[joosters]

You can do much better than that! The simplest way to profit from a 51% attack is to send some coins to a crypto exchange, wait for the required 'n' blocks to confirm your deposit, then 51% attack the chain to remove your original payment. At the same time, you can either withdraw your ill-gotten coins from the exchange, or trade them for something else and withdraw onto a different blockchain entirely.

This mechanism lets you double your money (minus the cost of the 51% attack)

[SilasX]

But don't most exchanges have a bigger delay for withdrawing? In order to escape with anything, you would have to withdraw before the exchange was aware of the double-spend, and at that point, they'd freeze your account. The exchange might be on the hook for honoring the phantom coins other users think they bought, but you haven't profited. Am I missing something?

[InvaderFizz]

This would require funding, exchanging, and double spending all in the span of a few minutes.

The more confirmations you have to unwind, the more work you have to do to catch up with the long chain.

All that will happen is exchanges require a greater number of confirmations before allowing you to trade deposits.

[joosters]

All true, but it is exactly what happened in this latest attack!

And yes, reportedly exchanges have increased the number of confirmations required to confirm a deposit as a result. But it's too late for the ones who got ripped off by the double spend!

[bduerst]

And most exchanges require numerous confirmations for this reason.

[csomar]

From my understanding this is not a 51% attack but rather a chain-rewrite which require significantly way more hash rate than the bitcoin network has. (which is why exchanges consider 3-6 confirmations as the safe number).