Tens of thousands of smartphone apps are running ads from rogue advertising networks that change smartphone settings and take contact information without permission, according to a new study released Monday.

Aggressive ad networks can disguise ads as text message notifications or app icons, and sometimes change browser settings and bookmarks. Often, the ads will upload your contacts list to the ad network's servers -- information the ad network can then sell to marketers

Sounds scary? It's not a giant problem yet, but it's a growing one. As many as 5% of free mobile apps use an "aggressive" ad network to make money, according to Lookout, a San Francisco-based mobile security company.

With millions of mobile apps in stores, that small sliver adds up to a big number. The study found that 19,200 of the 384,000 apps it tested used malicious ad networks. Those apps have been downloaded a whopping 80 million times.

PhoneLiving was one of the most prevalent app developers to use these kinds of ad networks, according to Lookout -- their dozens of talking animal apps have been downloaded several million times.

PhoneLiving says it has mended its ways. The company acknowledged using invasive techniques to make money from its apps, but says it dropped those methods at the start of this month because of bad reviews and declining downloads.