Internet Security

Common Computer Virus Basics 104-4

Partition Table

PC disks are often split in logical blocks known as partitions. Information required to access these partitions, as well as a flag which indicates which partition should be used to boot the system (the active partition) is stored in the Master Boot Record. View also boot sector, boot sector and MBR viruses.

Polymorphism

A virus is said to be polymorphic if its code appears to be different every time it replicates (though generally each replication of the virus is functionally identical). This is usually achieved by encrypting the body of the virus, and adding a decryption routine which is different for each replication. When a polymorphic virus replicates, a portion of the decryption code is modified. Additionally, random, do-nothing blocks of code can be embedded in the program and are shuffled around to further vary the signature. In essence, it looks like a different program to virus scanners.

Stealth Virus

These viruses actively hide themselves while running. The first common virus, the BRAIN (discovered in the wild in 1986), was a stealth virus. It infected the boot sector of a floppy diskette and any attempt to read the boot sector with BRAIN active would be redirected to a copy of the original boot sector someplace else on the diskette. View also File Stealth and Full Stealth.

Trojan, Trojan Horse

A Trojan (or Trojan Horse) is a program which carries out an unauthorized function while hidden inside an authorized program. It is designed to do something other than what it claims to, and frequently is destructive in its actions.

Tunneling

Viruses that use tunneling techniques redirect all hard drive calls between their location in RAM and the operating system. This allows them to bypass any anti-viral products in memory at that time.

Virus

A virus is an independent program which reproduces itself. It may attach to other programs; it may create copies of itself (View companion viruses). It may attach itself to any executable code, including but not limited to boot sectors and/or partition sectors of hard and/or floppy disks. It may damage, corrupt or destroy data, or degrade system performance.

Virus Simulator

A virus simulator is a program which creates files that "look like" viruses. Such files are questionable for testing purposes because they are not really infected. Command will not to be fooled by a simulator.

Virus Variant

A variant is a modification of a previously known virus, i.e., a variation.

Worm

A worm is a program which reproduces by copying itself over and over, system to system. Worms are self-contained and generally use networks to spread.

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