Adware is running rampant across the Android landscape, according to security firm Lookout. It says that as many as 6.5% of all free apps in the Google Play Store contain Adware. That's just under 50,000 apps. With so many apps containing adware, it is no surprise that more than one million U.S. Android users have unwittingly downloaded the stuff over the course of the last year.

As Lookout defines it, Adware is an app that "displays advertising outside of the normal app experience, harvests unusual personally identifiable information, or performs unexpected actions as a response to ad clicks."

Adware isn't affecting just U.S. users. Lookout estimates that 600,000 French users have downloaded adware, as have 400,000 German users, and 200,000 British users. The percentage of adware has climbed 5% year-over-year. It's much worse in app stores that aren't run by Google, naturally. Google does have some protection schemes built into the Play Store, but they are clearly not perfect.

[ Mobile ads have other consequences. See Mobile Ads Drain Battery Power. ]

Apps that help users customize their devices are the most likely to contain adware, said Lookout. Customization apps are those such as wallpaper or ringtone apps, of which some 26% contain adware. Lookout doesn't provide similar data for iOS apps, but Apple's stricter control over its ecosystem means there are fewer problem apps in the iTunes App Store. There is no comparable data for BlackBerry 10 or Windows Phone.

Many of today's apps are offered for free because they include advertising. Advertising networks are a vital part of the mobile app economy. Without display ads, many mobile apps couldn't be given away for free. Unfortunately, much of the adware comes from these advertising networks.

"Ad networks that overstep user privacy bounds or negatively affect user experience create conflict, compromising not only consumers' privacy and user experience, but also the developer community that relies on mobile ad networks to bring free products to market," explained Lookout.

Of course, Lookout sells software that helps to secure smartphones. It has a number of distribution deals with various handset makers and wireless network operators. Sprint, for example, pre-installs Lookout's mobile security app on most of its Android smartphones. The app can be used to scan for malware and protect against virulent downloads. There are free and paid versions of the application. The company has a vested interest in ringing the alarm bell.

Even so, its argument that there need to be better guidelines for mobile ad networks isn't a bad one.