AnyCable: Pusher REST API Does Not Verify Request Body MD5 Enabling Signed-Request Replay with Arbitrary Body

Summary

The Pusher-compatible REST API includes body_md5 in the HMAC signature string but never computes or verifies the MD5 of the received HTTP body, allowing anyone who observes a signed request to replay it with an entirely different body.

Details

In pusher/http.go, the Handler function extracts body_md5 from the URL query string (line 169) and includes it verbatim in stringToSign (line 175). It then verifies HMAC(stringToSign, secret) == auth_signature. After verification succeeds, handleEvents reads and parses r.Body (lines 201-212) without ever computing md5(body) and comparing it against the body_md5 that was signed. The Pusher protocol specification explicitly requires the server to verify this digest to prevent body-substitution attacks. There is also no auth_timestamp staleness check, so replays are valid indefinitely.

PoC

1. Capture a legitimate signed POST to /apps/<app_id>/events?auth_key=K&auth_timestamp=T&auth_version=1.0&body_md5=LEGIT_MD5&auth_signature=SIG carrying body {"name":"safe-event","channel":"ch","data":"..."} (e.g., from TLS-terminating load-balancer logs).
2. Send a new request with the same query string parameters but a different body:
   {"name":"injected-event","channel":"admin","data":"malicious-payload"}
3. The server accepts the request (HMAC over stringToSign matches the original) and broadcasts the injected event to all subscribers of admin.

Impact

An attacker who can read any single signed Pusher API request (from logs, a shared proxy, or a network tap) can broadcast arbitrary events to any channel indefinitely, potentially forging server-side events, corrupting application state, or delivering phishing messages to WebSocket clients.

Fix

After reading r.Body, compute hex(md5(body)) and compare it to the body_md5 query parameter using a constant-time comparison before proceeding. Additionally, reject requests whose auth_timestamp is more than 600 seconds from the current time.