Malware: Worms, Trojans, and Viruses
Malware is an abbreviated term for "malicious software". It refers to any software that is designed to specifically disrupt or damage a system. Malware includes such things as worms, Trojan horse, and viruses.
A Trojan horse is a program that appears to be useful or at the very least harmless. However, it has been designed to contain hidden code to exploit or damage a computer system. A Trojan horse neither replicates nor copies itself, but performs some illicit activity when it is run. It stays in the computer doing its damage or allows somebody from a remote site to take control of the computer.
There are other terms that are associated with Trojan Horses: Remote Access Trojans, and Rootkits. Remote Access Trojans are commonly referred as backdoors. They allow someone else to control your computer from a remote location. A rootkit is a collection of software programs that once installed allow someone to gain unauthorized remote access to your computer
A self-contained program (or set of programs) that is able to spread copies of itself to other computer systems. A worm can consume network or local system resources. They can cause a denial of service attack. A worm may also deliver other malware such as keyloggers in addition to spreading itself.
A virus is a program code that can cause damage to hardware, software or data. Virus code is usually buried within the code of another program (file, document or boot sector of a disk) and once executed it will attempt to replicate itself by infecting other hosts across the network. Some times viruses are used to deliver other types of malware such as a Trojan horse.
How Malware spreads
There are many different methods that malware will attempt to use to infect computer systems:
Remote vulnerability exploit: This occurs when a piece of malware attempts to gain access to a computer system by exploiting vulnerability in a service or an application. This is seen in worms.
Email: Email is an easy way for attackers to transport Malware to hundreds of thousands of people. It is very easy to trick users into opening e-mails or opening e-mail attachments using social engineering techniques.
Network Shares: Network files systems with poorly implemented security produces an environment where malware can spread to a large number of computer.
Removable media: This means of transport started with floppy disk and has spread to such devices as Universal Serial Bus(USB) and Firewire devices.
Network scanning : Malware writer use this mechanism to scan networks for vulnerable computers that are susceptible to remote exploits.
Peer-to-peer (P2P) networks - Users install a client component that makes their file system accessible to other computers across the network.