implementing-semgrep-for-custom-sast-rules

Write custom Semgrep SAST rules in YAML to detect application-specific vulnerabilities, enforce coding standards, and integrate into CI/CD pipelines. Use when writeing custom semgrep sast rules in yaml to detect application-specific.

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Letzte Quellaktivität: 31. Juli 2026 um 15:22
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name	implementing-semgrep-for-custom-sast-rules
description	Write custom Semgrep SAST rules in YAML to detect application-specific vulnerabilities, enforce coding standards, and integrate into CI/CD pipelines. Use when writeing custom semgrep sast rules in yaml to detect application-specific.
domain	cybersecurity
subdomain	devsecops
tags	["semgrep","sast","static-analysis","custom-rules","devsecops","code-security"]
version	1.0
author	oyi77
license	Apache-2.0
nist_csf	["PR.PS-01","GV.SC-07","ID.IM-04","PR.PS-04"]

Implementing Semgrep for Custom SAST Rules

Overview

Semgrep is an open-source static analysis tool that uses pattern-matching to find bugs, enforce code standards, and detect security vulnerabilities. Custom rules are written in YAML using Semgrep's pattern syntax, making it accessible without requiring compiler knowledge. It supports 30+ languages including Python, JavaScript, Go, Java, and C.

When to Use

Trigger phrases:

"implementing semgrep for custom sast rules"
"Write custom Semgrep SAST rules in YAML to detect application-specific vulnerabi"
When deploying or configuring implementing semgrep for custom sast rules capabilities in your environment
When establishing security controls aligned to compliance requirements
When building or improving security architecture for this domain
When conducting security assessments that require this implementation

Prerequisites

Python 3.8+ or Docker
Semgrep CLI installed
Target codebase in a supported language

Installation

# Install via pip
pip install semgrep

# Install via Homebrew
brew install semgrep

# Run via Docker
docker run -v "${PWD}:/src" returntocorp/semgrep semgrep --config auto /src

# Verify
semgrep --version

Running Semgrep

# Auto-detect rules for your code
semgrep --config auto .

# Use Semgrep registry rules
semgrep --config r/python.lang.security

# Use custom rule file
semgrep --config my-rules.yaml .

# Use multiple configs
semgrep --config auto --config ./custom-rules/ .

# JSON output
semgrep --config auto --json . > results.json

# SARIF output for GitHub
semgrep --config auto --sarif . > results.sarif

# Filter by severity
semgrep --config auto --severity ERROR .

Writing Custom Rules

This section covers writing custom rules for implementing semgrep for custom sast rules.

Ensure all prerequisites are met before proceeding
Follow the documented workflow steps in sequence

# rules/sql-injection.yaml
rules:
  - id: sql-injection-string-format
    languages: [python]
    severity: ERROR
    message: |
      Potential SQL injection via string formatting.
      Use parameterized queries instead.
    pattern: |
      cursor.execute(f"..." % ...)
    metadata:
      cwe: ["CWE-89"]
      owasp: ["A03:2021"]
      category: security
    fix: |
      cursor.execute("SELECT * FROM users WHERE id = %s", (user_id,))

rules:
  - id: hardcoded-secret-in-code
    languages: [python, javascript, typescript]
    severity: ERROR
    message: Hardcoded secret detected in source code
    patterns:
      - pattern-either:
          - pattern: $VAR = "..."
          - pattern: $VAR = '...'
      - metavariable-regex:
          metavariable: $VAR
          regex: (?i)(password|secret|api_key|token|aws_secret)
      - pattern-not: $VAR = ""
      - pattern-not: $VAR = "changeme"
      - pattern-not: $VAR = "PLACEHOLDER"
    metadata:
      cwe: ["CWE-798"]
      category: security

rules:
  - id: xss-taint-tracking
    languages: [python]
    severity: ERROR
    message: User input flows to HTML response without sanitization
    mode: taint
    pattern-sources:
      - pattern: request.args.get(...)
      - pattern: request.form.get(...)
      - pattern: request.form[...]
    pattern-sinks:
      - pattern: return render_template_string(...)
      - pattern: Markup(...)
    pattern-sanitizers:
      - pattern: bleach.clean(...)
      - pattern: escape(...)
    metadata:
      cwe: ["CWE-79"]
      owasp: ["A03:2021"]

rules:
  - id: insecure-random
    languages: [python, javascript, go, java]
    severity: WARNING
    message: |
      Using insecure random number generator. Use cryptographically
      secure alternatives for security-sensitive operations.
    pattern-either:
      # Python
      - pattern: random.random()
      - pattern: random.randint(...)
      # JavaScript
      - pattern: Math.random()
      # Go
      - pattern: math/rand.Intn(...)
      # Java
      - pattern: new java.util.Random()
    metadata:
      cwe: ["CWE-330"]

rules:
  - id: require-error-handling
    languages: [go]
    severity: WARNING
    message: Error return value not checked
    pattern: |
      $VAR, _ := $FUNC(...)
    fix: |
      $VAR, err := $FUNC(...)
      if err != nil {
        return fmt.Errorf("$FUNC failed: %w", err)
      }

  - id: no-console-log-in-production
    languages: [javascript, typescript]
    severity: WARNING
    message: Remove console.log before merging to production
    pattern: console.log(...)
    paths:
      exclude:
        - "tests/*"
        - "*.test.*"

rules:
  - id: jwt-none-algorithm
    languages: [python]
    severity: ERROR
    message: JWT decoded without algorithm verification - allows token forgery
    patterns:
      - pattern: jwt.decode($TOKEN, ..., algorithms=["none"], ...)
    metadata:
      cwe: ["CWE-347"]

  - id: jwt-no-verification
    languages: [python]
    severity: ERROR
    message: JWT decoded with verification disabled
    patterns:
      - pattern: jwt.decode($TOKEN, ..., options={"verify_signature": False}, ...)
    metadata:
      cwe: ["CWE-345"]

# rules/test-sql-injection.yaml
rules:
  - id: sql-injection-format-string
    languages: [python]
    severity: ERROR
    message: SQL injection via format string
    pattern: |
      cursor.execute(f"...{$VAR}...")

# Test annotation in test file:
# test-sql-injection.py
def bad_query(user_id):
    # ruleid: sql-injection-format-string
    cursor.execute(f"SELECT * FROM users WHERE id = {user_id}")

def good_query(user_id):
    # ok: sql-injection-format-string
    cursor.execute("SELECT * FROM users WHERE id = %s", (user_id,))

# Run rule tests
semgrep --test rules/

# Test specific rule
semgrep --config rules/sql-injection.yaml --test

name: Semgrep SAST
on: [pull_request]

jobs:
  semgrep:
    runs-on: ubuntu-latest
    container:
      image: returntocorp/semgrep
    steps:
      - uses: actions/checkout@v4

      - name: Run Semgrep
        run: |
          semgrep --config auto \
            --config ./custom-rules/ \
            --sarif --output results.sarif \
            --severity ERROR \
            .

      - name: Upload SARIF
        uses: github/codeql-action/upload-sarif@v3
        with:
          sarif_file: results.sarif

semgrep:
  stage: test
  image: returntocorp/semgrep
  script:
    - semgrep --config auto --config ./custom-rules/ --json --output semgrep.json .
  artifacts:
    reports:
      sast: semgrep.json

# .semgrep.yaml
rules:
  - id: my-org-rules
    # ... rules here

# .semgrepignore
tests/
node_modules/
vendor/
*.min.js

Rationalization	Reality
"We are too small to be targeted"	Automated attacks target everyone. Size does not matter.
"Security slows us down"	A breach slows you down 100x more. Build security in from the start.
"We will fix it after launch"	Vulnerabilities in production are exploited within hours. Fix before deploy.