webhook-security

Development

Implement secure webhook handling with signature verification, replay protection, and idempotency. Use when receiving webhooks from third-party services like Stripe, GitHub, Twilio, or building your own webhook system.

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Webhook Security

Production-ready webhook handling with defense in depth.

When to Use This Skill

Receiving webhooks from payment providers (Stripe, PayPal)
Integrating with GitHub, GitLab, or other dev tools
Building your own webhook delivery system
Any endpoint receiving external POST requests

Security Layers

┌─────────────────────────────────────────────────────┐
│                    Incoming Webhook                  │
└─────────────────────────────────────────────────────┘
                          │
                          ▼
┌─────────────────────────────────────────────────────┐
│  1. Signature Verification (HMAC-SHA256)            │
│     - Reject if signature invalid                   │
└─────────────────────────────────────────────────────┘
                          │
                          ▼
┌─────────────────────────────────────────────────────┐
│  2. Timestamp Validation                            │
│     - Reject if older than 5 minutes                │
└─────────────────────────────────────────────────────┘
                          │
                          ▼
┌─────────────────────────────────────────────────────┐
│  3. Idempotency Check                               │
│     - Skip if already processed                     │
└─────────────────────────────────────────────────────┘
                          │
                          ▼
┌─────────────────────────────────────────────────────┐
│  4. Process Webhook                                 │
│     - Handle business logic                         │
└─────────────────────────────────────────────────────┘

TypeScript Implementation

Signature Verification

// webhook-verifier.ts
import crypto from 'crypto';

interface WebhookConfig {
  secret: string;
  signatureHeader: string;
  timestampHeader?: string;
  tolerance?: number; // seconds
}

interface VerificationResult {
  valid: boolean;
  error?: string;
}

class WebhookVerifier {
  constructor(private config: WebhookConfig) {}

  verify(payload: string | Buffer, headers: Record<string, string>): VerificationResult {
    const signature = headers[this.config.signatureHeader.toLowerCase()];
    
    if (!signature) {
      return { valid: false, error: 'Missing signature header' };
    }

    // Check timestamp if configured
    if (this.config.timestampHeader) {
      const timestamp = headers[this.config.timestampHeader.toLowerCase()];
      if (!timestamp) {
        return { valid: false, error: 'Missing timestamp header' };
      }

      const timestampAge = Math.floor(Date.now() / 1000) - parseInt(timestamp, 10);
      const tolerance = this.config.tolerance || 300; // 5 minutes default

      if (Math.abs(timestampAge) > tolerance) {
        return { valid: false, error: 'Timestamp outside tolerance window' };
      }
    }

    // Compute expected signature
    const expectedSignature = this.computeSignature(payload, headers);
    
    // Constant-time comparison to prevent timing attacks
    const valid = crypto.timingSafeEqual(
      Buffer.from(signature),
      Buffer.from(expectedSignature)
    );

    return { valid, error: valid ? undefined : 'Invalid signature' };
  }

  private computeSignature(payload: string | Buffer, headers: Record<string, string>): string {
    const timestamp = this.config.timestampHeader 
      ? headers[this.config.timestampHeader.toLowerCase()] 
      : '';
    
    const signedPayload = timestamp ? `${timestamp}.${payload}` : payload.toString();
    
    return 'sha256=' + crypto
      .createHmac('sha256', this.config.secret)
      .update(signedPayload)
      .digest('hex');
  }
}

export { WebhookVerifier, WebhookConfig, VerificationResult };

Provider-Specific Verifiers

// providers/stripe.ts
import Stripe from 'stripe';

export function verifyStripeWebhook(
  payload: string | Buffer,
  signature: string,
  secret: string
): Stripe.Event {
  const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!);
  
  // Stripe's library handles verification
  return stripe.webhooks.constructEvent(payload, signature, secret);
}

// providers/github.ts
import crypto from 'crypto';

export function verifyGitHubWebhook(
  payload: string,
  signature: string,
  secret: string
): boolean {
  const expected = 'sha256=' + crypto
    .createHmac('sha256', secret)
    .update(payload)
    .digest('hex');
  
  return crypto.timingSafeEqual(
    Buffer.from(signature),
    Buffer.from(expected)
  );
}

// providers/twilio.ts
import crypto from 'crypto';

export function verifyTwilioWebhook(
  url: string,
  params: Record<string, string>,
  signature: string,
  authToken: string
): boolean {
  // Twilio uses URL + sorted params
  const data = url + Object.keys(params)
    .sort()
    .map(key => key + params[key])
    .join('');
  
  const expected = crypto
    .createHmac('sha1', authToken)
    .update(data)
    .digest('base64');
  
  return crypto.timingSafeEqual(
    Buffer.from(signature),
    Buffer.from(expected)
  );
}

Idempotency Handler

// idempotency.ts
import { Redis } from 'ioredis';

interface IdempotencyConfig {
  redis: Redis;
  keyPrefix?: string;
  ttlSeconds?: number;
}

class IdempotencyHandler {
  private redis: Redis;
  private keyPrefix: string;
  private ttl: number;

  constructor(config: IdempotencyConfig) {
    this.redis = config.redis;
    this.keyPrefix = config.keyPrefix || 'webhook:processed:';
    this.ttl = config.ttlSeconds || 86400; // 24 hours
  }

  async isProcessed(eventId: string): Promise<boolean> {
    const key = this.keyPrefix + eventId;
    const exists = await this.redis.exists(key);
    return exists === 1;
  }

  async markProcessed(eventId: string, result?: unknown): Promise<void> {
    const key = this.keyPrefix + eventId;
    const value = JSON.stringify({
      processedAt: new Date().toISOString(),
      result,
    });
    await this.redis.setex(key, this.ttl, value);
  }

  async getProcessedResult(eventId: string): Promise<unknown | null> {
    const key = this.keyPrefix + eventId;
    const value = await this.redis.get(key);
    if (!value) return null;
    return JSON.parse(value);
  }
}

export { IdempotencyHandler, IdempotencyConfig };

Complete Webhook Handler

// webhook-handler.ts
import { Request, Response, NextFunction } from 'express';
import { WebhookVerifier } from './webhook-verifier';
import { IdempotencyHandler } from './idempotency';

interface WebhookHandlerConfig {
  verifier: WebhookVerifier;
  idempotency: IdempotencyHandler;
  eventIdExtractor: (payload: unknown) => string;
}

function createWebhookHandler(config: WebhookHandlerConfig) {
  return async (req: Request, res: Response, next: NextFunction) => {
    // Get raw body (must use raw body parser)
    const rawBody = req.body;
    
    // 1. Verify signature
    const verification = config.verifier.verify(
      rawBody,
      req.headers as Record<string, string>
    );
    
    if (!verification.valid) {
      console.error('Webhook verification failed:', verification.error);
      return res.status(401).json({ error: verification.error });
    }

    // Parse payload
    const payload = JSON.parse(rawBody.toString());
    
    // 2. Check idempotency
    const eventId = config.eventIdExtractor(payload);
    
    if (await config.idempotency.isProcessed(eventId)) {
      console.log(`Webhook ${eventId} already processed, skipping`);
      return res.status(200).json({ status: 'already_processed' });
    }

    // 3. Attach parsed payload and continue
    req.body = payload;
    (req as any).webhookEventId = eventId;
    
    // 4. After processing, mark as processed
    res.on('finish', async () => {
      if (res.statusCode >= 200 && res.statusCode < 300) {
        await config.idempotency.markProcessed(eventId);
      }
    });

    next();
  };
}

export { createWebhookHandler };

Python Implementation

# webhook_security.py
import hmac
import hashlib
import time
from typing import Optional, Dict, Any
from dataclasses import dataclass
import redis

@dataclass
class VerificationResult:
    valid: bool
    error: Optional[str] = None

class WebhookVerifier:
    def __init__(
        self,
        secret: str,
        signature_header: str,
        timestamp_header: Optional[str] = None,
        tolerance: int = 300,
    ):
        self.secret = secret
        self.signature_header = signature_header.lower()
        self.timestamp_header = timestamp_header.lower() if timestamp_header else None
        self.tolerance = tolerance

    def verify(self, payload: bytes, headers: Dict[str, str]) -> VerificationResult:
        # Normalize headers to lowercase
        headers = {k.lower(): v for k, v in headers.items()}
        
        signature = headers.get(self.signature_header)
        if not signature:
            return VerificationResult(False, "Missing signature header")

        # Check timestamp
        if self.timestamp_header:
            timestamp = headers.get(self.timestamp_header)
            if not timestamp:
                return VerificationResult(False, "Missing timestamp header")
            
            timestamp_age = abs(int(time.time()) - int(timestamp))
            if timestamp_age > self.tolerance:
                return VerificationResult(False, "Timestamp outside tolerance")

        # Compute expected signature
        expected = self._compute_signature(payload, headers)
        
        # Constant-time comparison
        valid = hmac.compare_digest(signature, expected)
        
        return VerificationResult(valid, None if valid else "Invalid signature")

    def _compute_signature(self, payload: bytes, headers: Dict[str, str]) -> str:
        timestamp = headers.get(self.timestamp_header, "") if self.timestamp_header else ""
        signed_payload = f"{timestamp}.{payload.decode()}" if timestamp else payload
        
        if isinstance(signed_payload, str):
            signed_payload = signed_payload.encode()
        
        signature = hmac.new(
            self.secret.encode(),
            signed_payload,
            hashlib.sha256
        ).hexdigest()
        
        return f"sha256={signature}"


class IdempotencyHandler:
    def __init__(
        self,
        redis_client: redis.Redis,
        key_prefix: str = "webhook:processed:",
        ttl_seconds: int = 86400,
    ):
        self.redis = redis_client
        self.key_prefix = key_prefix
        self.ttl = ttl_seconds

    def is_processed(self, event_id: str) -> bool:
        key = f"{self.key_prefix}{event_id}"
        return self.redis.exists(key) == 1

    def mark_processed(self, event_id: str, result: Any = None) -> None:
        import json
        key = f"{self.key_prefix}{event_id}"
        value = json.dumps({
            "processed_at": time.time(),
            "result": result,
        })
        self.redis.setex(key, self.ttl, value)

FastAPI Middleware

# fastapi_webhook.py
from fastapi import Request, HTTPException, Depends
from functools import wraps

def webhook_protected(
    verifier: WebhookVerifier,
    idempotency: IdempotencyHandler,
    event_id_extractor: callable,
):
    async def dependency(request: Request):
        # Get raw body
        body = await request.body()
        
        # Verify signature
        result = verifier.verify(body, dict(request.headers))
        if not result.valid:
            raise HTTPException(status_code=401, detail=result.error)
        
        # Parse and check idempotency
        import json
        payload = json.loads(body)
        event_id = event_id_extractor(payload)
        
        if idempotency.is_processed(event_id):
            raise HTTPException(status_code=200, detail="Already processed")
        
        # Store for later marking
        request.state.webhook_event_id = event_id
        request.state.webhook_payload = payload
        
        return payload
    
    return Depends(dependency)


# Usage
@app.post("/webhooks/stripe")
async def stripe_webhook(
    payload: dict = webhook_protected(
        verifier=stripe_verifier,
        idempotency=idempotency_handler,
        event_id_extractor=lambda p: p["id"],
    )
):
    # Process webhook
    event_type = payload["type"]
    # ...
    
    # Mark as processed
    idempotency_handler.mark_processed(payload["id"])
    return {"status": "ok"}

Express Setup

// Important: Use raw body parser for webhooks
import express from 'express';

const app = express();

// Regular JSON parser for most routes
app.use(express.json());

// Raw body parser for webhook routes
app.use('/webhooks', express.raw({ type: 'application/json' }));

// Webhook route with verification
app.post('/webhooks/stripe', 
  createWebhookHandler({
    verifier: stripeVerifier,
    idempotency: idempotencyHandler,
    eventIdExtractor: (p: any) => p.id,
  }),
  async (req, res) => {
    const event = req.body;
    
    switch (event.type) {
      case 'checkout.session.completed':
        await handleCheckout(event.data.object);
        break;
      // ... other handlers
    }
    
    res.json({ received: true });
  }
);

Building Your Own Webhook Sender

// webhook-sender.ts
import crypto from 'crypto';

interface WebhookDelivery {
  url: string;
  event: string;
  payload: unknown;
  secret: string;
}

async function sendWebhook(delivery: WebhookDelivery): Promise<boolean> {
  const timestamp = Math.floor(Date.now() / 1000).toString();
  const body = JSON.stringify(delivery.payload);
  
  // Create signature
  const signedPayload = `${timestamp}.${body}`;
  const signature = 'sha256=' + crypto
    .createHmac('sha256', delivery.secret)
    .update(signedPayload)
    .digest('hex');

  try {
    const response = await fetch(delivery.url, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'X-Webhook-Signature': signature,
        'X-Webhook-Timestamp': timestamp,
        'X-Webhook-Event': delivery.event,
        'X-Webhook-ID': crypto.randomUUID(),
      },
      body,
    });

    return response.ok;
  } catch (error) {
    console.error('Webhook delivery failed:', error);
    return false;
  }
}

Best Practices

Always verify signatures: Never trust unverified payloads
Use constant-time comparison: Prevents timing attacks
Check timestamps: Prevents replay attacks
Implement idempotency: Same webhook may be delivered multiple times
Use raw body parser: JSON parsing before verification breaks signatures
Return 200 quickly: Process async to avoid timeouts

Common Mistakes

Parsing JSON before signature verification
Using regular string comparison for signatures
Not handling duplicate deliveries
Blocking on webhook processing (causes retries)
Exposing webhook secrets in logs

Security Checklist

Signature verification with HMAC-SHA256
Constant-time signature comparison
Timestamp validation (5 min tolerance)
Idempotency handling
Raw body parser for webhook routes
Secrets in environment variables
HTTPS only
Logging without exposing secrets