A computer worm is a type of malware that can automatically propagate or self-replicate without human interaction, enabling its spread to other computers across a network. Computer worms are standalone malware computer programs that replicate themselves in order to spread to other computers, often using a computer network to spread itself, relying on security failures on the target computer to access it. Worms use recursive methods to copy themselves without host programs and distribute themselves based on exploiting the advantages of exponential growth, thus controlling and infecting more and more computers in a short time. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.

Computer worms can spread across a network through the internet or LAN (Local Area Network) connection, and can infect USB sticks and external hard drives. Once a computer worm has breached a computers defenses, it can perform several malicious actions, such as dropping other malware like spyware or ransomware, consuming bandwidth, deleting files, overloading networks, stealing data, and opening a backdoor. Worms can also delete and modify files, inject more malicious software into a workstation or other device, and steal sensitive data.

Some worms may install a backdoor, allowing the computer to be remotely controlled by the worm author as a "zombie". Networks of such machines are often referred to as botnets and are very commonly used for a range of malicious purposes, including sending spam or performing DoS attacks.

There are several types of malicious computer worms, including email worms, file-sharing worms, instant messaging worms, and website worms. Successful email worms usually employ social engineering and phishing techniques to encourage users to open the attached file. Worms can target vulnerabilities in operating systems to install themselves into networks, gain access through backdoors built into software, through unintentional software vulnerabilities, or through flash drives.

To prevent computer worms, it is important to keep software up to date, use strong passwords, and avoid opening suspicious emails or attachments. Antivirus software can also help detect and remove worms.