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.

Cracked servers are popular among pirates, as this allows them to play Minecraft multiplayer without having to buy the game. However, offline-mode can also present a security risk, as it allows anyone to join the server with the username of an admin and thereby gain access to admin-only commands.

Because of this “username stealing” issue, most public cracked servers use online-mode=false in combination with an alternative authentication system, like AuthMe Reloaded or LoginSecurity for example.

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