Are a blockchain reorganization attack and a 51% attack the same thing? Is 51% of the hashing power necessary for a reorg attack?
With 51%, an attacker can do these things:
1. Reorg previous blocks, removing (and optionally replacing) transactions that have already been confirmed.
2. Censor any or all transactions.
3. Claim all of the block rewards.
An attacker is not required to have 51% in order to force a reorg, but success is not guaranteed without 51%.