Skills
skills/trailofbits/skills/semgrep

semgrep

SKILL.md

Semgrep Static Analysis

When to Use Semgrep

Ideal scenarios:

  • Quick security scans (minutes, not hours)
  • Pattern-based bug detection
  • Enforcing coding standards and best practices
  • Finding known vulnerability patterns
  • Single-file analysis without complex data flow
  • First-pass analysis before deeper tools

Consider CodeQL instead when:

  • Need interprocedural taint tracking across files
  • Complex data flow analysis required
  • Analyzing custom proprietary frameworks

When NOT to Use

Do NOT use this skill for:

  • Complex interprocedural data flow analysis (use CodeQL instead)
  • Binary analysis or compiled code without source
  • Custom deep semantic analysis requiring AST/CFG traversal
  • When you need to track taint across many function boundaries

Installation

# pip
python3 -m pip install semgrep

# Homebrew
brew install semgrep

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

# Update
pip install --upgrade semgrep

Core Workflow

1. Quick Scan

semgrep --config auto .                    # Auto-detect rules
semgrep --config auto --metrics=off .      # Disable telemetry for proprietary code

2. Use Rulesets

semgrep --config p/<RULESET> .             # Single ruleset
semgrep --config p/security-audit --config p/trailofbits .  # Multiple
Ruleset Description
p/default General security and code quality
p/security-audit Comprehensive security rules
p/owasp-top-ten OWASP Top 10 vulnerabilities
p/cwe-top-25 CWE Top 25 vulnerabilities
p/r2c-security-audit r2c security audit rules
p/trailofbits Trail of Bits security rules
p/python Python-specific
p/javascript JavaScript-specific
p/golang Go-specific

3. Output Formats

semgrep --config p/security-audit --sarif -o results.sarif .   # SARIF
semgrep --config p/security-audit --json -o results.json .     # JSON
semgrep --config p/security-audit --dataflow-traces .          # Show data flow

4. Scan Specific Paths

semgrep --config p/python app.py           # Single file
semgrep --config p/javascript src/         # Directory
semgrep --config auto --include='**/test/**' .  # Include tests (excluded by default)

Writing Custom Rules

Basic Structure

rules:
  - id: hardcoded-password
    languages: [python]
    message: "Hardcoded password detected: $PASSWORD"
    severity: ERROR
    pattern: password = "$PASSWORD"

Pattern Syntax

Syntax Description Example
... Match anything func(...)
$VAR Capture metavariable $FUNC($INPUT)
<... ...> Deep expression match <... user_input ...>

Pattern Operators

Operator Description
pattern Match exact pattern
patterns All must match (AND)
pattern-either Any matches (OR)
pattern-not Exclude matches
pattern-inside Match only inside context
pattern-not-inside Match only outside context
pattern-regex Regex matching
metavariable-regex Regex on captured value
metavariable-comparison Compare values

Combining Patterns

rules:
  - id: sql-injection
    languages: [python]
    message: "Potential SQL injection"
    severity: ERROR
    patterns:
      - pattern-either:
          - pattern: cursor.execute($QUERY)
          - pattern: db.execute($QUERY)
      - pattern-not:
          - pattern: cursor.execute("...", (...))
      - metavariable-regex:
          metavariable: $QUERY
          regex: .*\+.*|.*\.format\(.*|.*%.*

Taint Mode (Data Flow)

Simple pattern matching finds obvious cases:

# Pattern `os.system($CMD)` catches this:
os.system(user_input)  # Found

But misses indirect flows:

# Same pattern misses this:
cmd = user_input
processed = cmd.strip()
os.system(processed)  # Missed - no direct match

Taint mode tracks data through assignments and transformations:

  • Source: Where untrusted data enters (user_input)
  • Propagators: How it flows (cmd = ..., processed = ...)
  • Sanitizers: What makes it safe (shlex.quote())
  • Sink: Where it becomes dangerous (os.system())
rules:
  - id: command-injection
    languages: [python]
    message: "User input flows to command execution"
    severity: ERROR
    mode: taint
    pattern-sources:
      - pattern: request.args.get(...)
      - pattern: request.form[...]
      - pattern: request.json
    pattern-sinks:
      - pattern: os.system($SINK)
      - pattern: subprocess.call($SINK, shell=True)
      - pattern: subprocess.run($SINK, shell=True, ...)
    pattern-sanitizers:
      - pattern: shlex.quote(...)
      - pattern: int(...)

Full Rule with Metadata

rules:
  - id: flask-sql-injection
    languages: [python]
    message: "SQL injection: user input flows to query without parameterization"
    severity: ERROR
    metadata:
      cwe: "CWE-89: SQL Injection"
      owasp: "A03:2021 - Injection"
      confidence: HIGH
    mode: taint
    pattern-sources:
      - pattern: request.args.get(...)
      - pattern: request.form[...]
      - pattern: request.json
    pattern-sinks:
      - pattern: cursor.execute($QUERY)
      - pattern: db.execute($QUERY)
    pattern-sanitizers:
      - pattern: int(...)
    fix: cursor.execute($QUERY, (params,))

Testing Rules

Test File Format

# test_rule.py
def test_vulnerable():
    user_input = request.args.get("id")
    # ruleid: flask-sql-injection
    cursor.execute("SELECT * FROM users WHERE id = " + user_input)

def test_safe():
    user_input = request.args.get("id")
    # ok: flask-sql-injection
    cursor.execute("SELECT * FROM users WHERE id = ?", (user_input,))

semgrep --test rules/

CI/CD Integration (GitHub Actions)

name: Semgrep

on:
  push:
    branches: [main]
  pull_request:
  schedule:
    - cron: '0 0 1 * *'  # Monthly

jobs:
  semgrep:
    runs-on: ubuntu-latest
    container:
      image: returntocorp/semgrep

    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0  # Required for diff-aware scanning

      - name: Run Semgrep
        run: |
          if [ "${{ github.event_name }}" = "pull_request" ]; then
            semgrep ci --baseline-commit ${{ github.event.pull_request.base.sha }}
          else
            semgrep ci
          fi
        env:
          SEMGREP_RULES: >-
            p/security-audit
            p/owasp-top-ten
            p/trailofbits

Configuration

.semgrepignore

tests/fixtures/
**/testdata/
generated/
vendor/
node_modules/

Suppress False Positives

password = get_from_vault()  # nosemgrep: hardcoded-password
dangerous_but_safe()  # nosemgrep

Performance

semgrep --config rules/ --time .    # Check rule performance
ulimit -n 4096                       # Increase file descriptors for large codebases

Path Filtering in Rules

rules:
  - id: my-rule
    paths:
      include: [src/]
      exclude: [src/generated/]

Third-Party Rules

pip install semgrep-rules-manager
semgrep-rules-manager --dir ~/semgrep-rules download
semgrep -f ~/semgrep-rules .

Rationalizations to Reject

Shortcut Why It's Wrong
"Semgrep found nothing, code is clean" Semgrep is pattern-based; it can't track complex data flow across functions
"I wrote a rule, so we're covered" Rules need testing with semgrep --test; false negatives are silent
"Taint mode catches injection" Only if you defined all sources, sinks, AND sanitizers correctly
"Pro rules are comprehensive" Pro rules are good but not exhaustive; supplement with custom rules for your codebase
"Too many findings = noisy tool" High finding count often means real problems; tune rules, don't disable them

Resources

Weekly Installs
25
Install
$ npx skills add trailofbits/skills --skill "semgrep"
Repository
trailofbits/skills
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