A regression test is a repeatable check used to verify that a bug fixed today does not reappear after later code, configuration, or dependency changes. In software security, these tests are often added to a test suite or CI pipeline so teams can quickly confirm that a patch still blocks the original attack condition.
Regression testing matters because many vulnerabilities return when code is refactored or when a parser, proxy, or cache is tuned. For example, after fixing an HTTP desynchronization issue, defenders may keep malformed request cases in a regression set to ensure every layer still agrees on request boundaries. Attackers benefit when a fix is incomplete or a later change reopens the same edge case; defenders benefit when regression tests catch that drift before it reaches production. In practice, strong regression tests turn one-off bug fixes into durable security controls.