"Mobile devices are scary because people generally have no idea what the software they download will do, whether they get it from an app store or it comes with the phone," says Schmidt. "The apps on mobile devices are not at all transparent. A lot of software gets installed on them that users don't understand."

Smartphones have become an effective way for criminals to distribute malware because it's harder to recognize on a smartphone than it is on a PC. "Screen real estate is very limited on these devices," he says. "The visual cues we're used to on PCs [when we download a virus] are not available in a mobile environment. Even to sophisticated users, it's not entirely clear what's happening behind the scenes."

Faster connectivity and more powerful devices further complicate security. Schmidt says both factors make it easier to download malware more quickly, without the user knowing. "That makes a compromised device more valuable to a bad guy," he adds.

It also makes smartphones more susceptible to drive-by downloads.

How Drive-By Downloads Work on Your Smartphone