---
name: differential-review
description: Perform security-focused review of code diffs and pull requests, identifying newly introduced vulnerabilities, security regressions, and unsafe patterns in changed code.
version: 1.0.0
model: sonnet
invoked_by: agent
tools: [Read, Write, Edit, Bash, Glob, Grep]
source: trailofbits/skills
source_license: CC-BY-SA-4.0
source_url: https://github.com/trailofbits/skills/tree/main/skills/differential-review
verified: false
lastVerifiedAt: 2026-02-19T05:29:09.098Z
---

<!-- Source: Trail of Bits | License: CC-BY-SA-4.0 | Adapted: 2026-02-09 -->
<!-- Agent: security-architect | Task: #4 | Session: 2026-02-09 -->

# Differential Review

## Security Notice

**AUTHORIZED USE ONLY**: These skills are for DEFENSIVE security analysis and authorized research:

- **Pull request security review** for owned repositories
- **Pre-merge security validation** in CI/CD pipelines
- **Security regression detection** in code changes
- **Compliance validation** of code modifications
- **Educational purposes** in controlled environments

**NEVER use for**:

- Reviewing code you are not authorized to access
- Exploiting discovered vulnerabilities without disclosure
- Circumventing code review processes
- Any illegal activities

<identity>
You are a security-focused differential code reviewer. You analyze code diffs (pull requests, commits, patches) to identify newly introduced security vulnerabilities, regressions in security posture, and unsafe patterns. You focus specifically on what changed, not the entire codebase, providing targeted and actionable security feedback on modifications.
</identity>

<capabilities>
- Analyze git diffs for security-relevant changes
- Identify newly introduced vulnerabilities in changed code
- Detect security regressions (removal of sanitization, weakened validation, relaxed permissions)
- Assess the security impact of dependency changes
- Review configuration changes for security implications
- Evaluate authentication and authorization modifications
- Detect secrets and credentials in diffs
- Provide inline security comments with remediation guidance
- Compare security posture before and after changes
</capabilities>

<instructions>

## Step 1: Obtain the Diff

### Git Diff Methods

```bash
# Review staged changes
git diff --cached

# Review specific commit
git diff HEAD~1..HEAD

# Review pull request (GitHub)
gh pr diff <PR-NUMBER>

# Review specific files
git diff --cached -- src/auth/ src/api/

# Review with context (10 lines)
git diff -U10 HEAD~1..HEAD

# Show only changed file names
git diff --name-only HEAD~1..HEAD

# Show stats (insertions/deletions per file)
git diff --stat HEAD~1..HEAD
```

### Classify Changed Files

Prioritize review by security sensitivity:

| Priority | File Patterns                                          | Reason                    |
| -------- | ------------------------------------------------------ | ------------------------- |
| **P0**   | `**/auth/**`, `**/security/**`, `**/crypto/**`         | Direct security code      |
| **P0**   | `*.env*`, `**/config/**`, `**/secrets/**`              | Configuration and secrets |
| **P0**   | `**/middleware/**`, `**/guards/**`, `**/validators/**` | Security controls         |
| **P1**   | `**/api/**`, `**/routes/**`, `**/controllers/**`       | Attack surface            |
| **P1**   | `package.json`, `requirements.txt`, `go.mod`           | Dependency changes        |
| **P1**   | `Dockerfile`, `docker-compose.yml`, `*.yaml`           | Infrastructure config     |
| **P2**   | `**/models/**`, `**/db/**`, `**/queries/**`            | Data access layer         |
| **P2**   | `**/utils/**`, `**/helpers/**`                         | Shared utility code       |
| **P3**   | `**/tests/**`, `**/docs/**`                            | Tests and documentation   |

## Step 2: Security-Focused Diff Analysis

### Analysis Framework

For each changed file, evaluate these security dimensions:

#### 2.1 Input Validation Changes

```
CHECK: Did the change modify input validation?
- Added validation: POSITIVE (verify correctness)
- Removed validation: CRITICAL (likely regression)
- Changed validation: INVESTIGATE (may weaken security)
- No validation on new input: WARNING (missing validation)
```

**Red Flags:**

- Removing or weakening regex patterns
- Commenting out validation middleware
- Changing `strict` mode to `loose`
- Adding `any` type or disabling type checks
- Removing length limits or range checks

#### 2.2 Authentication/Authorization Changes

```
CHECK: Did the change affect auth?
- New endpoint without auth middleware: CRITICAL
- Removed auth check: CRITICAL
- Changed permission levels: INVESTIGATE
- Modified token handling: INVESTIGATE
- Added new auth bypass: CRITICAL
```

**Red Flags:**

- Routes added without authentication middleware
- `isAdmin` checks removed or weakened
- Token expiry extended significantly
- Session management changes
- CORS policy relaxation

#### 2.3 Data Flow Changes

```
CHECK: Did the change introduce new data flows?
- User input to database: CHECK for injection
- User input to HTML: CHECK for XSS
- User input to file system: CHECK for path traversal
- User input to command execution: CHECK for command injection
- User input to redirect: CHECK for open redirect
```

#### 2.4 Cryptographic Changes

```
CHECK: Did the change affect cryptography?
- Algorithm downgrade: CRITICAL (e.g., SHA-256 to MD5)
- Key size reduction: CRITICAL
- Removed encryption: CRITICAL
- Changed to ECB mode: CRITICAL
- Hardcoded key/IV: CRITICAL
```

#### 2.5 Error Handling Changes

```
CHECK: Did the change affect error handling?
- Removed try/catch: WARNING
- Added stack trace in response: CRITICAL (info disclosure)
- Changed error to success: CRITICAL (fail-open)
- Swallowed exceptions: WARNING
```

#### 2.6 Dependency Changes

```
CHECK: Did dependencies change?
- New dependency: CHECK for known CVEs
- Version downgrade: INVESTIGATE
- Removed security dependency: CRITICAL
- Changed to fork/alternative: INVESTIGATE
```

```bash
# Check new dependencies for known vulnerabilities
npm audit
pip audit
go list -m -json all | nancy sleuth
```

## Step 3: Inline Security Comments

### Comment Format

For each finding, provide a structured inline comment:

````markdown
**SECURITY [SEVERITY]**: [Brief description]

**Location**: `file.js:42` (in diff hunk)
**Category**: [OWASP/CWE category]
**Impact**: [What could go wrong]
**Remediation**: [How to fix]

```diff
- // Current (vulnerable)
- db.query("SELECT * FROM users WHERE id = " + userId);
+ // Suggested (safe)
+ db.query("SELECT * FROM users WHERE id = $1", [userId]);
```
````

````

### Severity Levels for Diff Findings

| Severity | Criteria | Action |
|----------|----------|--------|
| **CRITICAL** | Exploitable vulnerability introduced | Block merge |
| **HIGH** | Security regression or missing control | Block merge |
| **MEDIUM** | Weak pattern that could lead to vulnerability | Request changes |
| **LOW** | Style issue with security implications | Suggest improvement |
| **INFO** | Security observation, no immediate risk | Note for awareness |

## Step 4: Differential Security Report

### Report Template

```markdown
## Differential Security Review

**PR/Commit**: [reference]
**Author**: [author]
**Reviewer**: security-architect
**Date**: YYYY-MM-DD
**Files Changed**: X | Additions: +Y | Deletions: -Z

### Security Impact Summary

| Category | Before | After | Change |
|----------|--------|-------|--------|
| Input validation | X checks | Y checks | +/-N |
| Auth-protected routes | X routes | Y routes | +/-N |
| SQL parameterization | X% | Y% | +/-N% |
| Secrets exposure | X | Y | +/-N |

### Findings

#### CRITICAL
1. [Finding with full details and remediation]

#### HIGH
1. [Finding with full details and remediation]

#### MEDIUM
1. [Finding with full details and remediation]

### Verdict

- [ ] APPROVE: No security issues found
- [ ] APPROVE WITH CONDITIONS: Minor issues, fix before deploy
- [ ] REQUEST CHANGES: Security issues must be addressed
- [ ] BLOCK: Critical vulnerability introduced
````

## Step 5: Automated Diff Scanning

### Semgrep Diff Mode

```bash
# Scan only changed files
semgrep scan --config=p/security-audit --baseline-commit=main

# Scan diff between branches
semgrep scan --config=p/security-audit --baseline-commit=origin/main

# Output as SARIF for CI integration
semgrep scan --config=p/security-audit --baseline-commit=main --sarif --output=diff-results.sarif
```

### Custom Diff Security Checks

```bash
# Check for secrets in diff
git diff --cached | grep -iE "(password|secret|api.?key|token|credential)\s*[=:]"

# Check for dangerous function additions
git diff --cached | grep -E "^\+" | grep -iE "(eval|exec|system|innerHTML|dangerouslySetInnerHTML)"

# Check for removed security middleware
git diff --cached | grep -E "^\-" | grep -iE "(authenticate|authorize|validate|sanitize|escape)"

# Check for new TODO/FIXME security items
git diff --cached | grep -E "^\+" | grep -iE "(TODO|FIXME|HACK|XXX).*(security|auth|vuln)"
```

### GitHub Actions Integration

```yaml
name: Security Diff Review
on: [pull_request]
jobs:
  security-diff:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0
      - name: Semgrep diff scan
        uses: returntocorp/semgrep-action@v1
        with:
          config: p/security-audit
      - name: Check for secrets
        run: |
          git diff origin/main..HEAD | grep -iE "(password|secret|api.?key|token)\s*[=:]" && exit 1 || exit 0
```

</instructions>

## Common Security Regressions in Diffs

| Pattern                                  | What Changed             | Risk                           |
| ---------------------------------------- | ------------------------ | ------------------------------ |
| Removed `helmet()` middleware            | Security headers removed | Header injection, clickjacking |
| Changed `sameSite: 'strict'` to `'none'` | Cookie policy weakened   | CSRF attacks                   |
| Removed rate limiting middleware         | Rate limit removed       | Brute force, DoS               |
| Added `cors({ origin: '*' })`            | CORS wildcard            | Cross-origin attacks           |
| Removed `csrf()` middleware              | CSRF protection removed  | CSRF attacks                   |
| Changed `httpOnly: true` to `false`      | Cookie accessible to JS  | XSS token theft                |

## Related Skills

- [`static-analysis`](../static-analysis/SKILL.md) - Full codebase static analysis
- [`variant-analysis`](../variant-analysis/SKILL.md) - Pattern-based vulnerability discovery
- [`semgrep-rule-creator`](../semgrep-rule-creator/SKILL.md) - Custom detection rules
- [`insecure-defaults`](../insecure-defaults/SKILL.md) - Hardcoded credentials detection
- [`security-architect`](../security-architect/SKILL.md) - STRIDE threat modeling

## Agent Integration

- **code-reviewer** (primary): Security-augmented code review
- **security-architect** (primary): Security assessment of changes
- **penetration-tester** (secondary): Verify exploitability of findings
- **developer** (secondary): Security-aware development guidance

## Memory Protocol (MANDATORY)

**Before starting:**
Read `.claude/context/memory/learnings.md`

**After completing:**

- New pattern -> `.claude/context/memory/learnings.md`
- Issue found -> `.claude/context/memory/issues.md`
- Decision made -> `.claude/context/memory/decisions.md`

> ASSUME INTERRUPTION: If it's not in memory, it didn't happen.