Yes, it is possible for a a group or person to do a 51% attack, but is it realistic, even at 10%? Right now for a person to buy up even 10% of NXT would be, $4,000,000 and that is IF they could buy it at market value, but to buy that much would surely cause a huge spike in price as the buyer would be buying out all the sell walls, and new walls would go up at even higher prices. So in reality it would be much much higher than $4,000,000 to buy 10%, and then said buyer would have to offload their 10% which would cause a huge huge drop in price, probably crashing it to next to nothing. All this so that they could try to double spend a few coins when now the market is basically bottomed out. Does this make sense or am I missing something?
What you are missing here is that you dont need 51% of stake and a person doesnt need to buy 10% of the tokens. Just needs access to token history of some %.... PoS can be even attacked by even less than coins given enough trials since the attacked incurs no costs by just trying. This account mints 1 out of 4 blocks (last 100 blocks)
http://87.230.14.1/nxt/nxt.cgi?action=3000&acc=4747512364439223888 Do you think this person does not have the power to double spend again and again and again with 25% of the minting power? He can keep trying. No cost if he fails thousands of times... That 25% minting power is just 5% of total stake.... Not sure if you understand the difference. Also using 5% to earn so small earnings for forging is probably uneconomical for the rational forger who will prefer to put the coins in better use... i.e. deposit them in a just-dice.com type of service to earn more. If such services/stock markets compete for capital you should expect the effective stake for forging to be much much less