Local Rule

What a local rule does in Semgrep

A local rule is a team-owned Semgrep rule that extends the scan to patterns your standard rule packs do not cover. Because it is authored and managed locally, it can reflect an organisation’s own frameworks, code conventions, and risk priorities without waiting for an upstream rule release.

That makes local rules especially useful for catching proprietary idioms, internal helper functions, wrapper libraries, and security-sensitive patterns that only exist in one codebase or product family. They are also a practical way to translate policy intent into code checks when a generic rule would be too broad or too noisy.

How local rules are defined and reused

Local rules can be created for a one-off review or stored in YAML for repeated use. In practice, the difference is less about the syntax and more about operational ownership: ephemeral rules are often used to answer a narrow question during investigation, while reusable rules become part of a team’s ongoing scan library.

That reuse matters because teams can version-control the rule, review it like code, and refine it as the application evolves. A local rule is therefore both a detection artifact and a living decision about what the organisation considers important enough to check continuously.

Why local rules matter for coverage and signal quality

Standard rule sets are necessarily general, so they will miss some organisation-specific behaviors. Local rules close that gap by allowing teams to encode context the scanner cannot infer on its own, such as internal authentication helpers, custom deserialisation paths, or unsafe wrapper functions that appear benign in isolation.

They also help reduce false negatives and improve alert quality when used carefully. A well-designed local rule can focus on the exact code shape the team cares about, rather than forcing analysts to sift through generic findings that are technically valid but operationally unhelpful.

Where local rules fit in a security programme

Local rules work best as part of a wider secure development and code review practice, not as a substitute for upstream rule coverage. They are most valuable when teams treat them as maintained controls: tested against real code, reviewed when patterns change, and retired when the underlying risk disappears.

They can also surface governance issues. If a team needs many local rules for the same class of weakness, that often indicates a recurring design or library choice that should be addressed upstream in architecture, shared components, or training rather than patched repeatedly at scan time.

Risk and Threat Considerations

Local rules can become a blind spot if they are poorly maintained, because they only protect the patterns the team has explicitly encoded. Over time, stale rules may miss new code paths, while overly broad rules can bury real findings in noise and train reviewers to ignore alerts.

Failure mechanism: Attackers or bugs exploit code constructs that are unique to the organisation, but the scanner does not flag them because no local rule captures the pattern, or because the rule is too imprecise to be actionable.

Impact: Security reviews lose coverage exactly where custom application logic is most likely to differ from public examples, which can allow unsafe input handling, privilege checks, or secret handling issues to persist into production.

Practitioner Guidance

Governance implication: Treat local rules as owned security content, not ad hoc analyst notes. Give them a clear maintainer, review cadence, and naming convention so teams can tell which rules are authoritative and which are temporary investigation aids.

What to watch for: Repeated creation of similar local rules is often a sign that the same weakness is reappearing across services or repositories. When that happens, promote the pattern into a shared rule or address the underlying coding practice instead of letting each team reinvent its own detection.

Related resources from NHI Mgmt Group

• Why are local .env files and config notes risky in Microsoft 365?
• How should teams respond to a local Linux privilege escalation flaw in shared environments?
• What is the difference between global identity strategy and local governance?
• How should security teams handle local accounts in cloud and SaaS apps?