PostHog Webhooks & Events

Overview

Securely handle PostHog webhooks with signature validation and replay protection.

Prerequisites

•PostHog webhook secret configured
•HTTPS endpoint accessible from internet
•Understanding of cryptographic signatures
•Redis or database for idempotency (optional)

Webhook Endpoint Setup

Express.js

typescript

import express from 'express';
import crypto from 'crypto';

const app = express();

// IMPORTANT: Raw body needed for signature verification
app.post('/webhooks/posthog',
  express.raw({ type: 'application/json' }),
  async (req, res) => {
    const signature = req.headers['x-posthog-signature'] as string;
    const timestamp = req.headers['x-posthog-timestamp'] as string;

    if (!verifyPostHogSignature(req.body, signature, timestamp)) {
      return res.status(401).json({ error: 'Invalid signature' });
    }

    const event = JSON.parse(req.body.toString());
    await handlePostHogEvent(event);

    res.status(200).json({ received: true });
  }
);

Signature Verification

typescript

function verifyPostHogSignature(
  payload: Buffer,
  signature: string,
  timestamp: string
): boolean {
  const secret = process.env.POSTHOG_WEBHOOK_SECRET!;

  // Reject old timestamps (replay attack protection)
  const timestampAge = Date.now() - parseInt(timestamp) * 1000;
  if (timestampAge > 300000) { // 5 minutes
    console.error('Webhook timestamp too old');
    return false;
  }

  // Compute expected signature
  const signedPayload = `${timestamp}.${payload.toString()}`;
  const expectedSignature = crypto
    .createHmac('sha256', secret)
    .update(signedPayload)
    .digest('hex');

  // Timing-safe comparison
  return crypto.timingSafeEqual(
    Buffer.from(signature),
    Buffer.from(expectedSignature)
  );
}

Event Handler Pattern

typescript

type PostHogEventType = 'resource.created' | 'resource.updated' | 'resource.deleted';

interface PostHogEvent {
  id: string;
  type: PostHogEventType;
  data: Record<string, any>;
  created: string;
}

const eventHandlers: Record<PostHogEventType, (data: any) => Promise<void>> = {
  'resource.created': async (data) => { /* handle */ },
  'resource.updated': async (data) => { /* handle */ },
  'resource.deleted': async (data) => { /* handle */ }
};

async function handlePostHogEvent(event: PostHogEvent): Promise<void> {
  const handler = eventHandlers[event.type];

  if (!handler) {
    console.log(`Unhandled event type: ${event.type}`);
    return;
  }

  try {
    await handler(event.data);
    console.log(`Processed ${event.type}: ${event.id}`);
  } catch (error) {
    console.error(`Failed to process ${event.type}: ${event.id}`, error);
    throw error; // Rethrow to trigger retry
  }
}

Idempotency Handling

typescript

import { Redis } from 'ioredis';

const redis = new Redis(process.env.REDIS_URL);

async function isEventProcessed(eventId: string): Promise<boolean> {
  const key = `posthog:event:${eventId}`;
  const exists = await redis.exists(key);
  return exists === 1;
}

async function markEventProcessed(eventId: string): Promise<void> {
  const key = `posthog:event:${eventId}`;
  await redis.set(key, '1', 'EX', 86400 * 7); // 7 days TTL
}

Webhook Testing

bash

# Use PostHog CLI to send test events
posthog webhooks trigger resource.created --url http://localhost:3000/webhooks/posthog

# Or use webhook.site for debugging
curl -X POST https://webhook.site/your-uuid \
  -H "Content-Type: application/json" \
  -d '{"type": "resource.created", "data": {}}'

Instructions

Step 1: Register Webhook Endpoint

Configure your webhook URL in the PostHog dashboard.

Step 2: Implement Signature Verification

Use the signature verification code to validate incoming webhooks.

Step 3: Handle Events

Implement handlers for each event type your application needs.

Step 4: Add Idempotency

Prevent duplicate processing with event ID tracking.

Output

•Secure webhook endpoint
•Signature validation enabled
•Event handlers implemented
•Replay attack protection active

Error Handling

Issue	Cause	Solution
Invalid signature	Wrong secret	Verify webhook secret
Timestamp rejected	Clock drift	Check server time sync
Duplicate events	Missing idempotency	Implement event ID tracking
Handler timeout	Slow processing	Use async queue

Examples

Testing Webhooks Locally

bash

# Use ngrok to expose local server
ngrok http 3000

# Send test webhook
curl -X POST https://your-ngrok-url/webhooks/posthog \
  -H "Content-Type: application/json" \
  -d '{"type": "test", "data": {}}'

Resources

Next Steps

For performance optimization, see posthog-performance-tuning.