How Antivirus Software Works

How Antivirus Works

An antivirus software typically uses a variety of strategies in detecting and removing viruses, worms and other malware programs. The following are the two most widely employed identification methods:

1. Signature-based dectection (Dictionary approach)

This is the most commonly employed method which involves searching for known patterns of virus within a given file. Every antivirus software will have a dictionary of sample malware codes called signatures in it’s database. Whenever a file is examined, the antivirus refers to the dictionary of sample codes present within it’s database and compares the same with the current file. If the piece of code within the file matches with the one in it’s dictionary then it is flagged and proper action is taken immediately so as to stop the virus from further replicating. The antivirus may choose to repair the file, quarantine or delete it permanently based on it’s potential risk. 

As new viruses and malwares are created and released every day, this method of detection cannot defend against new malwares unless their samples are collected and signatures are released by the antivirus software company. Some companies may also encourage the users to upload new viruses or variants, so that the virus can be analyzed and the signature can be added to the dictionary.

Signature based detection can be very effective, but requires frequent updates of the virus signature dictionary. Hence the users must update their antivirus software on a regular basis so as to defend against new threats that are released daily.

2. Heuristic-based detection (Suspicious behaviour approach)

Heuristic-based detection involves identifying suspicious behaviour from any given program which might indicate a potential risk. This approach is used by some of the sophisticated antivirus softwares to identify new malware and variants of known malware. Unlike the signature based approach, here the antivirus doesn’t attempt to identify known viruses, but instead monitors the behavior of all programs.

For example, malicious behaviours like a program trying to write data to an executable program is flagged and the user is alerted about this action. This method of detection gives an additional level of security from unidentified threats.

File emulation: This is another type of heuristic-based approach where a given program is executed in a virtual environment and the actions performed by it are logged. Based on the actions logged, the antivirus software can determine if the program is malicious or not and carry out necessary actions in order to clean the infection.

Most commercial antivirus softwares use a combination of both signature-based and heuristic-based approaches to combat malware.

Issues of concern

Zero-day threats: A zero-day (zero-hour ) threat or attack is where a malware tries to exploit computer application vulnerabilities that are yet unidentified by the antivirus software companies. These attacks are used to cause damage to the computer even before they are identified. Since patches are not yet released for these kind of new threats, they can easily manage to bypass the antivirus software and carry out malicious actions. However most of the threats are identified after a day or two of it’s release, but damage caused by them before identification is quite inevitable.

Daily Updates: Since new viruses and threats are released everyday, it is most essential to update the antivirus software so as to keep the virus definitions up-to-date. Most softwares will have an auto-update feature so that the virus definitions are updated whenever the computer is connected to the Internet.

Effectiveness: Even though an antivirus software can catch almost every malware, it is still not 100% foolproof against all kinds of threats. As explained earlier, a zero-day threat can easily bypass the protective shield of the antivirus software. Also virus authors have tried to stay a step ahead by writing “oligomorphic“, “polymorphic” and, more recently, “metamorphic” virus codes, which will encrypt parts of themselves or otherwise modify themselves as a method of disguise, so as to not match virus signatures in the dictionary.

Thus user education is as important as antivirus software; users must be trained to practice safe surfing habits such as downloading files only from trusted websites and not blindly executing a program that is unknown or obtained from an untrusted source. I hope this article will help you understand the working of an antivirus software.