Late one recent Sunday night, an executive at a midsized financial services firm received the kind of call everyone in the industry dreads: a demand for $1 million, or else the brokerage's network would crash the next day with a surreptitiously installed program.

The firm's security team spent a frenzied night searching for the pernicious code but failed to find it, and the system went down for an hour in the morning. The executive's phone rang once more: The caller threatened to crash the system again, but this time during peak trading hours. The brokerage, in this case, paid up.

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