What is a Reorg?
A reorg (chain reorganization) occurs when a blockchain discards one or more recently confirmed blocks in favor of an alternative chain with more cumulative work or stake support. During a reorg, transactions that appeared confirmed may become unconfirmed or replaced, potentially reversing what seemed like finalized state changes. Reorgs are a natural part of blockchain consensus but can have serious implications for users.
How Reorgs Happen
In proof-of-work systems, reorgs occur when two miners find valid blocks simultaneously, creating temporary forks. As one branch extends further, nodes abandon the shorter chain and adopt the longer one following the longest-chain rule. The orphaned blocks' transactions return to the mempool unless they conflict with the new chain.
Proof-of-stake systems like Ethereum can experience reorgs during network partitions, attestation delays, or when competing block proposals receive initially similar support. If validator attestations temporarily favor a non-canonical branch before correcting, a reorg results. Ethereum's Casper FFG provides finality that prevents reorgs of finalized blocks, but recent unfinalized blocks remain potentially reorganizable.
Intentional reorgs through 51% attacks allow malicious actors controlling majority hash power or stake to deliberately reorganize the chain, potentially enabling double-spend attacks on exchanges or other services that accept insufficient confirmations.
Reorg Impacts
Reorgs affect transaction validity and smart contract state. A swap that appeared successful might disappear if the block containing it gets reorged. More seriously, an attacker could deposit funds to an exchange, receive credit, withdraw, and then reorg to reverse the original deposit. Completing a double-spend attack.
MEV searchers sometimes attempt to trigger small reorgs to capture profitable transactions or reorganize block contents for extractable value. This "time-bandit" attack becomes economical when MEV opportunities exceed the cost of block manipulation.
Protection Strategies
Exchanges and high-value services require multiple confirmations before crediting deposits. Typically 35+ confirmations for Ethereum (about 7 minutes) and 6 for Bitcoin (about one hour). Waiting for finality on PoS chains provides absolute protection against reorgs.