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.

Mode	Coverage	Findings Reported
Run all	All rulesets, all severity levels	Everything
Important only	All rulesets, pre- and post-filtered	Security vulns only, medium-high confidence/impact

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:

Step	Action	Gate	Key Reference
1	Resolve output dir, detect languages + Pro availability	—	Use Glob, not Bash
2	Select scan mode + rulesets	—	rulesets.md
3	Present plan, get explicit approval	⛔ HARD	AskUserQuestion
4	Spawn parallel scan Tasks	—	scanner-task-prompt.md
5	Merge results and report	—	Merge script (below)

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

Agent	Tools	Purpose
static-analysis:semgrep-scanner	Bash	Executes parallel semgrep scans for a language category

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

Rationalizations to Reject

Shortcut	Why It's Wrong
"User asked for scan, that's approval"	Original request ≠ plan approval. Present plan, use AskUserQuestion, await explicit "yes"
"Step 3 task is blocking, just mark complete"	Lying about task status defeats enforcement. Only mark complete after real approval
"I already know what they want"	Assumptions cause scanning wrong directories/rulesets. Present plan for verification
"Just use default rulesets"	User must see and approve exact rulesets before scan
"Add extra rulesets without asking"	Modifying approved list without consent breaks trust
"Third-party rulesets are optional"	Trail of Bits, 0xdea, Decurity catch vulnerabilities not in official registry — REQUIRED
"Use --config auto"	Sends metrics; less control over rulesets
"One Task at a time"	Defeats parallelism; spawn all Tasks together
"Pro is too slow, skip --pro"	Cross-file analysis catches 250% more true positives; worth the time
"Semgrep handles GitHub URLs natively"	URL handling fails on repos with non-standard YAML; always clone first
"Cleanup is optional"	Cloned repos pollute the user's workspace and accumulate across runs
"Use . or relative path as target"	Subagents need absolute paths to avoid ambiguity
"Let the user pick an output dir later"	Output directory must be resolved at Step 1, before any files are created

Reference Index

File	Content
rulesets.md	Complete ruleset catalog and selection algorithm
scan-modes.md	Pre/post-filter criteria and jq commands
scanner-task-prompt.md	Template for spawning scanner subagents

Workflow	Purpose
scan-workflow.md	Complete 5-step scan execution process

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/