I guess in theory, malicious actors have some options trying to target and overload all of the Fediverse, but I don’t think any are really feasible. At the point, where you could take out the (vast majority of) nodes with something like a DDOS, you already had enough resources to spare, to take out just significant parts of the overall internet altogether.

0-day exploits could of course be problematic, as they are for anything, but even then, using one on all (or then majority of) nodes simultaneously, and/or distributing a payload to all/the majority of nodes is also just so much effort. Also, chances are quite low you will get an exploit for all platform software, and if it targets something in ActivityPub, then all you can do at best is stifling federation, I guess, still allowing for local content to remain up.

Not only censorship resilient and with dynamic interlocking communities, also pretty damn resilient when it comes to overall uptime security.