2 Network security threats
2.1 Malware: viruses, worms, trojan horses and zombies

Malicious software (malware) is software deliberately designed to infiltrate or damage a computer system without the owner's consent. It can cause losses or damages in the system. Computer viruses are a large class of malicious codes that can spread among computers and perform detrimental operations.

The execution of malware can cause the disruption of computer operations and can be also used to gather sensitive information or gain unauthorized access to computer systems. Malware is not the same as defective software, which is software that has a legitimate purpose but contains harmful bugs that were not noticed before release. In fact, computer viruses are actually a subset within the larger malware family, like other specimens such as worms, Trojan horses, adware, spyware, adware, rootkits, etc...

According to PandaLabs during 2014 more than 75 million of new malware samples were detected, representing the 34 % of all malware and also is more than twice higher than the amount recorded in the previous year, 30 million. Following, there is a definition of some of the most relevant types of malware:

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Fig. 2.1 – Types of new malware created during 2014