most activitypub implementations don't really validate fetches: knowing an activity/object id will allow anyone to resolve it on most fedi software. this is of course unacceptable: "security through obscurity" just doesn't work
μpub correctly and rigorously implements and enforces access control on each object based on its addressing
most instances will have "authorized fetch" which kind of makes the issue less bad, but anyone can host an actor, have any server download their pubkey and then start fetching
μpub may be considered to have "authorized fetch" permanently on, except it depends on each post:
* all posts marked public (meaning, addressed to "https://www.w3.org/ns/activitystreams#Public"), will be fetchable without any authorization
* all posts not public will require explicit addressing and authentication: for example if post A is addressed to example.net/actor
* anonymous fetchers will receive 404 on GET /posts/A
* local users must authenticate and will be given said post only if it's addressed to them
* remote servers will be given access to all posts from any of their users once they have authenticated themselves (with http signing)
note that followers get expanded: addressing to example.net/actor/followers will address to anyone following actor that the server knows of, at that time
μpub, sometimes stylyzed `upub`, is pronounced `mu-pub` (the `μ` stands for [micro](https://en.wikipedia.org/wiki/International_System_of_Units#Prefixes))
