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npx versuz@latest install mukul975-anthropic-cybersecurity-skills-skills-implementing-semgrep-for-custom-sast-rulesgit clone https://github.com/mukul975/Anthropic-Cybersecurity-Skills.gitcp Anthropic-Cybersecurity-Skills/SKILL.MD ~/.claude/skills/mukul975-anthropic-cybersecurity-skills-skills-implementing-semgrep-for-custom-sast-rules/SKILL.md---
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.
domain: cybersecurity
subdomain: devsecops
tags:
- semgrep
- sast
- static-analysis
- custom-rules
- devsecops
- code-security
version: '1.0'
author: mahipal
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
- 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
```bash
# 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
```bash
# 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
### Basic Pattern Matching
```yaml
# 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,))
```
### Pattern Operators
```yaml
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
```
### Taint Analysis
```yaml
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"]
```
### Multiple Language Rule
```yaml
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"]
```
### Enforce Coding Standards
```yaml
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.*"
```
### JWT Security Rules
```yaml
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"]
```
## Rule Testing
```yaml
# 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,))
```
```bash
# Run rule tests
semgrep --test rules/
# Test specific rule
semgrep --config rules/sql-injection.yaml --test
```
## CI/CD Integration
### GitHub Actions
```yaml
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
```
### GitLab CI
```yaml
semgrep:
stage: test
image: returntocorp/semgrep
script:
- semgrep --config auto --config ./custom-rules/ --json --output semgrep.json .
artifacts:
reports:
sast: semgrep.json
```
## Configuration File
```yaml
# .semgrep.yaml
rules:
- id: my-org-rules
# ... rules here
# .semgrepignore
tests/
node_modules/
vendor/
*.min.js
```
## Best Practices
1. **Start with auto config** then add custom rules for org-specific patterns
2. **Test rules** with `# ruleid:` and `# ok:` annotations
3. **Use taint mode** for data flow vulnerabilities (XSS, SQLi, SSRF)
4. **Include metadata** (CWE, OWASP) for vulnerability classification
5. **Provide fix suggestions** with the `fix` key where possible
6. **Exclude test files** to reduce false positives
7. **Version control rules** in a shared repository
8. **Run in CI as a blocking check** for ERROR severity findings