This question already has an answer here:

Without security deposits, there is no dis-incentive for signing multiple blocks on multiple chains, and so the validators incentive is to sign off forks when they come up.

What attacks would be possible, without security deposits as a solution to "nothing at stake"?

marked as duplicate by Badr Bellaj, Rob Hitchens - B9lab, Jeff Coleman, AdrianClv, niksmac Jun 13 '17 at 2:16

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