Which type of malware spreads quickly and targets networks without needing to attach itself to existing programs?

The selection of the worm as the correct answer is based on its defining characteristics and behavior within computer networks. A worm is a type of malware designed to replicate itself and spread to other computers without the need for a host program, distinguishing it from other types of malware. This autonomous propagation enables worms to infect systems rapidly, often exploiting vulnerabilities in network protocols or security flaws.

In contrast, a Trojan horse masquerades as legitimate software but requires human intervention for installation since it does not replicate itself. A virus is also reliant on a host program and spreads by attaching itself to legitimate files or software. Spyware, meanwhile, is designed to gather user information without their consent, but it does not focus on self-replication across networks.

These differences highlight why the worm is specifically recognized for its ability to spread quickly across networks on its own, making it a significant concern for maintaining network security.