Malware (malicious software) is any software intentionally designed to cause damage to a computer, server, client, or computer network (by contrast, software that causes unintentional harm due to some deficiency is typically described as a software bug). A wide variety of malware types exist, including computer viruses, worms, Trojan horses, ransomware, spyware, adware, rogue software, wiper and scareware.

Hex dump of the Blaster worm, showing a message left for Microsoft co-founder Bill Gates by the worm’s programmer

Programs are also considered malware if they secretly act against the interests of the computer user. For example, at one point Sony music Compact discs silently installed a rootkit on purchasers’ computers with the intention of preventing illicit copying, but which also reported on users’ listening habits, and unintentionally created extra security vulnerabilities.

A range of antivirus software, firewalls and other strategies are used to help protect against the introduction of malware, to help detect it if it is already present, and to recover from malware-associated malicious activity and attacks.