semgrep-custom-rule-writing

Semgrep Custom Rule Writing

When to Use

• During a white-box penetration test or source code review when looking for bespoke vulnerabilities that generic SAST tools fail to detect.
• To codify and hunt for organization-specific anti-patterns (e.g., calling an internal API without passing the authentication context).

Prerequisites

• Authorized scope and target URLs from bug bounty program
• Burp Suite Professional (or Community) configured with browser proxy
• Familiarity with OWASP Top 10 and common web vulnerability classes
• SecLists wordlists for fuzzing and enumeration

Workflow

Phase 1: Understanding Semgrep Patterns

# Concept: Semgrep syntax rules:
  - id: simple-eval-detection
    pattern: eval(...)
    message: "Avoid using eval() as it can lead to code injection."
    languages: [javascript, python]
    severity: ERROR

Phase 2: Utilizing Metavariables

# rules:
  - id: python-exec-with-variable
    pattern: exec($X)
    message: "Using exec() on variable $X is dangerous."
    languages: [python]
    severity: WARNING

Phase 3: Pattern-Inside and Pattern-Not (Contextual Matching)

# rules:
  - id: missing-auth-check-flask
    patterns:
      - pattern-inside: |
          @app.route(...)
          def $FUNC(...):
            ...
      - pattern: return $RENDER(...)
      - pattern-not-inside: |
          @login_required
          def $FUNC(...):
            ...
      - pattern-not-inside: |
          if current_user.is_authenticated:
            ...
    message: "Flask route missing @login_required or explicit authentication check."
    languages: [python]
    severity: ERROR

Phase 4: Testing Rules via Semgrep CLI

# # semgrep --config custom-rules.yml src/

Decision Point 🔀

flowchart TD
    A[Identify Pattern ] --> B{Write Rule ]}
    B -->|Yes| C[Test Rule ]
    B -->|No| D[Refine Scope ]
    C --> E[Execute Scan ]

🔵 Blue Team Detection & Defense

• CI/CD Integration Pipeline Validation: Centralized Rule Repositories: Developer Education: Key Concepts | Concept | Description | |---------|-------------|