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.. . .

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.

"We figured out how the person got in and patched the system," said Ed Skoudis, a hacking expert at security firm Predictive Systems, which was called in to fortify the company's networks. "We deal with about two intrusions per month, and we're just one of the many teams out there doing this work. We're not dealing with denial-of-service attacks or script kiddies playing around, but skilled financial intrusions."

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