[lordnacho]

> A 51% attack doesn’t let you steal money from anyone.

1. You deposit BTC at an exchange. The exchange credits you the amount in their non-BTC ledger.

2. You send off a chain of blocks overwriting the the original deposit so that you never did it.

3. You fill in the form to withdraw your credited amount from the exchange.

Now you have 2x the coins.

Of course there are a LOT of details to this that I won't get into, and a number of mitigations for the exchange. But that's the basic outline.

[pat2man]

Yeah exchanges and all other external systems need to handle this. Effectively they should look at the possibility of a deep reorg and the potential cost to them and use that to adjust how many transactions they require until the risk is mitigated.