/differential-review

name: differential-review
description: "Performs security-focused differential review of code changes. Adapts analysis depth to codebase size, uses git blame for context, calculates blast radius by counting callers, checks test coverage of modified code, and generates a markdown report. Use when reviewing a PR, commit, or diff for security vulnerabilities, checking whether a change re-introduces a previously fixed bug, asking what else a change could break, or finding which modified code has no test covering it."
allowed-tools: Read Write Grep Glob Bash

Differential Security Review

Security-focused code review for PRs, commits, and diffs.

Core Principles

1. **Risk-First**: Focus on auth, crypto, value transfer, external calls 2. **Evidence-Based**: Every finding backed by git history, line numbers, attack scenarios 3. **Adaptive**: Scale to codebase size (SMALL/MEDIUM/LARGE) 4. **Honest**: Explicitly state coverage limits and confidence level 5. **Output-Driven**: Always generate comprehensive markdown report file

---

Rationalizations (Do Not Skip)

| Rationalization | Why It's Wrong | Required Action | |-----------------|----------------|-----------------| | "Small PR, quick review" | Heartbleed was 2 lines | Classify by RISK, not size | | "I know this codebase" | Familiarity breeds blind spots | Build explicit baseline context | | "Git history takes too long" | History reveals regressions | Never skip Phase 1 | | "Blast radius is obvious" | You'll miss transitive callers | Calculate quantitatively | | "No tests = not my problem" | Missing tests = elevated risk rating | Flag in report, elevate severity | | "Just a refactor, no security impact" | Refactors break invariants | Analyze as HIGH until proven LOW | | "I'll explain verbally" | No artifact = findings lost | Always write report |

---

Quick Reference

Codebase Size Strategy

| Codebase Size | Strategy | Approach | |---------------|----------|----------| | SMALL (<20 files) | DEEP | Read all deps, full git blame | | MEDIUM (20-200) | FOCUSED | 1-hop deps, priority files | | LARGE (200+) | SURGICAL | Critical paths only |

Risk Level Triggers

| Risk Level | Triggers | |------------|----------| | HIGH | Auth, crypto, external calls, value transfer, validation removal | | MEDIUM | Business logic, state changes, new public APIs | | LOW | Comments, tests, UI, logging |

---

Workflow Overview

Pre-Analysis → Phase 0: Triage → Phase 1: Code Analysis → Phase 2: Test Coverage
    ↓              ↓                    ↓                        ↓
Phase 3: Blast Radius → Phase 4: Deep Context → Phase 5: Adversarial → Phase 6: Report

---

Decision Tree

**Starting a review?**

├─ Need detailed phase-by-phase methodology?
│  └─ Read: methodology.md
│     (Pre-Analysis + Phases 0-4: triage, code analysis, test coverage, blast radius)
│
├─ Analyzing HIGH RISK change?
│  ├─ Read: adversarial.md
│  │  (Phase 5: Attacker modeling, exploit scenarios, exploitability rating)
│  └─ Or delegate to: differential-review:adversarial-modeler agent
│     (Autonomous attacker modeling with concrete exploit scenarios)
│
├─ Writing the final report?
│  └─ Read: reporting.md
│     (Phase 6: Report structure, templates, formatting guidelines)
│
├─ Looking for specific vulnerability patterns?
│  └─ Read: patterns.md
│     (Regressions, reentrancy, access control, overflow, etc.)
│
└─ Quick triage only?
   └─ Use Quick Reference above, skip detailed docs

---

Agents

**`differential-review:adversarial-modeler`** — Models attacker perspectives and builds exploit scenarios for HIGH RISK code changes. Follows the 5-step adversarial methodology (attacker model, attack vectors, exploitability rating, exploit scenario, baseline cross-reference) and produces structured vulnerability reports. Delegate to this agent when Phase 5 analysis is needed on high-risk changes, passing that full namespaced name as `subagent_type` — a bare `adversarial-modeler` is unregistered and the dispatch fails at runtime.

---

Quality Checklist

Before delivering:

• [ ] All changed files analyzed
• [ ] Git blame on removed security code
• [ ] Blast radius calculated for HIGH risk
• [ ] Attack scenarios are concrete (not generic)
• [ ] Findings reference specific line numbers + commits
• [ ] Report file generated
• [ ] User notified with summary

---

Integration

**audit-context-building skill:**

• Pre-Analysis: Build baseline context
• Phase 4: Deep context on HIGH RISK changes

**issue-writer skill:**

• Transform findings into formal audit reports
• Command: `issue-writer --input DIFFERENTIAL_REVIEW_REPORT.md --format audit-report`

---

Example Usage

Quick Triage (Small PR)

Input: 5 file PR, 2 HIGH RISK files
Strategy: Use Quick Reference
1. Classify risk level per file (2 HIGH, 3 LOW)
2. Focus on 2 HIGH files only
3. Git blame removed code
4. Generate minimal report
Time: ~30 minutes

Standard Review (Medium Codebase)

Input: 80 files, 12 HIGH RISK changes
Strategy: FOCUSED (see methodology.md)
1. Full workflow on HIGH RISK files
2. Surface scan on MEDIUM
3. Skip LOW risk files
4. Complete report with all sections
Time: ~3-4 hours

Deep Audit (Large, Critical Change)

Input: 450 files, auth system rewrite
Strategy: SURGICAL + audit-context-building
1. Baseline context with audit-context-building
2. Deep analysis on auth changes only
3. Blast radius analysis
4. Adversarial modeling
5. Comprehensive report
Time: ~6-8 hours

---

When NOT to Use This Skill

• **Greenfield code** (no baseline to compare)
• **Documentation-only changes** (no security impact)
• **Formatting/linting** (cosmetic changes)
• **User explicitly requests quick summary only** (they accept risk)

For these cases, use standard code review instead.

---

Red Flags (Stop and Investigate)

**Immediate escalation triggers:**

• Removed code from "security", "CVE", or "fix" commits
• Access control modifiers removed (on