrain from listing files whose names begin with Virgil-. Viruses can also hide in unusual and unsuspected places, such as the bad sector list on the disk or the Windows registry (an in-memory database available for programs to store uninterpreted strings). The flash ROM used to hold the BIOS and the CMOS memory are also possibilities although the former is hard to write and the latter is quite small. And, of course, the main workhorse of the virus world is infecting executable files and documents on the hard disk.

Virus Scanners

Clearly, the average garden-variety user is not going to find many viruses that do their best to hide, so a market has developed for antivirus software. Below we will discuss how this software works. Antivirus software companies have laboratories in which dedicated scientists work long hours tracking down and understanding new viruses. The first step is to have the virus infect a program that does nothing, often called a goat file, to get a copy of the virus in its purest form. The next step is to make an exact listing of the virus' code and enter it into the database of known viruses. Companies compete on the size of their databases. Inventing new viruses just to pump up your database is not considered sporting.

Once an antivirus program is installed on a customer's machine, the first thing it does is scan every executable file on the disk looking for any of the viruses in the database of known viruses. Most antivirus companies have a Web site from which customers can download the descriptions of newly-discovered viruses into their databases. If the user has 10,000 files and the database has 10,000 viruses, some clever programming is needed to make it go fast, of course.

Since minor variants of known viruses pop up all the time, a fuzzy search is needed, so a 3-byte change to a virus does not let it escape detection. However, fuzzy searches are not only slower than exact searches, but they may turn up false alarms, that is, warnings about legitimate files that happen to contain some code vaguely similar to a virus reported in Pakistan 7 years ago. What is the user supposed to do with the message:

WARNING! File xyz.exe may contain the lahore-9x virus. Delete?

The more viruses in the database and the broader the criteria for declaring a hit, the more false alarms there will be. If there are too many, the user will give up in disgust. But if the virus scanner insists on a very close match, it may miss some modified viruses. Getting it right is a delicate heuristic balance. Ideally, the lab should try to identify some core code in the virus that is not likely to change and use this as the virus signature to scan for.

Just because the disk was declared virus free last week does not mean that it still is, so the virus scanner has to be run frequently. Because scanning is slow, it is more efficient to check only those files that have been changed since the date of the last scan. The trouble is, a clever virus will reset the date of an infected file to its original date to avoid detection. The antivirus program's response to that is to check the date the enclosing directory was last changed. The virus' response to that is to reset the directory's date as well. This is the start of the cat-and-mouse game alluded to above.

Another way for the antivirus program to detect file infection is to record and store on the disk the lengths of all files. If a file has grown since the last check, it might be infected, as shown in Fig. 9-4(a-b). However, a clever virus can avoid detection by compressing the program and padding out the file to its original length. To make this scheme work, the virus must contain both compression and decompression procedures, as shown in Fig. 9-4(c). Another way for the virus to try to escape detection is to make sure its representation on the disk does not look at all like its representation in the antivirus software's database. One way to achieve this goal is to encrypt itself with a different key for each file infected. Before making a new copy, the virus generates a random 32-bit encryption key, for example by XORing the current time with the contents of, say, memory words 72,008 and 319,992. It then XORs its code with this key, word by word to produce the encrypted virus stored in the infected file, as illustrated in Fig. 9-4(d).

Figure 9-4 (a) A program. (b) An infected program. (c) A compressed infected program. (d) An encrypted virus. (e) A compressed virus with encrypted compression code.

The key is stored in the file. For secrecy purposes, putting the key in the file is not ideal, but the goal here is to foil the virus scanner, not prevent the dedicated scientists at the antivirus lab from reverse engineering the code. Of course, to run, the virus has to first decrypt itself, so it needs a decrypting procedure in the file as well.

This scheme is still not perfect because the compression, decompression, encryption, and decryption procedures are the same in all copies, so the antivirus program can just use them as the virus signature to scan for. Hiding the compression, decompression, and encryption procedures is easy: they are just encrypted along with the rest of the virus, as shown in Fig. 9-4(e). The decryption code cannot be encrypted, however. It has to actually execute on the hardware to decrypt the rest of the virus so it must be present in plaintext form. Antivirus programs know this, so they hunt for the decryption procedure.

However, Virgil enjoys having the last word, so he proceeds as follows. Suppose that the decryption procedure needs to perform the calculation

     X = (A + B + C - 4) 

The straightforward assembly code for this calculation for a generic two-address computer is shown in Fig. 9-5(a). The first address is the source; the second is the destination, so MOV A,R1 moves the variable A to the register R1. The code in Fig. 9-5(b) does the same thing, only less efficiently due to the NOP (no operation) instructions interspersed with the real code.

But we are not done yet. It is also possible to disguise the decryption code.

Figure 9-5 Examples of a polymorphic virus.

There are many ways to represent NOP. For example, adding 0 to a register, ORing it with itself, shifting it left 0 bits, and jumping to the next instruction all do nothing. Thus the program of Fig. 9-5(c) is functionally the same as the one of Fig. 9-5(a). When copying itself, the virus could use Fig. 9-5(c) instead of Fig. 9-5(a) and still work later when executed. A virus that mutates on each copy is called a polymorphic virus.

Now suppose that R5 is not needed during this piece of the code. Then Fig. 9-5(d) is also equivalent to Fig. 9-5(a). Finally, in many cases it is possible to swap instructions without changing what the program does, so we end up with Fig. 9-5(e) as another code fragment that is logically equivalent to Fig. 9-5(a). A piece of code that can mutate a sequence of machine instructions without changing its functionality is called a mutation engine, and sophisticated viruses contain them to mutate the decryptor from copy to copy. The mutation engine itself can be hidden by encrypting it along with the body of the virus.

Asking the poor antivirus software to realize that Fig. 9-5(a) through Fig. 9-5(e) are all functionally equivalent is asking a lot, especially if the mutation engine has many tricks up its sleeve. The antivirus software can analyze the code to see what it does, and it can even try to simulate the operation of the code, but remember it may have thousands of viruses and thousands of files to analyze so it does not have much time per test or it will run horribly slowly.

As an aside, the store into the variable Y was thrown in just to make it harder to detect the fact that the code related to R5 is dead code, that is, does not do anything. If other code fragments read and write Y, the code will look perfectly legitimate. A well-written mutation engine that generates good polymorphic code can give antivirus software writers nightmares. The only bright side is that such an engine is hard to write, so Virgil's friends all use his code, which means there are not so many different ones in circulation—yet.

So far we have talked about just trying to recognize viruses in infected executable files. In addition, the antivirus scanner has to check the MBR, boot sectors, bad sector list, flash ROM, CMOS memory, etc., but what if there is a memory-resident virus currently running? That will not be detected. Worse yet, suppose the running virus is monitoring all system calls. It can easily detect that the antivirus program is reading the boot sector (to check for viruses). To thwart the antivirus program, the virus does not make the system call. Instead it just returns the true boot sector from its hiding place in the bad block list. It also makes a mental note to reinfect all the files when the virus scanner is finished.

To prevent being spoofed by a virus, the antivirus program could make hard reads to the disk, bypassing the operating system. However this requires having built-in device drivers for IDE, SCSI, and other common disks, making the antivirus program less portable and subject to failure on computers with unusual disks. Furthermore, since bypassing the operating system to read the boot sector is possible, but bypassing it to read all the executable files is not, there is also some danger that the virus can produce fraudulent data about executable files as well.

Integrity Checkers

A completely different approach to virus detection is integrity checking. An antivirus program that works this way first scans the hard disk for viruses. Once it is convinced that the disk is clean, it computes a checksum for each executable file and writes the list of checksums for all the relevant files in a directory to a file, checksum, in that directory. The next time it runs, it recomputes all the checksums and sees if they match what is in the file checksum. An infected file will show up immediately.

The trouble is Virgil is not going to take this lying down. He can write a virus that removes the checksum file. Worse yet, he can write a virus that computes the checksum of the infected file and replaces the old entry in the checksum file. To protect against this kind of behavior, the antivirus program can try to hide the checksum file, but that is not likely to work since Virgil can study the antivirus program carefully before writing the virus. A better idea is to encrypt it to make tampering easier to detect. Ideally, the encryption should involve use of a smart card with an externally stored key that programs cannot get at.

Behavioral Checkers

A third strategy used by antivirus software is behavioral checking. With this approach, the antivirus program lives in memory while the computer is running and catches all system calls itself. The idea is that it can then monitor all activity and try to catch anything that looks suspicious. For example, no normal program should attempt to overwrite the boot sector, so an attempt to do so is almost certainly due to a virus. Likewise, changing the flash ROM is highly suspicious.

But there are also cases that are less clear cut. For example, overwriting an executable file is a peculiar thing to do—unless you are a compiler. If the antivirus software detects such a write and issues a warning, hopefully the user knows whether overwriting an executable makes sense in the context of the current work. Similarly, Word overwriting a .doc file with a new document full of macros is not necessarily the work of a virus. In Windows, programs can detach from their executable file and go memory resident using a special system call. Again, this might be legitimate, but a warning might still be useful.

Viruses do not have to passively lie around waiting for an antivirus program to kill them, like cattle being led off to slaughter. They can fight back. A particularly interesting battle can occur if a memory-resident virus and a memory-resident antivirus meet up on the same computer. Years ago there was a game called Core Wars in which two programmers faced off by each dropping a program into an empty address space. The programs took turns probing memory, with the object of the game being to locate and wipe out your opponent before he wiped you out. The virus-antivirus confrontation looks a little like that, only the battlefield is the machine of some poor user who does not really want it to happen there. Worse yet, the virus has an advantage because its writer can find out a lot about the antivirus program by just buying a copy of it. Of course, once the virus is out there, the antivirus team can modify their program, forcing Virgil to go buy a new copy.

Virus Avoidance

Every good story needs a moral. The moral of this one is

Better safe than sorry.

Avoiding viruses in the first place is a lot easier than trying to track them down once they have infected a computer. Below are a few guidelines for individual users, but also some things that the industry as a whole can do to reduce the problem considerably.

What can users do to avoid a virus infection? First, choose an operating system that offers a high degree of security, with a strong kernel-user mode boundary and separate login passwords for each user and the system administrator. Under these conditions, a virus that somehow sneaks in cannot infect the system binaries.

Second, install only shrink-wrapped software bought from a reliable manufacturer. Even this is no guarantee since there have been cases where disgruntled employees have slipped viruses onto a commercial software product, but it helps a lot. Downloading software from Web sites and bulletin boards is risky behavior.

Third, buy a good antivirus software package and use it as directed. Be sure to get regular updates from the manufacturer's Web site.

Fourth, do not click on attachments to email and tell people not to send them to you. Email sent as plain ASCII text is always safe but attachments can start viruses when opened.

Fifth, make frequent backups of key files onto an external medium, such as floppy disk, CD-recordable, or tape. Keep several generations of each file on a series of backup media. That way, if you discover a virus, you may have a chance to restore files as they were before they were infected. Restoring yesterday's infected file does not help, but restoring last week's version might.

The industry should also take the virus threat seriously and change some dangerous practices. First, make simple operating systems. The more bells and whistles there are, the more security holes there are. That is a fact of life.

Second, forget active content. From a security point of view, it is a disaster. Viewing a document someone sends you should not require your running their program. JPEG files, for example, do not contain programs, and thus cannot contain viruses. All documents should work like that.

Third, there should be a way to selectively write protect specified disk cylinders to prevent viruses from infecting the programs on them. This protection could be implemented by having a bitmap inside the controller listing the write protected cylinders. The map should only be alterable when the user has flipped a mechanical toggle switch on the computer's front panel.

Fourth, flash ROM is a nice idea, but it should only be modifiable when an external toggle switch has been flipped, something that will only happen when the user is consciously installing a BIOS update. Of course, none of this will be taken seriously until a really big virus hits. For example, one that hit the financial world and reset all bank accounts to 0. Of course, by then it would be too late.

Recovery from a Virus Attack

When a virus is detected, the computer should be halted immediately since a memory-resident virus may still be running. The computer should be rebooted from a CD-ROM or floppy disk that has always been write protected, and which contains the full operating system to bypass the boot sector, hard disk copy of the operating system, and disk drivers, all of which may now be infected. Then an antivirus program should be run from its original CD-ROM, since the hard disk version may also be infected.

The antivirus program may detect some viruses and may even be able to eliminate them, but there is no guarantee that it will get them all. Probably the safest course of action at this point is to save all files that cannot contain viruses (like ASCII and JPEG files). Those files that might contain viruses (like Word files) should be converted to another format that cannot contain viruses, such as flat ASCII text (or at least the macros should be removed). All the saved files should be saved on an external medium. Then the hard disk should be reformatted using a format program taken from a write-protected floppy disk or a CD-ROM to insure that it itself is not infected. It is especially important that the MBR and boot sectors are also fully erased. Then the operating system should be reinstalled from the original CD-ROM. When dealing with virus infections, paranoia is your best friend.

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