> There is one problem the author mentioned which applies to XMPP but not to email/ActivityPub: presence tracking. [...] clients and servers to advertise the presence status (online, away) to the entire world, and that's a huge metadata leak.

They don't advertise it to the entire world, but to authorized contacts (and typically groups that you join). With the rise of mobile clients, statuses other than 'online' are far less common than they used to be, because they are growing irrelevant.

Advertising online status to the whole world is what WhatsApp does ( https://news.ycombinator.com/item?id=26809770 ).

> and typically groups that you join

So more or less to the entire world if you join public chats. Even if you use different nicknames, occupant-id (used for message edits) may be used to track your presence across rooms.

> Advertising online status to the whole world is what WhatsApp does

If we're pointing the finger, i'd recommend reading this document about what Signal does wrong in terms of privacy and security. It's overall a good read of what not to do in private messenger UX:

https://north-shore.info/wp-content/uploads/2019/06/signalfa...

>They don't advertise it to the entire world, but to authorized contacts (and typically groups that you join)

But in the article they showed how the server administrator can easily add contacts to a user's contact list. So my understanding is that the user isn't really in control of who can see this information, as the server admin could add a new contact that can then see the presence status. Correct?

Edit: formatting

Valid question, not sure why the downvotes. You're right, the server can modify the contact list (and this is used extensively within organizations, and in Snikket to ensure family members can see each other by default).

But this isn't the cause for concern here - if the server is malicious it can broadcast this info without the need to modify your contact list. This again is independent of protocol - if someone wants to log or share the times and durations you are actively connected to their server, they can do so quite easily. It doesn't require XMPP.

> the user isn't really in control of who can see this information

Server-side parties (e.g., the admin) can see the status information without adding somebody to the group. It is in cleartext, passing the server.

Strange criticism. Advertising presence information is the _goal_ of XMPP protocol. The "P" in XMPP stands for Presence.

Indeed! However, some things have changed since XMPP initially came out. For example, Message Archive Management (and equivalents in the PubSub/MIX specs) has been introduced so that messages can be stored reliably, reducing the need for presence. Multi-client usage has also grown, especially with mobile computers ("smartphones") with Internet access everywhere. Then, we now know and have proof that metadata leaks are used to actually harm people and that presence information can be used to establish correlations between several identities.

To be clear, I'm not arguing against presence being supported on a protocol level. However, i believe the protocol and clients/servers should support the usecase where presence information should not be leaked: that people see a message when i send it is fine, but that they know when my computer is online/offline, or when i'm "away" is way too much metadata leak for a privacy-conscious usecase.

My favorite way to deal with that would be to have a "private browsing" mode on IM clients. So that you could turn on/off privacy settings globally with a single switch. I'm guessing a lot of people would choose the "private" mode if prompted for it on first run. I know i would :)

It is quite possible not to send presence information at all. I'll say more, we're working on an 'omnipresence' protocol that will advertise only the desired status to the external contacts, and will not leak per-device presences.

This Presence thing works well for always connected clients (desktop), but becomes a huge problem on clients that can't be always connected (iOS ones).

Desktop is not always connected. People turn it off, or suspend it to save on electricity bills.

From the point of view of xmpp development, it is always connected. Thing is, xmpp session start is a very costly operation. Desktop generally does it once and has it easy from then on. iOS device connects every time a user opens an app. By the time it'll finish receiving presence information, a user will already close it, and the OS will terminate app process.

Unless the protocol is connectionless, the server will always know at least the user's online status.

I have to use Slack for work, and the way I solve this is to have my client not send typing notifications, and have the client running on my server and repeatedly sending activity notifications all the time, so I always appear online and present to Slack servers.

I can prevent Slack from knowing when I'm really connected only by being connected all the time and always pinging my activity status in periodic fashion 24/7, so that from PoV of the server it's not possible to determine anything about my actual activity or online status on my workstation, because there's no relation to real-life status.

Would these leaks be solved by implementing a VPN at the router instead of the host?