Fix Insecure Webhooks in Sanic
Insecure webhooks are an invitation for unauthorized data injection and state manipulation. If your Sanic endpoint processes incoming payloads without cryptographic verification, you are trusting unauthenticated input from the open web. To secure this, you must implement HMAC signature verification to ensure the request originated from a trusted provider and hasn't been tampered with in transit.
The Vulnerable Pattern
from sanic import Sanic, responseapp = Sanic(“VulnerableWebhook”)
@app.post(“/webhook”) async def handle_webhook(request): # CRITICAL: No authentication or integrity check # An attacker can spoof this request easily data = request.json execute_business_logic(data) return response.json({“status”: “processed”})
The Secure Implementation
The secure implementation introduces a mandatory HMAC-SHA256 signature check. 1. Shared Secret: Both the sender and receiver know a secret key. 2. Raw Body: Verification uses `request.body` (the raw bytes) because `request.json` can be altered by parser normalization, breaking the hash. 3. Constant-Time Comparison: `hmac.compare_digest` is used instead of a standard equality check (`==`) to eliminate timing side-channel attacks that could allow an attacker to brute-force the signature character by character. 4. Strict Enforcement: If the signature is missing or invalid, the app returns a 403 Forbidden immediately.
import hmac
import hashlib
from sanic import Sanic, response, exceptions
app = Sanic(“SecureWebhook”)
Store this in environment variables, never hardcode
WEBHOOK_SECRET = b”your_secure_shared_secret”
def verify_hmac(body, signature):
# Calculate the expected hash using the raw request body
expected_signature = hmac.new(WEBHOOK_SECRET, body, hashlib.sha256).hexdigest()
# Use compare_digest to prevent timing attacks
return hmac.compare_digest(expected_signature, signature)
@app.post(“/webhook”)
async def handle_webhook(request):
signature = request.headers.get(“X-Payload-Signature”)
if not signature or not verify_hmac(request.body, signature):
raise exceptions.Forbidden("Invalid or missing signature")
data = request.json
return response.json({"status": "verified"})</code></pre>
Your Sanic API
might be exposed to Insecure Webhooks
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