Malicious code is software, script, or embedded logic written to behave in a harmful or unauthorized way. It may steal data, weaken security controls, create backdoors, sabotage systems, or change how legitimate software works. The code can be obvious malware, but it can also hide inside trusted packages, build scripts, updates, or automation tools.
In cyber security, malicious code matters because it can ride along with normal business processes and inherit trust from signed releases, approved accounts, or routine maintenance. Attackers often introduce it by compromising an identity, altering source code, or poisoning a software supply chain. Defenders look for unusual changes, unexpected network activity, suspicious dependencies, and release behavior that does not match normal patterns. Rapid review, strong authentication, code signing, and rollback procedures help limit the damage when malicious code is discovered.