| name | differential-review |
| description | Performs security-focused differential review of code changes (PRs, commits, diffs). Adapts analysis depth to codebase size, uses git history for context, calculates blast radius, checks test coverage, and generates comprehensive markdown reports. Automatically detects and prevents security regressions.
|
| source | https://github.com/trailofbits/skills/tree/main/plugins/differential-review |
| allowed-tools | ["Read","Write","Grep","Glob","Bash"] |
Differential Security Review
Security-focused code review for PRs, commits, and diffs.
Core Principles
- Risk-First: Focus on auth, crypto, value transfer, external calls
- Evidence-Based: Every finding backed by git history, line numbers, attack scenarios
- Adaptive: Scale to codebase size (SMALL/MEDIUM/LARGE)
- Honest: Explicitly state coverage limits and confidence level
- 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)
│
├─ 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
Quality Checklist
Before delivering:
Integration
audit-context-building skill:
- Pre-Analysis: Build baseline context
- Phase 4: Deep context on HIGH RISK changes
GitHub issue tracking:
- File findings as tracked issues so they can be triaged and closed independently
- Command:
gh issue create --title "<finding title>" --body-file DIFFERENTIAL_REVIEW_REPORT.md
- Confirm with the user before posting to repos with other collaborators
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 (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.
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)
Supporting Documentation
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.
