Yeah, there's also this:

A more recent issue came about when Pixelfed’s creator, Daniel Supernault made the details of a vulnerability public before server operators had a chance to update, which would have left the fediverse vulnerable to bad actors, she says. (Supernault has already apologized publicly for his handling of the issue that had affected private accounts.)

In the case of the Pixelfed issue, for instance, the Hachyderm Mastodon server, which has over 9,500 members, decided it needed to defederate (or disconnect from) other Pixelfed servers that hadn’t been updated in order to protect their users.

It is weird to spend almost half the words in this, pretending that something in Pixelfed that wasn't a problem on Pixelfed's side was. This is the weirdest "vulnerability" in the world to pick if you want to pick one to hold up extensively as an example.

1. This is nothing to do with ActivityPub. It's to do with Mastodon's custom implementation of "private" posts.
2. Making it extremely clear to everyone that random server software can expose Mastodon's "private" posts is absolutely the right way to handle disclosure here. Dan didn't even try to do that, he just fixed the bug, but if he had made a big post saying "hey this is not my fault Mastodon private posts are not private, here's full explanation about what's going on" I think that would have been completely fine. This is not a "vulnerability" in the traditional sense like a buffer overflow, it's just a design flaw in Mastodon which other softwares are by convention agreeing to cater to. I think the culture of security (and the level of clue in general) in the Fediverse has wandered into territory where "let's all pretend that these posts are secure and get mad at anyone who reveals that they are not" is widely accepted now, but that doesn't make it right.

Maybe I’m wrong, but shouldn’t posts only be insecure if they’re propagated to the insecure instance?

"Insecure" in this case simply means any server that doesn't implement Mastodon's custom handling for "private" posts. With that definition, the answer to your question is yes. It has been mentioned by Mastodon people that this is a significant problem for the ability to actually keep these private posts private in the real world. The chance of it going wrong is small (depending on your follower count) but the potential for harm is very large. I would therefore go further, and say that it's a very bad thing that Mastodon is telling people that these posts are "private" when the mechanism which is supposed to keep them private is so unreliable.

marrus-sh.github.io/mastodon-i…

github.com/mastodon/mastodon/i…

Is any private post visible to people on servers that the poster doesn’t have followers on?

It is not. If you're sufficiently careful with approving your followers, making sure that each of them is on an instance that's going to handle private posts the way you expect, then you're probably fine.

Could I curl the uri of a post thats “private” and get the post’s content?

If it's been federated to an insecure server then yes. If not then I think no.

custom federation levels (at the very least, for private posts) · Issue #712 · mastodon/mastodon

pretty much every user i've seen around who makes use of private posts wants the option for them to federate i understand that there was backlash over this when it was implemented, but i think havi...

^GitHub

Mastodon really is the internet explorer of the fediverse.
In any case, I don't think its that bad. I would compare it to an email provider accidentially leaking messages. Still bad, but its not a reason to abandon email as a means of communication.
We should encrypt posts, like diaspora does. Like how we should pgp encrypt emails, but no one will.

also, I just checked myself, a random "private" post I made isn't accessible over AP if I curl it unauthenticated.
Running curl.exe https://calckey.world/notes/a63slz8j6l -H "Accept: application/activity+json" returns nothing, but replacing the uri with a public post does show it.
An insecure server's copy of the post isn't accessible over AP, only the original post's link should return anything.