Tens of thousands of smartphone applications are running ads from the overwhelming  advertising networks that change smartphone settings and copy people's contact information without permission. Aggressive ad networks can disguise  ads as text message notifications  , and sometimes change browser settings and bookmarks. Often, the ads will upload your contacts list to the ad network's servers--- information that 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. PhoneLiving was one of the most popular app developers to use these kinds of ad networks; their dozens of talking animal apps have been downloaded several million times. Later, PhoneLiving says it has mended its ways. The company admittedusing _ techniques to make money from its apps, but said it dropped those methods at the start of July because of bad reviews and declining downloads. The most popular type of apps that use aggressive ad networks are "Personalisation" apps, which include wallpapers. Comic and entertainment apps are also among the most likely to have rogue  ad networks running behind the scenes. Like aggressive pop-ups on PCs, the bad software isn't easy to get rid of. Though the damage can typically be removed by deleting the app, it can be hard to tell which app is causing the problems. When developers create free mobile apps, they usually make money through ads displayed within the app. App makers don't usually tell people which ad network they are using, which makes it hard to avoid the known offenders. The best defense is to read reviews and avoid downloading apps that have attracted many complaints. What is the main idea of the passage?
Mobile ads can attack your phone and steal your contacts. Mobile ads have become popular in recent years. Smartphones have been damaged by mobile ads. You may download some aggressive apps unconsciously.