A controversial developer circumvented one of Mastodon's primary tools for blocking bad actors, all so that his servers could connect to Threads.
Authorized Fetch (also referred to as Secure Mode in Mastodon) was recently circumvented by a stupidly easy solution: just sign your fetch requests with some other domain name.
Fixing this in general is not so easy as ActivityPub wasn't designed to prevent such things and AFAIK without some fundamental changes like proposed in Spritely or implemented in the Zot protocol it can't really prevent this from happening.