Several incidents have occurred with a legacy web application that has had little development work...

Outdated librariesin a legacy web applicationintroduce security vulnerabilities, as theylack modern patchesandcontain known exploits​.

Option A (Misconfigured WAF)can contribute to security issues but is notinherent to legacy applications.

Option B (Data integrity failure)is apotential impactbut not a direct cause of recurring incidents.

Option D (Insufficient logging)affects detection, but theroot causeisinsecure, outdated components.

Thus,C (Outdated libraries) is the correct answer, aslegacy applications frequently suffer from unpatched vulnerabilities​.