GHSA-5p54-whvp-x327

### 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.