[dkasak]

The risk being accepted is that a homeserver can currently add members to the group with the group being notified of this.

This risk will be removed completely once TOFU and signed control events are implemented, which is planned (and was planned before this research). It's just more work than could fit in the disclosure timeline, especially because it's a large change needing ecosystem coordination.

[tptacek]

I don't think "the group being notified" that an unauthorized member can decrypt all their messages is quite the mitigation that Matrix advocates think it is.

This is the fundamental task of any secure group messenger. It has really one job: don't let unauthorized people read messages for the group. Here, Matrix has apparently accepted the risk that their group messenger can't do that job if the server is compromised. If you know where to look and your group is small enough, you can constantly watch to see if your homeserver has decided to stop protecting your group, but either way: your homeserver can spontaneously decide to stop protecting your group. Matrix, you had one job!

At the point where you accept this risk, you might as well just use Slack.

[throwawayKiwi9]

Your suggesting to use Slack, where a similar compromise would reveal your entire account message history.. ? Just enforce proper key verification for now and you're fine..