A 51% attack will let you double spend and roll back transactions. If one wanted to steal an established name on Namecoin, they&#x27;d need to either roll back to before it&#x27;s registration, roll back to before it&#x27;s previous renewal and force an expiration, or mine past it&#x27;s next renewal to force expiration. Renewal limit is 36,000 blocks. For a name that consistently renews early every 18,000 blocks a 51% attack would need to go on for months to steal it. Still possible, but very difficult.<p>Namecoin is merge-mined with Bitcoin by many miners, so if you wanted to 51% it you&#x27;d need a pretty large chunk of Bitcoin&#x27;s hash power. An evil pool could do this, however it would be visible on the Bitcoin blockchain that they were doing it (although not what transactions are present&#x2F;not present in the attack chain).