Malicious Software

Malicious users use different methods to break into the systems. The software that is intentionally included into a system with the intention to harm the system is called malicious software. Viruses, Trojan horse, and Worms are examples of malicious programs. JavaScript’s and Java applets written with the purpose of attacking, are also malicious programs.

Virus

Virus is a software program that is destructive in nature. Virus programs have the following properties:

It can attach itself to other healthy programs.

It can replicate itself and thus can spread across a network.

It is difficult to trace a virus after it has spread across a network.

Viruses harm the computer in many ways—

· corrupt or delete data or files on the computer,

· change the functionality of software applications,

· use e-mail program to spread itself to other computers,

· erase everything on the hard disk, or,

· degrade performance of the system by utilizing resources such as memory or disk space.

· Virus infects an executable file or program. The virus executes when a program infected with virus is executed or you start a computer from a disk that has infected system files.

· Once a virus is active, it loads into the computer’s memory and may save itself to the hard drive or copies itself to applications or system files on the disk.

· However, viruses cannot infect write protected disks or infect written documents. Viruses do not infect an already compressed file. Viruses also do not infect computer hardware; they only infect software.

· Viruses are most easily spread by attachments in e-mail messages. Viruses also spread through download on the Internet.

Some examples of viruses are—“Melissa” and “I Love You”.

Worms

Worm is self-replicating software that uses network and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well. A worm is however different from a virus. A worm does not modify a program like a virus, however, it replicates so much that it consumes the resources of the computer and makes it slow. Some examples of worms are—“Code Red” and “Nimda”.

Trojan Horse

Trojan horse is destructive programs that masquerade as useful programs. The name “Trojan horse” is given because of the Greek soldiers who reached the city of Troy by hiding themselves inside a large wooden horse. The people of the city of Troy themselves pulled the horse inside their city, unaware of the fact that the Greek soldiers were hiding inside the horse. Similarly, users install Trojan horses thinking that it will serve a useful purpose such as a game or provide entertainment. However, Trojan horses contain programs that corrupt the data or damage the files. Trojan horses can corrupt software applications. They can also damage files and can contain viruses that destroy and corrupt data and programs. Trojan horse does not replicate themselves like viruses.

             Trojan horse

JavaScript’s, Java Applets and ActiveX Controls

Applets (Java programs), and ActiveX controls are used with Microsoft technology, which can be inserted in a Web page and are downloaded on the client browser for execution. Applets and ActiveX controls are generally used to provide added functionality such as sound and animation. However, these programs when designed with a malicious intention can be disastrous for the client machine. Java Applets have strong security checks that define what an applet can do and what it cannot. ActiveX controls do not have such security checks. Normally, ActiveX controls must be kept disabled while working on the Internet.

Javascript is a scripting language generally nested within HTML code. The client-side scripts on a HTML page execute inside the Web browser on the client computer. Javascript codes can be used to transfer files, send e-mails and write to local files. If used with a maligned intention, the scripts can be dangerous for the client machine.