When following someone on a different server on the Fediverse, the remote server decides whether you are allowed to do so. This enables features like private accounts. Due to an implementation mistake, Pixelfed ignores this and allows anyone to follow even private accounts on other servers. When a legitimate user from a Pixelfed instance follows you on your locked fediverse account, anyone on that Pixelfed instance can read your private posts. You don’t need to be a Pixelfed user to be affected.

Pixelfed admins should update to v1.12.5 ASAP, but upgrading can be a major hurdle.

Importantly, your Mastodon or GoToSocial instance isn’t handing your private posts to any random server, just because it asks. The problem only becomes apparent when you have at least one legit accepted follower from a Pixelfed server. Now that server is allowed to fetch all your private posts. And when it knows the posts, it has to decide who to show them. When you accept a follower, you not only place your trust to keep a secret on them, but also on their admin and the software they are running.

Edited to add the last block quote.

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[–] PhilipTheBucket@ponder.cat 2 points 5 days ago* (last edited 5 days ago)

content should be private by default, nowhere is stated otherwise

This is completely false. Read section 7.1, "Note: Silent and private activities". It specifically says that privacy behavior, for activities with no recipients at all, is undefined. It recommends not showing them to anyone, obviously, but that "behavior is not defined" has a very specific meaning in a specification document. It means, if you sent an activity of that type to someone, trusting that they would then keep it private, then you fucked up, because behavior in that area is undefined and cannot be relied upon.

That's not "rules lawyering." That is how specification documents work. That's an important note, which I suspect is why it is highlighted and in its own separate box. There are some similar parts of the document, involving the big word "MAY" in all caps where they had the option of writing "SHALL" or even "SHOULD", to indicate that a server had to keep certain things private, that follow the same philosophy.

None of that means you can't use some common sense. It's obviously not good to be handling intended-to-be-private information in some way that the sender doesn't expect, and that's why Dansup fixed it quickly when it was brought to his attention (particularly since the issue wasn't even directly related to access control on private posts, just in a subtle interaction involving approved-followers-only users and a setting that was failing to federate). My point was just on the broader issue, that if Mastodon is sending out "private" statuses to random servers, then this is at the root a Mastodon issue. The quick fix (regardless of whatever it was about that made the blog poster even more upset when Dansup took it seriously and fixed it quickly) puts the lie to your assertion that Dansup is "toxic" "ignoring what the federation requires" and so on.

I suspect that we're going to keep going around in circles on this forever. I have a new strategy when someone is just endlessly arguing with me about some weird minor issue. I just make a new post dealing with the issue in more depth, so that it's not just you and me endlessly going in circles deep in the comments at each other. You're welcome to come to that post, and continue the conversation there, if you'd like to:

https://sh.itjust.works/post/35210537