The SSE client→server POST endpoints are @csrf exempt and the SSE GET stream endpoint had no Origin check, so a cross-origin page could drive a victim-cookie-authenticated SSE session: force the victim's browser to GET the stream URL (which creates and mounts a LiveView as the victim) and POST to the message endpoint with credentials: include to fire state-changing event handlers as the victim. The URL session id is client-chosen (validated only for UUID format), so it is not a CSRF token, and a JSON body sent as text/plain is a CORS simple request with no preflight.

Fixed in djust 1.0.7. All three SSE endpoints validate the request Origin against ALLOWED HOSTS (mirroring the WebSocket CSWSH defense) and reject cross-origin requests with 403; the POST endpoints additionally require Content-Type: application/json (415 otherwise), closing the text/plain simple-request bypass.

Disable the SSE transport, or front it with a proxy that enforces an Origin allowlist.