It's become a familiar pattern in online security. A groundbreaking way to communicate emerges, spreads like wildfire, and then hackers find a way to use it to their advantage. Security companies react—but not before the problem has succeeded in wreaking havoc. It happened with e-mail and is happening now with instant messaging and mobile devices.

The next area that could be targeted: Voice over Internet Protocol, or VoIP, which lets people make low-priced phone calls using the same technology that delivers e-mail. And the results could be just as damaging, if not worse, than with other technologies, some security experts warn.

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