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Worms

Computer worms are a type of self-replicating malware that spread automatically across networks without requiring user interaction.

What is a worm?How it typically works?Common techniques & variantsImpactFurther reading

What is a worm?

A worm is different from Trojans or viruses because it does not rely on tricking users. Instead, worms spread by exploiting software vulnerabilities, weak configurations, or open network services. Once active, they scan for new targets and replicate themselves rapidly. Famous cases include the **Morris Worm** in 1988, one of the first to cause widespread disruption, and **Conficker**, which infected millions of systems globally by abusing Windows flaws.

How it typically works?

  1. Exploitation: the worm identifies a vulnerability in operating systems, applications, or network protocols.
  2. Replication: once inside, it copies itself to the infected machine.
  3. Propagation: the worm scans for new vulnerable systems and automatically spreads.
  4. Payload execution: in addition to replication, worms may drop ransomware, backdoors, or spyware.

Common techniques & variants

  • Email worms: spread by sending infected attachments to a victim’s contact list. Examples include ILOVEYOU and Mydoom.
  • Network worms: exploit network vulnerabilities to move laterally. SQL Slammer and Conficker are well-known cases.
  • Instant messaging worms: spread through chat platforms by sending infected links.
  • File-sharing worms: copy themselves into shared directories or removable drives.
  • Hybrid worms: combine multiple methods of spreading, often paired with Trojan functionality.

Impact

Worms can overwhelm networks, disable systems, and deliver additional malware at massive scale. The Conficker outbreak alone impacted government and enterprise systems worldwide for years. Worms like WannaCry demonstrated how quickly ransomware could spread using worm techniques. For SecOps teams, worms remain a critical concern because they can bypass user awareness training and cause automated, large-scale compromise within hours.

Further reading

Related topics

MalwareTrojansRansomwareSpywareFileless Malware