UnreadRoomsChannel streamed from a hardcoded global name, and every message
published its room id to it. Any authenticated user, including one with no
memberships at all, could subscribe and watch which rooms were active and exactly
when, across every closed room and direct conversation on the account. The browser
filters ids it doesn't recognise, but that happens after delivery.
Its sibling ReadRoomsChannel is already scoped per user; this mirrors it, and
message creation fans the notice out to the room's members instead of broadcasting
it to everyone. No message content was exposed either way, only the timing.
Message content is delivered over turbo streams, which ran on the stock
Turbo::StreamsChannel. That channel verifies the signature on the stream name and
nothing else. The name carries no expiry and no binding to a user, so one read off
the page while a member kept working after the membership was revoked.
Revocation made this worse rather than better. Membership#after_destroy_commit
disconnects the user with reconnect: true, and the client replays its subscriptions
on the new socket: RoomChannel re-checks membership and rejects, while the turbo
subscription re-verified only the signature and was accepted.
RoomMessagesChannel re-checks membership on every subscribe, deriving the room from
the verified stream name so there is no parameter to point elsewhere. Since the
subscriber names the channel it wants, the stock channel would otherwise be a way
around that check, so it now turns these stream names away and this is the only door.
Jeremy Daer
Kevin McConnell