The public WebRTC signaling WebSocket (`/public/signaling/{session_token}`)
validated only the session token and its expiry, not the embed token's
allowed-domain policy that the HTTP embed endpoints already enforce. A leaked
or replayed session token could therefore attach to the signaling path from
an arbitrary origin.
Validate the request origin against `embed_token.allowed_domains` (reusing the
existing `validate_origin` helper) before the signaling handoff, rejecting
disallowed origins with a 1008 close — mirroring the HTTP embed endpoints.
Closes#330
Co-authored-by: shiminshen <16914659+shiminshen@users.noreply.github.com>
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