Unknown hacked client able to crash server with low bandwidth attack

A player joined my server, it started crashing right away, every time it came back up, maybe a minute passed then it crashed again, as you can see from those log file names.

The crash was triggered by the server after detecting a tick took longer than 60 seconds (good feature that) and i have the server start command in a loop so it starts again if it crashes.

I do not have the client he had but after doing it multiple times he said this:
[04:43:18] [Server thread/INFO]: <§aBoomheadshotUK§r> Hope you enjoyed your server while it lasted. Goodnight my sweet prince~ Cyanit Hacking Sqaud.

And yes that was his player name.

I also have a packet capture using tcpdump of a couple of the last minutes he was there if that's useful, he did crash the server once after i started that command so i believe the packets he used are there, if you want it, let me know, it's 11 MB.