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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[–] SkaveRat@discuss.tchncs.de 8 points 1 week ago* (last edited 1 week ago)

Abolutely necessarily.

it works like this:

@privateuser@mastodon.example.com has a "followers only account".
@someuser@pixelfed.example.com is a friend of above account, requested access and was granted. This now causes mastodon.example.com to push all messages of @privateuser to pixelfed.example.com.
@anotheruser@pixelfed.example.com requests access, but gets ignored. But the pixelfed instance marks the user as "follows @privateuser"
In the interface of @someuser, the messages are shown as expected.
In the interface of @anotheruser, they are also shown. Because PF basically does a database "select messages of users that the user follows", without checking if the access was ever granted.

Important to note, that this would not happen, if the messages weren't already pushed to the server due to the "allowed" user