Once a player is docked at a planet, will it be easy for you to steal his ship, or will there be security measures?

No, no. Nope.

Sorry, prospective ship thieves. So if you only backed Star Citizen for like $5 because you're like, I'm just going to steal another player's ship from their hangar, it's not happening.

So once you land on a planet, your ship is safe?

That is the plan, as far as I know, in almost every place. There may be cases where it's not. For instance, if you're doing a boarding action, no, your ship is not safe, but that's different. But as far as landing on the planet, so far as I know, that's been the plan all along, and I haven't heard anything to change that.