Viruses are sometimes also categorized by how they infect. These categorizations often overlap the categories above and may even be included in the description (e.g., polymorphic file virus). These categories include:
- Polymorphic Viruses
Viruses that change their characteristics as they infect.
- Stealth Viruses and Rootkits
Viruses that try to actively hide themselves from anti-virus or system software.
- Fast and Slow Infectors
Viruses that infect in a particular way to try to avoid specific anti-virus software.
- Sparse Infectors
Viruses that don’t infect very often.
- Armored Viruses
Viruses that are programmed to make disassembly difficult.
- Multipartite Viruses
Viruses that may fall into more than one of the top classes.
- Spacefiller (Cavity) Viruses
Viruses that attempt to maintain a constant file size when infecting.
- Tunneling Viruses
Viruses that try to “tunnel” under anti-virus software while infecting.
- Camouflage Viruses
Viruses that attempted to appear as a benign program to scanners.
- Metamorphic Viruses
Viruses that change from one form to another.
- NTFS ADS Viruses
Viruses that ride on the alternate data streams in the NT File System.
- Buffer Overflow
Viruses can sneak into the system when data overflows a buffer holding it.
- Botnet
Botnets can spread viruses and other malware automatically or on command.
- Social Engineering
This is probably the fastest way for a virus, worm or Trojan to spread.
- Peer-to-Peer Network
File sharing carries many risks when you don’t know where files originate.
- Search Poisoning
Viruses can be spread by Web pages constructed to be high in search results.
- Trusted Software or Site
There are many cases of viruses being spread on media or on the websites of major companies.
 Types of Viruses |
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| Vulnerabilities | Polymorphic Viruses |