Semgrep

Semgrep Security Scan

Run a Semgrep scan with automatic language detection, parallel execution via Task subagents, and merged SARIF output.

Essential Principles

1. Always use --metrics=off — Semgrep sends telemetry by default; --config auto also phones home. Every semgrep command must include --metrics=off to prevent data leakage during security audits.
2. User must approve the scan plan (Step 3 is a hard gate) — The original "scan this codebase" request is NOT approval. Present exact rulesets, target, engine, and mode; wait for explicit "yes"/"proceed" before spawning scanners.
3. Third-party rulesets are required, not optional — Trail of Bits, 0xdea, and Decurity rules catch vulnerabilities absent from the official registry. Include them whenever the detected language matches.
4. Spawn all scan Tasks in a single message — Parallel execution is the core performance advantage. Never spawn Tasks sequentially; always emit all Task tool calls in one response.
5. Always check for Semgrep Pro before scanning — Pro enables cross-file taint tracking and catches ~250% more true positives. Skipping the check means silently missing critical inter-file vulnerabilities.

When to Use

• Security audit of a codebase
• Finding vulnerabilities before code review
• Scanning for known bug patterns
• First-pass static analysis

When NOT to Use

• Binary analysis → Use binary analysis tools
• Already have Semgrep CI configured → Use existing pipeline
• Need cross-file analysis but no Pro license → Consider CodeQL as alternative
• Creating custom Semgrep rules → Use semgrep-rule-creator skill
• Porting existing rules to other languages → Use semgrep-rule-variant-creator skill

Output Directory

All scan results, SARIF files, and temporary data are stored in a single output directory.

• If the user specifies an output directory in their prompt, use it as OUTPUT_DIR.
• If not specified, default to ./static_analysis_semgrep_1. If that already exists, increment to _2, _3, etc.

In both cases, always create the directory with mkdir -p before writing any files.

# Resolve output directory
if [ -n "$USER_SPECIFIED_DIR" ]; then
  OUTPUT_DIR="$USER_SPECIFIED_DIR"
else
  BASE="static_analysis_semgrep"
  N=1
  while [ -e "${BASE}_${N}" ]; do
    N=$((N + 1))
  done
  OUTPUT_DIR="${BASE}_${N}"
fi
mkdir -p "$OUTPUT_DIR/raw" "$OUTPUT_DIR/results"

The output directory is resolved once at the start of Step 1 and used throughout all subsequent steps.

$OUTPUT_DIR/
├── rulesets.txt                 # Approved rulesets (logged after Step 3)
├── raw/                         # Per-scan raw output (unfiltered)
│   ├── python-python.json
│   ├── python-python.sarif
│   ├── python-django.json
│   ├── python-django.sarif
│   └── ...
└── results/                     # Final merged output
    └── results.sarif

Prerequisites

Required: Semgrep CLI (semgrep --version). If not installed, see Semgrep installation docs.

Optional: Semgrep Pro — enables cross-file taint tracking, inter-procedural analysis, and additional languages (Apex, C#, Elixir). Check with:

semgrep --pro --validate --config p/default 2>/dev/null && echo "Pro available" || echo "OSS only"

Limitations: OSS mode cannot track data flow across files. Pro mode uses -j 1 for cross-file analysis (slower per ruleset, but parallel rulesets compensate).

Scan Modes

Select mode in Step 2 of the workflow. Mode affects both scanner flags and post-processing.

Important only applies two filter layers:

1. Pre-filter: --severity MEDIUM --severity HIGH --severity CRITICAL (CLI flag)
2. Post-filter: JSON metadata — keeps only category=security, confidence∈{MEDIUM,HIGH}, impact∈{MEDIUM,HIGH}

See scan-modes.md for metadata criteria and jq filter commands.

Orchestration Architecture

┌──────────────────────────────────────────────────────────────────┐
│ MAIN AGENT (this skill)                                          │
│ Step 1: Detect languages + check Pro availability                │
│ Step 2: Select scan mode + rulesets (ref: rulesets.md)           │
│ Step 3: Present plan + rulesets, get approval [⛔ HARD GATE]     │
│ Step 4: Spawn parallel scan Tasks (approved rulesets + mode)     │
│ Step 5: Merge results and report                                 │
└──────────────────────────────────────────────────────────────────┘
         │ Step 4
         ▼
┌─────────────────┐
│ Scan Tasks      │
│ (parallel)      │
├─────────────────┤
│ Python scanner  │
│ JS/TS scanner   │
│ Go scanner      │
│ Docker scanner  │
└─────────────────┘

Workflow

Follow the detailed workflow in scan-workflow.md. Summary:

Task enforcement: On invocation, create 5 tasks with blockedBy dependencies (each step blocks the previous). Step 3 is a HARD GATE — mark complete ONLY after user explicitly approves.

Merge command (Step 5):

uv run {baseDir}/scripts/merge_sarif.py $OUTPUT_DIR/raw $OUTPUT_DIR/results/results.sarif

Agents

Use subagent_type: static-analysis:semgrep-scanner in Step 4 when spawning Task subagents.

Rationalizations to Reject

Reference Index

Success Criteria

• Output directory resolved (user-specified or auto-incremented default)
• All generated files stored inside $OUTPUT_DIR
• Languages detected with file counts; Pro status checked
• Scan mode selected by user (run all / important only)
• Rulesets include third-party rules for all detected languages
• User explicitly approved the scan plan (Step 3 gate passed)
• All scan Tasks spawned in a single message and completed
• Every semgrep command used --metrics=off
• Approved rulesets logged to $OUTPUT_DIR/rulesets.txt
• Raw per-scan outputs stored in $OUTPUT_DIR/raw/
• results.sarif exists in $OUTPUT_DIR/results/ and is valid JSON
• Important-only mode: post-filter applied before merge; unfiltered results preserved in raw/
• Results summary reported with severity and category breakdown
• Cloned repos (if any) cleaned up from $OUTPUT_DIR/repos/

Source: trailofbits/skills → plugins/static-analysis/skills/semgrep/SKILL.md