How to find a Minecraft server seed
A world seed is the number the terrain generator started from. Knowing it lets anyone locate every structure in the world instantly, which is why servers guard it and players want it.
The command, when it is allowed
In single player and on a server where you have permission, this prints the seed:
/seedOn most multiplayer servers it needs operator level 2, so a normal player gets a permissions error. That is the intended state and not a bug.
Server owners can see it in server.properties under level-seed if it was set deliberately, or by running the command from the console, which always has full permission.
Why servers hide it
With the seed, anyone can load the same world offline and read the exact coordinates of every stronghold, ancient city, bastion, buried treasure and slime chunk. On a survival server that removes most of the exploration and a good deal of the point.
It also affects fairness. A player who knows where the strongholds are reaches the End on day one while everyone else is still throwing eyes of ender.
It does not expose anything about the server itself. A seed is not a password and leaking one is an annoyance rather than a security problem.
How seeds get worked out anyway
Terrain generation is deterministic, so world features are evidence. Given enough observed structure positions, biome boundaries or end pillar arrangements, the seed that produced them can be narrowed down and often recovered. Tools that do this are freely available and this is a well documented area.
Older versions are much easier because parts of the world generator used fewer bits of the seed, so features like the End pillar layout leak information directly. Newer versions are harder but not immune.
The practical point for a server owner: assume a long running world with many players is eventually crackable, and design around that rather than relying on secrecy.
What owners can actually do
Restrict the command. Make sure /seed is operator only, and check that no plugin exposes it in a different command. Our permissions guide covers locking individual commands.
Do not reuse a public seed. A seed from a popular seed list is already known, so it never needed cracking in the first place.
Pregenerate and set a border. A bounded world limits what anyone gains from knowing where distant structures are, and pregeneration is worth doing anyway for performance.
Accept it for the main world. The strongest answer is a resource world reset on a schedule, since a seed that changes every few weeks is worth very little. Our world reset guide covers setting that up.