[csomar]

The thing is, what you can do with a 51% attack is very limited and the big services are probably going to be aware of it. Best case scenario is that you halt transactions for a few hours. This will cost a few million dollars with, maybe, no possible reward.

The next problem is that this calculation is based on the current hash-rate cost. However, you don't have that much hardware and it'll be close to impossible to rent half of bitcoin mining rate. So such an attack will be order of magnitudes more expensive if even possible.

[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).