Antivirus software aims to detect, isolate and if necessary, delete malware on a computer before it can harm data. Antivirus software uses several techniques to identify malware, the two most common are known as signatures and heuristics.
Signatures
A malware’s signature is a distinctive pattern of data either in memory or in a file. An antivirus program may contain thousands of signatures, but it can only detect malware for which a signature has been identified and published by the antivirus program’s authors. As a result there is a period between a new piece of malware being released ‘into the wild’ and when its signature can be incorporated into antivirus products. During this period, the malware can propagate and attack unprotected systems, exploiting the so called ‘zero day’ vulnerabilities that exist until the systems are fixed and antivirus signatures are updated. It is not uncommon for several variants of a malware program to be published at intervals, each sufficiently different that they possess different signatures.
A second weakness of signatures is that more sophisticated malware has the ability to change its program (it is said to be polymorphic or metamorphic), disguising itself without affecting its operation.
Heuristics
Complementing signatures, heuristics use rules to identify viruses based on previous experience of known viruses. Heuristic detection may execute suspicious programs in a virtual machine (a software recreation of a physical computer) and analyse the program for operations typical of known malware (such as replicating itself or attempting to overwrite key operating system files); or it might revert the program back to its original source code and look for malware-like instructions. If the heuristic analysis considers that the file acts in a malware-like manner, it is flagged as potentially dangerous.
Unlike signatures, heuristics do not require specific knowledge about individual types of malware, they can detect new malware, for which signatures do not exist, simply by their behaviour. The drawback of heuristics is that they can only draw conclusions based on past experience, radically new malware (which appears all too regularly) can pass unnoticed.Issues with antivirus software
Although antivirus software is an essential part of protecting your computer, it is not a complete solution to malware problems.
Despite the best endeavours of its makers, antivirus software has occasionally proved to contain bugs with consequences such as being inaccurate, failing to update itself or simply consuming huge amounts of computer power. Fortunately, these problems are rare, easily fixed and much less serious than the risk from a malware attack.
