Leap to a definition:
Adware - Executable
Hybrid virus - Keylogger
Malware - Probe
Rootkit - Worm
A software application in which advertising banners are displayed while the program is running; sometimes, also tracks user information, which makes it also spyware.
Ask the experts: My adware is delaying reboots.
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A means of access to a computer system that bypasses security mechanisms, installed sometimes by an authorized person, sometimes by an attacker.
Tip: How attackers install backdoors and what to do about it
A bot (short for "robot") is a program that operates as an agent for a user or another program or simulates a human activity. On the Internet, the most ubiquitous bots are the programs, also called spiders or crawlers, that access Web sites and gather their content for search engine indexes.
Tip: Invasion of the bots
Type of attack that sends more data than a buffer was intended to hold; surplus data will overflow into adjacent buffers, corrupting or overwriting the valid data held in them or may. Data sent may include malicious code.
Tip: How buffer overflow attacks work
In a malware context, a program that tracks and processes data about the user's browsing behavior for marketing purposes.
Distributed denial-of-service attack
One in which a multitude of compromised systems attack a single target, thereby causing denial of service for users of the targeted system.
Expert response: How do I find a program that activates only when the system goes into hibernation?
Malicious computer code sent to you as an e-mail note attachment. The best two defenses against e-mail viruses for the individual user are (1) a policy of never opening an e-mail attachment (even from someone you know) unless you have been expecting the attachment and know what it contains, and (2) installing and using anti-virus software to scan any attachment before you open it.
Security school: Email security
Type of file containing a program that will start it to run; viruses are often sent in executable files that will run when the user opens the file.
Expert response: Virus causing your computer to reboot?
A virus that combines characteristics of more than one type of virus to infect both program files and system sectors. The virus may attack at either level and proceed to infect the other once it has established itself.
Learning guide: Malware
Malicious code that combines characteristics of both those types of malware, typically featuring the virus' ability to alter program code with the worm's ability to reside in live memory and to propagate without any action on the part of the user.
Tip: Eight steps to prevent malware outbreaks
Self-replicating malicious code that spreads in instant messaging networks.
Expert response: Scanning and removing worms received via IM messaging
In the wild
Malicious computer code that spreads in the real world as a result of normal day-to-day operations.
Type of spyware program that records the user's keystrokes invisibly and either transmits them to the attacker on an ongoing basis or saves them to a secret file in the user's computer to be sent at a later time.
Tip: Keylogger basics
Virus that infects a word processing application and causes a sequence of actions to be performed automatically when the application is started or something else triggers it. Macro viruses tend to be surprising but relatively harmless.
Tip: Securing your Office
The eventual effect of a software virus.
Series of messages sent by someone attempting to break into a computer to learn which computer network services, each associated with a "well-known" port number, the computer provides.
ITKnowledge exchange: Local LAN Vulnerabilities and Open Ports NAT
A collection of tools (programs) that enable administrator-level access to a computer or computer network. It allows an attacker to mask intrusion and gain root or privileged access to the computer and, possibly, other machines on the network.
Expert advice collection: Rootkit education
Derogatory term used to describe immature and unskilled -- but unfortunately still dangerous -- malware creators.
A non-technical kind of intrusion that relies heavily on human interaction and often involves tricking other people to break normal security procedures.
Tip: Online scams: Top 5 best of the worst
A virus that includes mechanisms that enable it to hide from anti-virus programs.
A piece of programming code usually disguised as something else that causes some unexpected and usually undesirable event. A virus is often designed so that it is automatically spread to other computer users. Generally, there are three main classes of viruses: file infectors, system or boot infectors, and macro viruses.
Expert response: The difference between antivirus and antispyware/malware software
A false warning about a computer virus. Virus hoaxes are usually forwarded using distribution lists and will typically suggest that the recipient forward the note to other distribution lists. If you get a message about a new virus, you can check it out by going to one of several Web sites that keep up with viruses and virus hoaxes.
Tip: Hoaxes: Nearly as deadly as a virus
Self-replicating virus that does not alter files but resides in active memory and duplicates itself. Worms use parts of an operating system that are automatic and usually invisible to the user. It is common for worms to be noticed only when their uncontrolled replication consumes system resources, slowing or halting other tasks.
Expert response: Displaying hidden windows