Appendix A. Types of Computer Threats

Herein, the term “threat” is defined as any kind of software potentially or directly capable of inflicting damage to a computer or network and compromising the user’s information or rights (that is, malicious and other unwanted software). In a wider sense, the term “threat” may be used to indicate any type of potential danger to the security of the computer or network (that is, vulnerabilities that can result in hacker attacks).

All of the program types stated below have the ability to endanger user data or confidentiality. Programs that do not conceal their presence in the system (e.g. spam distribution software and various traffic analyzers) are usually not considered as computer threats, although they can become threats under certain circumstances.

Computer Viruses

This type of computer threats is characterized by the ability to embed its code into other programs. Such implementation is called infection. In most cases, an infected file becomes a virus carrier and the embedded code does not necessarily match the original one. Most viruses are intended to damage or destroy data in the system.

In Doctor Web classification, viruses are divided by the type of objects they infect:

File viruses infect files of the operating system (usually executable files and dynamic libraries) and are activated when the infected file is launched.

macro-viruses are viruses that infect documents used by Microsoft® Office and some other applications supporting macro commands (for example, written in Visual Basic). Macro commands are a type of implemented programs (macros) written in a fully functional programming language. For instance, in Microsoft® Word, macros can be automatically initiated upon opening, closing, or saving a document.

Script viruses are created using script languages and usually infect other scripts (e.g. service files of an operating system). They are also able to infect other file formats that allow execution of scripts and thus take advantage of scripting vulnerabilities in web applications.

boot viruses infect boot records of disks and partitions or master boot records of hard drives. They do not require much memory and remain ready to continue performing their tasks until a system roll-out, restart or shut-down is performed.

Most viruses have some kind of protection against detection. Protection methods are being constantly improved, and ways to overcome them are constantly being developed. All viruses may also be classified according to protection type they use:

Encrypted viruses cipher their code upon every infection to hamper their detection in a file, boot sector or memory. All copies of such viruses contain only a small common code fragment (the decryption procedure) that can be used as a virus signature.

Polymorphic viruses also encrypt there code, but besides that they also generate a special decryption procedure that is different in every copy of the virus. This means that such viruses do not have byte signatures.

Stealth viruses perform certain actions to disguise their activity and thus conceal their presence in an infected object. Such viruses gather the characteristics of an object before infecting it and then plant these “dummy” characteristics that mislead the scanner searching for modified files.

Viruses can also be classified according to the programming language in which they are written (in most cases, it is Assembler, high-level programming languages, script languages, and so on) or according to affected operating systems.

Computer Worms

Recently, malicious programs of the “computer worm” type have become much more common than viruses and other types of malware. Just like viruses, such programs can make copies of themselves, however they do not infect other objects. A worm gets into a computer from a network (most frequently as an attachment to an email or from the internet) and sends the functioning copies of itself to other computers. To start their spread, worms can either rely on the computer user’s actions or can select and attack computers in an automatic mode.

Worms do not necessarily consist of only one file (the worm’s body). Many of them have an infectious part (the shellcode) that loads into the main memory (RAM) and then downloads the worm’s body as an executable file via the network. If only the shellcode is present in the system, the worm can be deleted by simply restarting the system (at which the RAM is erased and reset). However, if the worm’s body infiltrates the computer, then only an anti-virus program can cope with it.

Worms have the ability to cripple entire networks even if they do not bear any payload (i.e. do not cause any direct damage) due to their intensive distribution.

In Doctor Web classification, worms are divided by distribution method:

Net worms distribute their copies via various network and file-sharing protocols.

mail worms spread themselves using email protocols (POP3, SMTP, and so on).

chat worms use protocols of popular messengers and chat programs (ICQ, IM, IRC, and so on).

Trojan Programs (Trojans)

This type of threats cannot reproduce itself. A trojan substitutes a frequently-used program and performs its functions (or imitates its operation). Meanwhile, it performs some malicious actions in the system (damages or deletes data, sends confidential information, and so on) or makes it possible for hackers to access the computer without permission, for example, to harm the computer of a third party.

A trojan masking and malicious facilities are similar to those of a virus. A trojan may even be a component of a virus. However, most trojans are distributed as separate executable files (through file exchange servers, removable data carriers or email attachments) that are launched by users or system tasks.

It is very hard to classify trojans due to the fact that they are often distributed by viruses or worms and also because many malicious actions that can be performed by other types of threats are attributed to trojans only. Here are some trojan types which are distinguished as separate classes in Doctor Web:

backdoors are trojans that make it possible for an intruder to log on into the system or obtain privileged functions bypassing any existing access and security measures. Backdoors do not infect files, but they write themselves into the registry modifying the registry keys.

rootkits are used to intercept system functions of an operating system in order to conceal themselves. Besides, a rootkit can conceal processes of other programs (e.g. other threats), registry keys, folders and files. It can be distributed either as an independent program or as a component of another malicious program. There are two kinds of rootkits according to the mode of operation: User Mode Rootkits (UMR) that operate in user mode (intercept functions of the user mode libraries) and Kernel Mode Rootkits (KMR) that operate in kernel mode (intercept functions on the level of the system kernel, which makes it harder to detect).

keyloggers are used to log data that users enter by means of a keyboard. The aim of this is to steal personal information (i.e. network passwords, logins, credit card data, and so on).

clickers redirect hyperlinks to certain addresses (sometimes malicious) in order to increase traffic of websites or perform DDoS attacks.

Proxy trojans provide anonymous internet access through a victim’s computer.

In addition, trojans can also change the start page in a web browser or delete certain files. However, these actions can also be performed by other types of threats (viruses and worms).


Hacktools are programs designed to assist the intruder with hacking. The most common among them are port scanners that detect vulnerabilities in firewalls and other components of computer protection system. Besides hackers, such tools are used by administrators to check security of their networks. Occasionally, common software that can be used for hacking and various programs that use social engineering techniques are designated as among hacktools as well.


Usually, this term refers to a program code implemented into freeware programs that force display of advertisements to users. However, sometimes such codes can be distributed via other malicious programs and show advertisements in web browsers. Many adware programs operate with data collected by spyware.


Like adware, this type of minor threats can not be used to inflict any direct damage to the system. Joke programs usually just generate messages about errors that never occurred and threaten to perform actions that will lead to data loss. Their purpose is to frighten or annoy users.


These are special programs that are designed to scan a range of telephone numbers and find those where a modem answers. These numbers are then used to mark up the price of telephoning facilities or to connect the user to expensive telephone services.


These software applications were not created for malicious purposes, but due to their characteristics can pose a threat to the computer security. Riskware programs can not only damage or delete data, but they are also used by crackers (i.e. malevolent hackers) or by some malicious programs to harm the system. Among such programs, there are various remote chat and administrative tools, FTP-servers, and so on.

Suspicious objects

These are possible computer threats detected by the heuristic analyzer. Such objects can potentially be any type of threat (even unknown to IT security specialists) or turn out to be safe in case of false detection. It is recommended that you choose to move the files containing suspicious objects to the quarantine, they also should be sent to Doctor Web anti-virus laboratory for analysis.