What is a chain reorg?
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 5 min
In short
A reorg is when blocks already accepted are discarded and a different branch becomes the canonical chain. Shallow reorgs of a block or two fall within normal operation. Transactions in the discarded blocks return to pending, so treating a low-confirmation deposit as settled can cost you.
Key points
- Accepted blocks are dropped in favour of another branch
- Shallow reorgs occur during normal operation
- Reverted transactions go back to pending
- A deep reorg is a reason to suspect an attack
Definition
The replacement of part of the chain a node considered canonical with a competing branch that now takes priority. The displaced blocks and their transactions lose their accepted status.
When two blocks appear at almost the same time, the network briefly holds two branches. As soon as a further block extends one of them, nodes switch to that side and undo the blocks they had accepted. That is a reorg, and at a depth of one or two it is unremarkable.
Transactions from the undone blocks return to the mempool and wait again. Usually they confirm in the next block without incident — but if a competing transaction spending the same funds confirms first, the original never takes effect. This is precisely why confirmations matter.
Deep reorgs are a different matter. Rolling back many blocks requires enough hash power or stake to overtake the existing chain, which points to deliberate attack. The smaller a chain's total hash rate or staked value, the cheaper that attack becomes.
Watch out for
- · Releasing goods or another asset on a low-confirmation deposit leaves you exposed if it reverses
- · Small chains are cheaper to reorg deeply, so the same confirmation count is not equally safe everywhere
- · A transaction vanishing from an explorer may still confirm later — do not act on the first reading