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"You could punch any block coordinate in the entire map, and 2b2t would either:

-Tell you what block is placed there, if the chunk is currently loaded

-Or ignore you, if the chunk is not currently loaded.

2b2t would immediately reveal whether or not any chunk coordinate that you ask about is currently loaded. If it is currently loaded, it tells you which specific block is there at the coordinate you asked about."

This allowed the hackers to find every player's coordinates and where their bases were.

There's a section called "The entire thing summed up in three sentences" in the linked GitHub README.

Its a typical case of Software developer not validating player input 'because thats too taxing on the server'. Minecraft server was programmed to reply to all, even stupid or impossible queries.