Malware, a portmanteau word from the words malicious and software, is software designed to infiltrate or damage a computer system without the owner's informed consent. The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code.

Many computer users are unfamiliar with the term, and often use "computer virus" for all types of malware, including true viruses.

Software is considered malware based on the perceived intent of the creator rather than any particular features. Malware includes computer viruses, worms, trojan horses, most rootkits, spyware, dishonest adware, crimeware and other malicious and unwanted software. In law, malware is sometimes known as a computer contaminant, for instance in the legal codes of several American states, including California and West Virginia.

Malware is not the same as defective software, that is, software which has a legitimate purpose but contains harmful bugs.

Preliminary results from Symantec sensors published in 2008 suggested that "the release rate of malicious code and other unwanted programs may be exceeding that of legitimate software applications." According to F-Secure, "As much malware 1 produced in 2007 as in the previous 20 years altogether." Malware's most common pathway from criminals to users is through the Internet, by email and the World Wide Web.

Purposes

Many early infectious programs, including the first Internet Worm and a number of MS-DOS viruses, were written as experiments or pranks generally intended to be harmless or merely annoying rather than to cause serious damage to computers. In some cases the perpetrator did not realize how much harm their creations could do. Young programmers learning about viruses and the techniques used to write them only to prove that they could or to see how far it could spread. As late as 1999, widespread viruses such as the Melissa virus appear to have been written chiefly as pranks.

Hostile intent related to vandalism can be found in programs designed to cause harm or data loss. Many DOS viruses, and the Windows ExploreZip worm, were designed to destroy files on a hard disk, or to corrupt the file system by writing junk data. Network-borne worms such as the 2001 Code Red worm or the Ramen worm fall into the same category. Designed to vandalize web pages, these worms may seem like the online equivalent to graffiti tagging, with the author's alias or affinity group appearing everywhere the worm goes.

However, since the rise of widespread broadband Internet access, malicious software has come to be designed for a profit motive, either more or less legal (forced advertising) or criminal. For instance, since 2003, the majority of widespread viruses and worms have been designed to take control of users' computers for black-market exploitation. Fact: date=February 2007 Infected "zombie computers" are used to send email spam, to host contraband data such as child pornography, or to engage in distributed denial-of-service attacks as a form of extortion.