A hacker (or intruder) is an individual who intends to gain unauthorized access to a computer system. Within the hacking community, the term cracker is typically used to denote a hacker with criminal intent, although in the public press, the terms hacker and cracker are used interchangeably.
Hacker activities have broadened beyond mere system intrusion to include theft of goods and information, as well as system damage and cyber vandalism. Cyber vandalism is the intentional
disruption, defacement, or even destruction of a Web site or corporate information system. Hackers create security problems in a number of ways as given below.
Spoofing and sniffing – Spoofing is, generally, the act of one person pretending to be someone else. Hackers attempt to hide their true identity by using fake email address or masquerading as someone else. Spoofing can also involve redirecting a Web link to an address different from the intended one, with a site masquerading as the intended destination. A sniffer is a type of eavesdropping program that monitors information traveling over a network. Sniffers enable hackers to steal proprietary information from anywhere on a network, including email messages, company files, and confidential reports.
Denial of service attacks – In denial of service (DoS) attack, hackers flood a network server or Web server with many thousands of false communications or requests for services to crash the network. A distributed denial service (DDoS) attack uses numerous computers to inundate and overwhelm the network form numerous launch points.
Identity theft – With the growth of the Internet and electronic commerce, identity theft has become especially troubling. Identity theft is a crime in which an imposter obtains key pieces of personal information, such as social security identification numbers, driver‘s license numbers, or credit card numbers, to impersonate someone else.
Cyber terrorism and cyber warfare – Concern is mounting that the vulnerabilities of the Internet or other networks could be exploited by terrorists, foreign intelligence services, or other groups to create widespread disruption and harm.