It appears the saying "there's no such thing as a free lunch", which warns us that things which appear to be free usually come with strings attached, can now be applied to free apps as well; in 5% of those apps at least, and the number is rising according to a recent study by Lookout, a mobile security firm.

Free apps which run ads from aggressive ad networks now number in the thousands, and have been downloaded 80 million times. Those networks can not only infiltrate your phone to change settings and steal your personal data, they can even install malicious programs on your phone, all without your knowledge and certainly without your consent.

The benefit to app developers using these networks is they make more money from these ads, and some app developers aren't shy about admitting their use or former use of such ad networks.

One such company is PhoneLiving, which makes various talking animal apps. The company admitted they had been using invasive ads with their apps for some time to generate more revenue, but have since stopped as the negative reviews climbed, and the number of downloads of their apps subsequently dwindled.

"We have removed all of the notification/icon ads from all of our talking apps," a company spokesman said. "We have made this switch to benefit our users despite the lower profits involved in other types of ads."

Like malware on PC's, it isn't always easy to get rid of the offending programs or files once they're on your phone. While it's usually as simple as deleting the app that contains the rogue ads, it often takes investigation to discover which app is to blame, especially if you download apps in bunches.

"Many apps are ad-supported, there is nothing wrong with it, but users should know what is their trade-off. People want to have confidence and trust that they're not being compromised while on devices that have access to their most personal information," said Jules Polonetsky, co-chair of Future of Privacy Forum.

Making it more difficult for users is that most apps don't declare which ad networks they use, giving you very little defense, even if you know which ad networks to avoid (and Lookout avoided singling out particularly heinous ad networks in the hopes they would mend their ways). All you can do is read the reviews of an app and look for the warning signs that the free app may not be quite so free after all.