Reorg Attack

A reorg attack, or chain reorganization, occurs when an attacker with significant hashing power creates a longer chain of blocks than the honest network, causing the network to switch to the attacker's chain. This allows the attacker to revert confirmed transactions and potentially double spend assets.

By effectively rewriting history, the attacker invalidates transactions that the recipient believed were final. This is a primary risk in proof-of-work networks with low total hash rate.

The defense against such attacks is to wait for a sufficient number of confirmations, making it exponentially more expensive for an attacker to rewrite the history. It highlights the importance of chain depth in determining the security of a transaction.

Reorgs are a major concern for exchanges and platforms that rely on fast settlement.