differential-review – Coding skill for AI agents

Differential Security Review

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

When to Use

• You need a security-focused review of a PR, commit range, or diff rather than a general code review.
• The changes touch auth, crypto, external calls, value transfer, permissions, or other high-risk logic.
• You need findings backed by code evidence, attack scenarios, and an explicit report artifact.

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

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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 |

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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 |

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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

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Supporting material availability

Only SKILL.md is bundled. methodology.md, adversarial.md, reporting.md and patterns.md are unavailable here; obtain inspected upstream material or use the inline quick reference and explicitly report limited depth. Classify by actual data flow: UI/logging/docs can contain XSS, sensitive disclosure or unsafe instructions and are not automatically low risk.

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)
│
├─ 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

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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

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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

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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

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When NOT to Use This Skill

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

For these cases, use standard code review instead.

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Red Flags (Stop and Investigate)

Immediate escalation triggers:

• Removed code from "security", "CVE", or "fix" commits
• Access control modifiers removed (onlyOwner, internal → external)
• Validation removed without replacement
• External calls added without checks
• High blast radius (50+ callers) + HIGH risk change

These patterns require adversarial analysis even in quick triage.

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Tips for Best Results

Do:

• Start with git blame for removed code
• Calculate blast radius early to prioritize
• Generate concrete attack scenarios
• Reference specific line numbers and commits
• Be honest about coverage limitations
• Always generate the output file

Don't:

• Skip git history analysis
• Make generic findings without evidence
• Claim full analysis when time-limited
• Forget to check test coverage
• Miss high blast radius changes
• Output report only to chat (file required)

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Supporting Documentation

• methodology.md - Detailed phase-by-phase workflow (Phases 0-4)
• adversarial.md - Attacker modeling and exploit scenarios (Phase 5)
• reporting.md - Report structure and formatting (Phase 6)
• patterns.md - Common vulnerability patterns reference

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For first-time users: Start with methodology.md to understand the complete workflow.

For experienced users: Use this page's Quick Reference and Decision Tree to navigate directly to needed content.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.