Next.js/React Security Analysis

Comprehensive security patterns for Next.js and React applications, covering both client-side and server-side attack surfaces.

Architecture Overview

┌─────────────────────────────────────────────────────────────┐
│                     Next.js Application                      │
├─────────────────────────────────────────────────────────────┤
│                                                              │
│  Client-Side (Browser)          Server-Side (Node.js)       │
│  ┌──────────────────┐          ┌──────────────────────┐    │
│  │ React Components │          │ Server Components    │    │
│  │ Client Actions   │◄────────►│ Server Actions       │    │
│  │ useEffect/State  │          │ Route Handlers       │    │
│  └──────────────────┘          │ Middleware           │    │
│                                │ getServerSideProps   │    │
│                                └──────────────────────┘    │
│                                                              │
└─────────────────────────────────────────────────────────────┘

Attack Surface Categories

1. Server Actions ("use server")

• SSRF via redirect() with Host header manipulation
• Insecure direct object references
• Missing authentication/authorization
• SQL injection in database operations

2. Route Handlers (app/api/**/route.ts)

• Unauthenticated API endpoints
• Mass assignment vulnerabilities
• Rate limiting bypass
• CORS misconfiguration

3. Server Components

• Data exposure in serialized props
• Sensitive data in __NEXT_DATA__
• Server-side XSS in rendered content

4. Middleware

• Path-based bypass (normalization)
• Authentication bypass
• Header injection

5. Client Components

• XSS via unsanitized rendering
• Prototype pollution
• Open redirects

Detection Workflow

Step 1: Map the Application

# Find all Server Actions
grep -rn '"use server"' --include="*.ts" --include="*.tsx"

# Find all Route Handlers
find . -path "*/app/api/*" \( -name "route.ts" -o -name "route.js" \)

# Find middleware
find . -name "middleware.ts" -o -name "middleware.js"

# Find page components
find ./app -name "page.tsx" -o -name "page.js"

Step 2: Identify Entry Points

# Server Actions called from forms
grep -rn "action=" --include="*.tsx" | grep -v node_modules

# Server Actions called programmatically
grep -rn "startTransition\|useTransition" --include="*.tsx"

# Route Handler methods
grep -rn "export.*function\s\+\(GET\|POST\|PUT\|DELETE\|PATCH\)" --include="route.ts"

Step 3: Check Authentication

# Find auth patterns
grep -rn "getServerSession\|getSession\|auth\(\)" --include="*.ts" --include="*.tsx"

# Find unprotected handlers (no auth import)
for f in $(find . -path "*/app/api/*" -name "route.ts"); do
  if ! grep -q "getServerSession\|auth\|verify" "$f"; then
    echo "Potentially unprotected: $f"
  fi
done

Step 4: Trace Data Flow

# Find request data access
grep -rn "request\.json\|request\.formData\|request\.text" --include="*.ts"

# Find database operations
grep -rn "prisma\.\|db\.\|sql\`\|query\(" --include="*.ts"

# Find external API calls
grep -rn "fetch\(\|axios\.\|got\(" --include="*.ts"

Key Patterns

SSRF via Server Action Redirect

See framework-patterns/nextjs-patterns.md for detailed pattern.

Unprotected Route Handler

// VULNERABLE: No auth check
export async function DELETE(req: Request) {
  const { id } = await req.json();
  await db.user.delete({ where: { id } });  // Anyone can delete!
  return Response.json({ success: true });
}

Server Component Data Leak

// VULNERABLE: Sensitive data exposed
async function UserProfile({ userId }: { userId: string }) {
  const user = await db.user.findUnique({ where: { id: userId } });
  // Full user object including password hash goes to client!
  return <ClientProfile user={user} />;
}

Middleware Bypass

// VULNERABLE: Case-sensitive matching
export const config = {
  matcher: '/admin/:path*'  // /Admin/secret bypasses!
}

Integration with Chain Detection

Next.js vulnerabilities often enable chains:

Next.js Vulnerability	Chains To
Server Action SSRF	Internal Flask/Django SSTI
Server Action SSRF	Cloud metadata (169.254.169.254)
Route Handler SQLi	Data exfiltration, auth bypass
Middleware Bypass	Admin panel access
Data Leak	Credential theft, session hijacking

Remediation Checklist

• All Server Actions validate authentication
• All Route Handlers check authorization
• Middleware uses case-insensitive matching
• Server Components filter sensitive fields before passing to client
• redirect() calls validate Host header or use absolute URLs
• Database queries use parameterized statements
• File operations validate paths