Security Review Skill for Claude

Security Review Skill

Run a two-layer security review over the current git changes: a deterministic

regex scan for known vulnerability classes, then an LLM-depth Security review of

the diff. Report a single BLOCK / FIX / APPROVE verdict.

The LLM-depth review runs inside the current Claude session — the same

subscription that loaded this skill. There is no separate model call, no

ANTHROPIC_API_KEY, no Agent SDK, and no network request. The "reviewer" is the

session agent executing the steps below, exactly like every other skill here.

Detection reaches parity with Anthropic's security-guidance plugin: the scanner

ports its 25 deterministic patterns, and the LLM pass applies its full review

taxonomy (loaded on demand from references/coverage.md).

Reference Loading Table

| Signal | Load | Why |

|--------|------|-----|

| Running Phase 3 (LLM-depth review); classifying a finding; needing the vuln taxonomy, severity rubric, FP exclusions, or per-language guidance | references/coverage.md | 40 vulnerability classes + 4-tier severity + false-positive exclusions + per-language guidance + the 12 high-miss reviewer classes + the finding output schema. |

Instructions

Phase 1: SCOPE

Goal: Determine the changed files to review before scanning.

Step 1: List changed files — scope to the working-tree and staged changes so

the review covers exactly what the user is about to commit, not the whole repo.

# Tracked changes (working tree + index) plus staged adds:
git diff --name-only HEAD
git diff --cached --name-only --diff-filter=ACM

Step 2: Read repository CLAUDE.md to load project conventions the reviewer

must respect (e.g. secrets-handling rules, allowed patterns).

Gate: Changed files listed. When the list is empty, report "no changes to

review" and stop — there is nothing to scan.

Phase 2: DETERMINISTIC SCAN

Goal: Run the regex engine first so judgment time is spent on real signal,

not on patterns a script catches deterministically.

Step 1: Run the scanner over the changed files. It is the single source of

detection rules (secrets, SQL injection, shell injection, dangerous eval,

unsafe deserialization). Exit 1 means at least one HIGH/CRITICAL finding.

# Staged-files convenience (matches the commit-time hook):
python3 scripts/security-review-scan.py --staged --format json

# Or an explicit list from Phase 1:
python3 scripts/security-review-scan.py --files <changed-files> --format json

Step 2: Record the findings by severity. CRITICAL and HIGH are blocking-class;

MEDIUM is advisory. Keep the file:line and rule for each.

Gate: Scanner ran and JSON parsed. Proceed with the findings in hand.

Phase 3: LLM-DEPTH REVIEW

Goal: Catch what regex cannot — authorization gaps, injection through data

flow, missing input validation, secrets in non-obvious forms. This is the

session agent's review of the diff; compose the existing parallel-code-review

Security reviewer over the changed files.

Step 1: Load references/coverage.md — the full review taxonomy (40

vulnerability classes, the 4-tier severity rubric, the false-positive exclusion

list, per-language guidance, and the 12 high-miss reviewer classes). Review to

this taxonomy so the session-agent pass reaches parity with the plugin's reviewer.

If claude-security-guidance.md exists (precedence: ~/.claude/ →

<cwd>/.claude/ → <cwd>/.claude/*.local.md), read it as ADDITIVE context — it

may add checks or raise a class's severity, and must not suppress findings.

Step 2: Dispatch the Security reviewer (the Reviewer 1 — Security role from

parallel-code-review) over the changed files via the Task tool, applying the

coverage.md taxonomy. Surface medium and above. Output: findings in the

coverage.md schema (`filePath, category, vulnerableCode, explanation, fix,

severity) with file:line` references.

Step 3: Merge the LLM findings with the Phase 2 scanner findings.

Deduplicate — when both flag the same file:line, keep one entry at the higher

severity. Independent confirmation by both layers raises confidence.

Gate: Security reviewer returned results and findings are merged. Issue a

verdict only from a completed review — a missing reviewer may hold the only

CRITICAL finding.

Phase 4: VERDICT

Goal: Produce a single clear recommendation.

Step 1: Determine the verdict from the merged findings:

| Condition | Verdict |

|-----------|---------|

| Any CRITICAL finding | BLOCK |

| HIGH findings, no CRITICAL | FIX (resolve before commit) |

| Only MEDIUM/LOW findings | APPROVE (with suggestions) |

Step 2: Output the structured report:

## Security Review Complete

### Severity Matrix
| Severity | Count | Source (scanner / reviewer / both) |
|----------|-------|------------------------------------|
| Critical | N | ... |
| High     | N | ... |
| Medium   | N | ... |

### Findings
#### CRITICAL (Block)
1. [source] description — file:line

#### HIGH (Fix before commit)
1. [source] description — file:line

#### MEDIUM (Should fix)
1. [source] description — file:line

### VERDICT
**BLOCK / FIX / APPROVE** — [1-2 sentence rationale]

Gate: Structured report delivered with an explicit verdict. Review complete.

Automatic Coverage (hooks)

This skill is the on-demand (PULL) path. The same review also runs automatically

(PUSH) via hooks/security-review-hook.py, wired in .claude/settings.json:

| Event | Behavior |

|-------|----------|

| PreToolUse (Bash git commit) | Scans STAGED files with the same scanner. A HIGH/CRITICAL finding blocks the commit (deny). Clean commits pass. |

| Stop | Re-wakes the session with the working-tree diff and an instruction to run this pipeline. Advisory — never blocks. |

Bypass / kill switches (commit-time block only, deliberate overrides):

| Env var | Effect |

|---------|--------|

| VEXJOY_SECURITY_REVIEW_SKIP=1 | Allow a commit through despite findings (one-off override). |

| VEXJOY_SECURITY_REVIEW_DISABLE=1 | Disable the hook entirely (both events). |

The hook fails open on any internal error — a hook crash never blocks a commit.

Extensibility (custom rules + project guidance)

Both extension points are additive and discovered in this precedence order:

~/.claude/<name> → <cwd>/.claude/<name> → <cwd>/.claude/<name>.local.<ext>.

| File | Effect |

|------|--------|

| security-patterns.{yaml,json} | Custom regex/substring rules merged into the scanner's built-ins. Shape: {"patterns": [{"rule_name", "regex"\|"substrings", "severity"?, "paths"?, "exclude_paths"?}]}. Capped at 50. ReDoS-prone or invalid rules are skipped with a stderr warning (non-fatal). PyYAML is used only if importable — JSON always works (stdlib-only). |

| claude-security-guidance.md | Markdown surfaced to the Phase 3 review as ADDITIVE context. It may add checks or raise a class's severity; it must not suppress findings — if it says to ignore a class, flag the vulnerability anyway and note the conflict. |

Built-in scanner rules always run and cannot be disabled by a config file.

Error Handling

Scanner reports findings but the code is intentional

Cause: A regex rule flagged a test fixture, an example, or a deliberately

hardcoded local value.

Solution: Confirm context in Phase 3. Downgrade in the report with a one-line

justification. For a commit the user knows is safe, document the

VEXJOY_SECURITY_REVIEW_SKIP=1 override rather than editing the scanner rules.

Security reviewer times out or returns nothing

Cause: Task agent exceeded its budget, or the diff was too large.

Solution: Report the Phase 2 scanner findings immediately (a partial review

beats no review), note the LLM-depth gap in the verdict, and offer to re-run the

Security reviewer on a reduced file set.

Scanner unavailable

Cause: scripts/security-review-scan.py missing from the working tree.

Solution: Run the Phase 3 LLM-depth review alone and state in the verdict that

the deterministic layer did not run.

References

• Detection rules: scripts/security-review-scan.py (single source of truth)
• Review taxonomy (40 classes + severity + FP filters + language guidance): references/coverage.md
• Security reviewer role: skills/review/parallel-code-review/SKILL.md (Reviewer 1)
• Auto-run hook: hooks/security-review-hook.py
• Design contract: adr/local-security-review.md