Anti-Virus

 

In the virus dictionary approach, when the anti-virus software examines a file, it refers to a dictionary of known viruses that the authors of the anti-virus software have identified. If a piece of code in the file matches any virus identified in the dictionary, then the anti-virus software can take one of the following actions:
attempt to repair the file by removing the virus itself from the file
quarantine the file (such that the file remains inaccessible to other programs and its virus can no longer spread)
delete the infected file

To achieve consistent success in the medium and long term, the virus dictionary approach requires periodic (generally online) downloads of updated virus dictionary entries. As civically minded and technically inclined users identify new viruses "in the wild", they can send their infected files to the authors of anti-virus software, who then include information about the new viruses in their dictionaries.

Dictionary-based anti-virus software typically examines files when the computer's operating system creates, opens, closes or e-mails them. In this way it can detect a known virus immediately upon receipt. Note too that a System Administrator can typically schedule the anti-virus software to examine (scan) all files on the user's hard disk on a regular basis.

Although the dictionary approach can effectively contain virus outbreaks in the right circumstances, virus authors have tried to stay a step ahead of such software by writing "oligomorphic", "polymorphic" and more recently "metamorphic" viruses, which encrypt parts of themselves or otherwise modify themselves as a method of disguise, so as to not match the virus's signature in the dictionary.
Suspicious behavior approach

The suspicious behavior approach, by contrast, doesn't attempt to identify known viruses, but instead monitors the behavior of all programs. If one program tries to write data to an executable program, for example, the anti-virus software can flag this suspicious behavior, alert a user and ask what to do.

Unlike the dictionary approach, the suspicious behavior approach therefore provides protection against brand-new viruses that do not yet exist in any virus dictionaries. However, it can also sound a large number of false positives, and users probably become desensitized to all the warnings. If the user clicks "Accept" on every such warning, then the anti-virus software obviously gives no benefit to that user. This problem has worsened since 1997, since many more nonmalicious program designs came to modify other .exe files without regard to this false positive issue. Thus, most modern anti-virus software uses this technique less and less.
Other ways to detect viruses

Some antivirus-software uses of other types of heuristic analysis. For example, it could try to emulate the beginning of the code of each new executable that the system invokes before transferring control to that executable. If the program seems to use self-modifying code or otherwise appears as a virus (if it immediately tries to find other executables, for example), one could assume that a virus has infected the executable. However, this method could result in a lot of false positives.

Yet another detection method involves using a sandbox. A sandbox emulates the operating system and runs the executable in this simulation. After the program has terminated, software analyzes the sandbox for any changes which might indicate a virus. Because of performance issues, this type of detection normally only takes place during on-demand scans.

Some virus scanners can also warn a user if a file is likely to contain a virus based on the file type.
Issues of concern
The spread of e-mail viruses (arguably the most destructive and widespread computer viruses) could be inhibited far more inexpensively and effectively, and without the need to install anti-virus software, if bugs in the e-mail clients, which relate to the execution of downloaded code and to the ability of executables to spread and wreak havoc, were fixed.
User education can effectively supplement anti-virus software; simply training users in safe computing practices (such as not downloading and executing unknown programs from the Internet) would slow the spread of viruses and obviate the need of much anti-virus software.
Computer users should not always run with administrator access to their own machine. If they would simply run in user mode then some types of viruses could not spread (or at least the damage caused by viruses could be reduced).
The dictionary approach to detecting viruses does not always suffice -- due to the continual creation of new viruses -- yet the suspicious behavior approach does not work well due to the false positive problem; hence, the current understanding of anti-virus software will never conquer computer viruses.
Various methods exist of encrypting and packing malicious software which will make even well-known viruses undetectable to anti-virus software. Detecting these "camouflaged" viruses requires a powerful unpacking engine, which can decrypt the files before examining them. Unfortunately, many popular anti-virus programs do not have this and thus are often unable to detect encrypted viruses.
The ongoing writing and spreading of viruses and of panic about them gives the vendors of commercial anti-virus software a financial interest in the ongoing existence of viruses.
Some anti-virus software can considerably reduce performance. Users may disable the anti-virus protection to overcome the performance loss, thus increasing the risk of infection. For maximum protection the anti-virus software needs to be enabled all the time - often at the cost of slower performance (see also Software bloat). Some anti-virus software has less impact on performance.
It is sometimes necessary to temporarily disable virus protection when installing major updates such as Windows Service Packs or updating graphics card drivers for example. Having anti-virus protection running at the same time as installing a major update may prevent the update installing properly or at all.