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Chapter: Cryptography and Network Security Principles and Practice : System Security : Malicious Software

Computer Viruses: Nature, Classification, Virus Kits

· The Nature of Viruses · Viruses Classification · Virus Kits · Macro Viruses · E-Mail Viruses


The Nature of Viruses

A computer virus is a piece of software that can “infect” other programs by modifying them; the modification includes injecting the original program with a routine to make copies of the virus program, which can then go on to infect other programs. Computer viruses first appeared in the early 1980s, and the term itself is attributed to Fred Cohen in 1983. Cohen is the author of a groundbreaking book on the subject [COHE94].

Biological viruses are tiny scraps of genetic code—DNA or RNA—that can take over the machinery of a living cell and trick it into making thousands of flaw- less replicas of the original virus. Like its biological counterpart, a computer virus carries in its instructional code the recipe for making perfect copies of itself. The typical virus becomes embedded in a program on a computer. Then, whenever the infected computer comes into contact with an uninfected piece of software, a fresh copy of the virus passes into the new program. Thus, the infection can be spread from computer to computer by unsuspecting users who either swap disks or send programs to one another over a network. In a network environment, the ability to access applications and system services on other computers provides a perfect cul- ture for the spread of a virus.

A virus can do anything that other programs do. The difference is that a virus attaches itself to another program and executes secretly when the host program is run. Once a virus is executing, it can perform any function, such as erasing files and programs that is allowed by the privileges of the current user.

A computer virus has three parts [AYCO06]:


Infection mechanism: The means by which a virus spreads, enabling it to repli- cate. The mechanism is also referred to as the infection vector.

Trigger: The event or condition that determines when the payload is activated or delivered.

Payload: What the virus does, besides spreading. The payload may involve damage or may involve benign but noticeable activity.


During its lifetime, a typical virus goes through the following four  phases:


Dormant phase: The virus is idle. The virus will eventually be activated by some event, such as a date, the presence of another program or file, or the capacity of the disk exceeding some limit. Not all viruses have this stage.

Propagation phase: The virus places a copy of itself into other programs or into certain system areas on the disk. The copy may not be identical to the propa- gating version; viruses often morph to evade detection. Each infected program will now contain a clone of the virus, which will itself enter a propagation phase.

Triggering phase: The virus is activated to perform the function for which it was intended. As with the dormant phase, the triggering phase can be caused  by a variety of system events, including a count of the number of times that   this copy of the virus has made copies of itself.


Execution phase: The function is performed. The function may be harmless, such as a message on the screen, or damaging, such as the destruction of programs and data files.

Most viruses carry out their work in a manner that is specific to a particular oper- ating system and, in some cases, specific to a particular hardware platform. Thus, they are designed to take advantage of the details and weaknesses of particular systems.

VIRUS STRUCTURE A virus can be prepended or postpended to an executable program, or it can be embedded in some other fashion. The key to its operation is that the infected program, when invoked, will first execute the virus code and then execute the original code of the program.

A very general depiction of virus structure is shown in Figure 21.1 (based on [COHE94]). In this case, the virus code, V, is prepended to infected programs, and it is assumed that the entry point to the program, when invoked, is the first line of the program.

The infected program begins with the virus code and works as follows. The first line of code is a jump to the main virus program. The second line is a special marker that is used by the virus to determine whether or not a potential victim program has already been infected with this virus. When the program is invoked, control is imme- diately transferred to the main virus program. The virus program may first seek out uninfected executable files and infect them. Next, the virus may perform some action, usually detrimental to the system. This action could be performed every time the program is invoked, or it could be a logic bomb that triggers only under certain conditions. Finally, the virus transfers control to the original program. If the infection

phase of the program is reasonably rapid, a user is unlikely to notice any difference between the execution of an infected and an uninfected program.

A virus such as the one just described is easily detected because an infected version of a program is longer than the corresponding uninfected one. A way to thwart such a simple means of detecting a virus is to compress the executable file so that both the infected and uninfected versions are of identical length. Figure 21.2 [COHE94] shows in general terms the logic required. The key lines in this virus are numbered, and Figure 21.3 [COHE94] illustrates the operation. We assume that program P1 is infected with the virus CV. When this program is invoked, control passes to its virus, which performs the following steps:

1.                        For each uninfected file P2 that is found, the virus first compresses that file to produce P¿2 , which is shorter than the original program by the size of the virus.

2.                        A copy of the virus is prepended to the compressed program.

3.                        The compressed version of the original infected program, P1¿ , is uncompressed.

4.                        The uncompressed original program is executed.

In this example, the virus does nothing other than propagate. As previously mentioned, the virus may include a logic bomb.


INITIAL INFECTION Once a virus has gained entry to a system by infecting a single program, it is in a position to potentially infect some or all other executable files on that system when the infected program executes. Thus, viral infection can be completely prevented by preventing the virus from gaining entry in the first place. Unfortunately, prevention is extraordinarily difficult because a virus can be part of any program outside a system. Thus, unless one is content to take an absolutely bare piece of iron and write all one’s own system and application programs, one is vulnerable. Many forms of infection can also be blocked by denying normal users the right to modify programs on the system.

The lack of access controls on early PCs is a key reason why traditional machine code based viruses spread rapidly on these systems. In contrast, while it is easy enough to write a machine code virus for UNIX systems, they were almost never seen in practice because the existence of access controls on these systems pre- vented effective propagation of the virus. Traditional machine code based viruses are now less prevalent, because modern PC OSs do have more effective access con- trols. However, virus creators have found other avenues, such as macro and e-mail viruses, as discussed subsequently.


Viruses  Classification

There has been a continuous arms race between virus writers and writers of antivirus software since viruses first appeared. As effective countermeasures are developed for existing types of viruses, newer types are developed. There is no simple or universally agreed upon classification scheme for viruses, In this section, we follow [AYCO06] and classify viruses along two orthogonal axes: the type of target the virus tries to infect and the method the virus uses to conceal itself from detection by users and antivirus software.

A virus classification by target includes the following categories:


                           Boot sector infector: Infects a master boot record or boot record and spreads when a system is booted from the disk containing the virus.

                           File infector: Infects files that the operating system or shell consider to be executable.

                           Macro virus: Infects files with macro code that is interpreted by an applica- tion.

A virus classification by concealment strategy includes the following categories:


Encrypted virus: A typical approach is as follows. A portion of the virus cre- ates a random encryption key and encrypts the remainder of the virus. The key is stored with the virus. When an infected program is invoked, the virus uses the stored random key to decrypt the virus. When the virus replicates, a differ- ent random key is selected. Because the bulk of the virus is encrypted with a different key for each instance, there is no constant bit pattern to observe.


Stealth virus: A form of virus explicitly designed to hide itself from detection by antivirus software. Thus, the entire virus, not just a payload is hidden.

Polymorphic virus: A virus that mutates with every infection, making detec- tion by the “signature” of the virus impossible.

Metamorphic virus: As with a polymorphic virus, a metamorphic virus mutates with every infection. The difference is that a metamorphic virus rewrites itself completely at each iteration, increasing the difficulty of detection. Metamorphic viruses may change their behavior as well as their appearance.

One example of a stealth virus was discussed earlier: a virus that uses com- pression so that the infected program is exactly the same length as an uninfected version. Far more sophisticated techniques are possible. For example, a virus can place intercept logic in disk I/O routines, so that when there is an attempt to read suspected portions of the disk using these routines, the virus will present back the original, uninfected program. Thus, stealth is not a term that applies to a virus as such but, rather, refers to a technique used by a virus to evade detection.

A polymorphic virus creates copies during replication that are functionally equivalent but have distinctly different bit patterns. As with a stealth virus, the pur- pose is to defeat programs that scan for viruses. In this case, the “signature” of the virus will vary with each copy. To achieve this variation, the virus may randomly insert superfluous instructions or interchange the order of independent instructions. A more effective approach is to use encryption. The strategy of the encryption virus is followed. The portion of the virus that is responsible for generating keys and performing encryption/decryption is referred to as the mutation engine. The muta- tion engine itself is altered with each use.


Virus Kits

Another weapon in the virus writers’ armory is the virus-creation toolkit. Such a toolkit enables a relative novice to quickly create a number of different viruses. Although viruses created with toolkits tend to be less sophisticated than viruses designed from scratch, the sheer number of new viruses that can be generated using a toolkit creates a problem for antivirus schemes.


Macro Viruses

In the mid-1990s, macro viruses became by far the most prevalent type of virus. Macro viruses are particularly threatening for a number of reasons:

1.                        A macro virus is platform independent. Many macro viruses infect Microsoft Word documents or other Microsoft Office documents. Any hardware plat- form and operating system that supports these applications can be infected.

2.                        Macro viruses infect documents, not executable portions of code. Most of the information introduced onto a computer system is in the form of a document rather than a program.

3.                        Macro viruses are easily spread. A very common method is by electronic mail.

4.                        Because macro viruses infect user documents rather than system programs, tra- ditional file system access controls are of limited use in preventing their spread.

Macro viruses take advantage of a feature found in Word and other office applications such as Microsoft Excel, namely the macro. In essence, a macro is an executable program embedded in a word processing document or other type of file. Typically, users employ macros to automate repetitive tasks and thereby save keystrokes. The macro language is usually some form of the Basic programming language. A user might define a sequence of keystrokes in a macro and set it up so that the macro is invoked when a function key or special short combination of keys is input.

Successive releases of MS Office products provide increased protection against macro viruses. For example, Microsoft offers an optional Macro Virus Protection tool that detects suspicious Word files and alerts the customer to the potential risk of opening a file with macros. Various antivirus product vendors have also developed tools to detect and correct macro viruses. As in other types of viruses, the arms race continues in the field of macro viruses, but they no longer are the predominant virus threat.


            ail Viruses

A more recent development in malicious software is the e-mail virus. The first rapidly spreading e-mail viruses, such as Melissa, made use of a Microsoft Word macro embedded in an attachment. If the recipient opens the e-mail attachment, the Word macro is activated. Then

1.                                       The e-mail virus sends itself to everyone on the mailing list in the user’s e-mail package.

2.                                       The virus does local damage on the user’s system.

In 1999, a more powerful version of the e-mail virus appeared. This newer version can be activated merely by opening an e-mail that contains the virus rather than opening an attachment. The virus uses the Visual Basic scripting language supported by the e-mail package.

Thus we see a new generation of malware that arrives via e-mail and uses e-mail software features to replicate itself across the Internet. The virus propagates itself as soon as it is activated (either by opening an e-mail attachment or by opening the e-mail) to all of the e-mail addresses known to the infected host. As a result, whereas viruses used to take months or years to propagate, they now do so in hours.This makes it very difficult for antivirus software to respond before much damage is done. Ultimately, a greater degree of security must be built into Internet utility and applica- tion software on PCs to counter the growing threat.


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