Custom rules are user-defined detection logic that teach a scanner what suspicious code looks like in a specific environment. Instead of relying only on built-in checks, a security team writes patterns, conditions, and exceptions that match the languages, frameworks, and coding styles used in its own applications. In static analysis tools such as Semgrep, these rules help convert generic scanning into targeted application security testing.
They matter because default detections often miss organization-specific risks or generate too much noise. A well-written custom rule can flag insecure API use, dangerous deserialization, hardcoded secrets, or unsafe data handling before the code is deployed. Poorly written rules, however, can create false positives and reduce trust in the scanner. In real defenses, teams maintain custom rules like code: they review them, refine them, and retire them when the application changes. That maintenance is what keeps scanning useful against real attack paths.