Can a Successful 51% Attack Break Immutability?

Yes, a successful 51% attack can temporarily break the probabilistic immutability of a blockchain. By controlling the majority of the hashrate, an attacker can create a longer, private chain that reorganizes the transaction history, reversing recent transactions (double-spending).

While the entire history of the chain is not permanently rewritten, the recent transactions that were considered finalized are reversed, undermining the network's core promise of transaction finality and immutability for those specific transactions.