# Differential Review

Performs security-focused differential review of code changes.

- Repository: https://github.com/trailofbits/skills (7935 stars)
- Category: Testing
- License: CC-BY-SA-4.0
- Last updated: 2026-10-28
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add trailofbits/skills --skill differential-review -g -a claude-code -y`
Codex: `npx skills add trailofbits/skills --skill differential-review -g -a codex -y`
Cursor: `npx skills add trailofbits/skills --skill differential-review -g -a cursor -y`
Gemini CLI: `npx skills add trailofbits/skills --skill differential-review -g -a gemini-cli -y`
Claude Code plugin marketplace: `/plugin marketplace add trailofbits/skills`

## 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.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# 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

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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

The rest: https://github.com/trailofbits/skills/blob/HEAD/plugins/differential-review/skills/differential-review/SKILL.md