AI-assisted security code reviewer, built on a hybrid architecture: Semgrep does detection, Claude does triage. Claude never invents a finding Semgrep didn't already flag.

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Why hybrid, not pure-LLM

Asking an LLM to freely hunt for vulnerabilities produces too many false positives and false negatives to be credible — that's the classic failure mode of "AI security tools." vuln-hunter splits the job instead:

• Semgrep (a real, widely-used static analysis engine) does detection against known rule patterns — the ground truth.
• Claude is strictly downstream of that: it explains why a matched finding is risky in context, rates exploitability, and suggests a concrete fix. It does not go looking for problems Semgrep didn't already surface.

The one deliberate exception is the business-logic reasoning pass (below) — and even that is kept honestly separate and clearly labeled, never mixed in with rule-confirmed findings.

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What's Included

Detection — scanner.py

Wraps semgrep scan, enriches every finding with the real source snippet around the match. Community rule packs (p/security-audit, p/secrets) alone missed common raw-Python patterns entirely — three custom rules (rules/custom-python-security.yml) close that gap: SQL injection via string concatenation into cursor.execute(), shell injection via os.system(), and hardcoded secrets by variable-name heuristic. Verified against a planted 4-vulnerability fixture: 1/4 caught with community packs alone → 4/4 with custom rules, 0 false positives on a matched clean file.

Triage — triage.py

Claude explains, rates exploitability, and suggests a fix for each finding Semgrep already flagged — system-prompt-constrained so it can't wander off and invent new ones.

Business-logic reasoning pass — business_logic.py

The most novel piece: a second Claude pass that reasons about intent — missing authorization/ownership checks, the kind of bug that's structurally invisible to pattern-matching. Every finding from this pass is tagged finding_type: "ai_reasoning" (vs "rule_confirmed" from the Semgrep path) and shown with a distinct badge in the frontend — never silently blended in. Verified against three fixtures: a real IDOR bug with zero ownership check (caught, 2/2, high confidence), the identical function shape with a real ownership check added (correctly zero findings — proof it's reasoning about the actual check, not the function name), and an unrelated clean fixture (zero manufactured noise).

This pass found a real bug in vuln-hunter's own code: no ownership check on the /ignore and /scan endpoints, plus the ignore-list leaking suppressed-finding info to any caller. Investigating why that mattered surfaced a separate, more serious issue — the server was bound to 0.0.0.0 (every network interface) instead of localhost, meaning anyone on the same network could have reached those unauthenticated endpoints. Fixed: defaults to 127.0.0.1 now, override via API_HOST.

MCP server — mcp_server.py

Exposes scan_repo, scan_diff, ignore_finding, and list_ignored as MCP tools, registered globally so any Claude Code session can call them directly — not just this repo. Verified end to end with real Anthropic API calls against a planted vulnerability fixture, not just imported and assumed working.

SARIF 2.1.0 output

The format GitHub's Security tab and most CI tooling consume. Output is validated against the real official schema (schemastore.org/sarif-2.1.0.json) with jsonschema, not eyeballed.

Suppression / ignore list

Persistent per-repo .vulnhunter-ignore.json, fingerprinted on the exact matched code (not line numbers, not the padded display snippet) so marking a finding safe survives unrelated nearby edits.

Diff-only scanning

Scans only files changed vs. a ref instead of the whole repo — practical for CI/PR use. Unions git diff with git ls-files --others --exclude-standard so brand-new, never-git add-ed files aren't invisible to it.

Never-read guarantee for credentials

.env, *.pem, *.key, id_rsa, credentials.json, and similar are excluded at three layers — passed as --exclude to Semgrep itself (confirmed via Semgrep's own scanned-paths list, not just trusted), a defensive filter on returned findings, and again at the source-snippet read call site. Regression-tested with fake credential fixtures in a temp dir.

Frontend

Next.js 16 + Tailwind 4 dashboard — repo-path input, scan button, findings list with exploitability-colored badges, expandable per-finding detail (source snippet, explanation, markdown-rendered fix, CWE/OWASP).

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Quick Start

# Backend
cd backend
pip install -r requirements.txt
python main.py          # binds to 127.0.0.1:8001 by default

# Frontend (separate terminal)
cd frontend
npm install
npm run dev

The API is local-only by default — it can scan arbitrary filesystem paths and has no authentication, so it must never be reachable from the network without adding auth first.

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Known Limitations

• Only Python has custom rules written/tested so far (community packs cover other languages, but with the same blind spots noted above)
• No formal test suite yet — manual smoke tests only (test_*.py in backend/)
• Not yet deployed anywhere — runs locally
• deep_review (the business-logic pass) is API-only right now; no frontend toggle for it yet, and the diff-scan endpoint has no dedicated UI either

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License

Not yet licensed — all rights reserved by default until a license is chosen.

Author

Chris Brown
Albatross AI
Portfolio