The public one at echo.websocket.org tends to be slow at times,
frequently causing timeouts. Ideally we would run a local websocket
server but for now we can make do with a self-hosted alternative.
Previously, we would only keep the cell that must survive alive, but
none of it's edges.
This cropped up with a GC UAF in must_survive_garbage_collection of
WebSocket in .NET's SignalR frontend implementation, where an
out-of-scope WebSocket had it's underlying EventTarget properties
garbage collected, and must_survive_garbage_collection read from the
destroyed EventTarget properties.
See: https://github.com/dotnet/aspnetcore/blob/main/src/SignalR/clients/ts/signalr/src/WebSocketTransport.ts#L81
Found on https://www.formula1.com/ during a live session.
Co-Authored-By: Tim Flynn <trflynn89@pm.me>