Semgrep is a static analysis tool that scans source code by matching code patterns, much like a security-aware search engine for developers. It can run on many languages and is often used as part of SAST, or Static Application Security Testing, to catch risky code before it is deployed.
In security programs, Semgrep matters because it helps teams write their own rules for the patterns they care about most, such as unsafe function use, insecure input handling, or policy violations in internal frameworks. In real defenses, that means fewer blind spots than a generic scanner. In real attacks, the same idea is useful in reverse: defenders can create rules to detect suspicious code changes, hard-coded secrets, or exploit-prone constructs during review. Its value depends on rule quality, because weak rules create noisy false positives and reduce trust in the scanner.