I reported this to Clubhouse in February, no response whatsoever (I am not involved in this leak, just to be extra clear). Essentially anyone with the token from the iOS app (MITMproxy + SSL kill switch) can query through the entire public (records are cleaned) user profile database. It supports wildcard queries and just responds with some 20M records you can page through if you have the time. It luckily (!) doesn't expose e-mail and phone number, which is why I also agree with others here that this is only mildly interesting. The news won't care, however. I think at around 4M users or so they switched from auto-increasing IDs to a better numbering format, until then all records remain as-is (increasing).

I think Clubhouse can fix this quite easily (limit the records returned in search!!!) and apply some harsher rate limits on a per-token basis (tokens never expire, that's another thing).

I think they relied a bit too much on certificate pinning. Once that's bypassed, it's relatively easy to query your way through the data. If you managed to grab someone else's token (which doesn't expire), you impersonate them (without logging the other session out), and continue to show up/talk in rooms using the Agora SDK as that person.

They also do upload phone numbers of the address book in clear-text (non-hashed), although I can see that there's not too much of a point because reverse-hashes can maybe work around this easily if not salted.