Legacy IT is technology that remains in use even though it is older, harder to maintain, or no longer a good fit for current business needs. The label is not just about age: a system can become legacy when documentation is thin, specialists are scarce, patch support is ending, or changes are too risky to make quickly. In practice, this often includes old applications, operating systems, middleware, and terminal-style interfaces.
In cyber security, legacy IT matters because weak maintainability increases exposure. Unsupported components may miss security fixes, while fragile custom code can break during updates and delay remediation. Attackers often target legacy systems for known vulnerabilities, weak authentication, and poor monitoring. Defenders reduce risk by inventorying assets, prioritizing patching, segmenting critical services, limiting access, and planning modernization before the system becomes too costly to change safely.