Fix Insecure Webhooks in CherryPy
Webhooks are effectively unauthenticated POST endpoints exposed to the public internet. If you aren't verifying the cryptographic signature of the incoming payload, you're inviting attackers to trigger internal logic, manipulate data, or achieve RCE via spoofed requests. In CherryPy, the fix involves implementing HMAC signature verification using a pre-shared secret and constant-time comparison to thwart timing attacks.
The Vulnerable Pattern
import cherrypy
class WebhookHandler: @cherrypy.expose @cherrypy.tools.json_in() def index(self): # VULNERABLE: No signature verification. # Anyone can POST any JSON to this endpoint. data = cherrypy.request.json if data.get(‘event’) == ‘push’: self.trigger_build(data.get(‘repo’)) return “Accepted”
The Secure Implementation
The secure implementation addresses three critical vectors: 1) It forces the use of a pre-shared secret known only to the provider and the consumer. 2) It uses the raw request body for HMAC calculation, as JSON re-serialization can change byte-order and break signatures. 3) It utilizes `hmac.compare_digest` to ensure the comparison takes the same amount of time regardless of how many characters match, preventing attackers from brute-forcing the signature character-by-character via timing analysis.
import cherrypy
import hmac
import hashlib
WEBHOOK_SECRET = b’your_high_entropy_secret_here’
class SecureWebhookHandler:
@cherrypy.expose
def index(self):
# 1. Read raw bytes before any JSON parsing occurs
body = cherrypy.request.body.read()
signature = cherrypy.request.headers.get(‘X-Hub-Signature-256’)
if not signature:
raise cherrypy.HTTPError(401, "Missing Signature")
# 2. Compute the expected HMAC using the raw body
expected_sig = 'sha256=' + hmac.new(WEBHOOK_SECRET, body, hashlib.sha256).hexdigest()
# 3. Use constant-time comparison to prevent timing side-channels
if not hmac.compare_digest(expected_sig, signature):
raise cherrypy.HTTPError(403, "Invalid Signature")
# 4. Proceed only after validation
return "Verified and Processed"</code></pre>
Your CherryPy API
might be exposed to Insecure Webhooks
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