Custom rules & RE2 safety

Any caller can attach ad-hoc rules to a single /scan call via customRules, using the same shape as a catalog entry minus the maintainer-only fields (level, mandatory, defaultEnabled). A tenant can also persist a reusable rule library with jaasctl guardrails push.

Example custom rule

{
  "id": "custom:acme:no-internal-hostname",
  "name": "No internal hostname",
  "category": "SUPPLY_CHAIN",
  "severity": "WARN",
  "kind": "regex_file_scan",
  "config": {
    "scope": "all_files",
    "patterns": [{ "name": "host", "regex": "\\.acme\\.internal\\b" }]
  }
}

Two deliberate restrictions, both security boundaries. A custom rule can only use an existing executor kind — it can never run arbitrary code, the same line Semgrep and gitleaks draw between “rule” and “engine.” And its regex is compiled with RE2, not Python’s re — guaranteed linear-time matching, so no caller-supplied pattern can trigger catastrophic backtracking (CWE-1333 / ReDoS). This means a custom pattern cannot use backreferences or lookaround ((?=, (?!, (?<=, (?<!); both /validate-rule and /scan reject such a pattern with a 400 rather than silently falling back to backtracking re. Catalog rules are unaffected — maintainer-reviewed before shipping, they keep using re and may use lookaround.

The guardrails service never persists a custom rule on its own — it validates and executes whatever arrives with a given request.

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Custom rules & RE2 safety