The Billion-Dollar Bet That Better Software Can Back Off Hackers

 About the only thing rising as fast as online mischief is the stock of firms trying to thwart it.

Companies from FireEye Inc. to Palo Alto Networks Inc. have taken off in 2015, extending the gain in a four-year-old index tracking network security firms past 200 percent. An exchange-traded fund tied to the shares just surpassed $1 billion in market value, having doubled in size since the start of April.