The online-mode=false option in server.properties.

A cracked server is a server that doesn't use Mojang's authentication system, meaning that any player can join the server with any username, even if they don't actually own the corresponding Minecraft account.

Server owners achieve this by setting the online-mode option to false in the server.properties file.

• PlanetMinecraft: "Offline Mode/Cracked servers - What are they?"