It's great having all these Android apps, but be sure you're not downloading one that a hacker has got his dirty paws on

One culprit is a hacked version of the app Steamy Window, said by security firm Symantec to be running up text message charges on the wireless bills of unsuspecting users.

Hackers, understood to be in China, downloaded the Steamy Window app and added a virus to the source code called Android.Pjapps. It was then re-released on third-party Android app websites unregulated by Google. Symantec spokesman Mario Ballano said in a blog post on the Steamy Windows mischief,"It is difficult to differentiate the legitimate version from the malicious one once it is installed. However, during installation it is possible to identify the malicious version by the excessive permissions it requests."

Once downloaded, the virus gets to work installing other applications, taking over your Web browser, adding bookmarks, surfing to other infected websites, as well as hijacking text messaging abilities.

The malicious app continuously sends out texts without telling you - or signalling when it has passed your text message limit, Ballano said.

Android is the world's most popular mobile operating system. It is built on an open-source platform, making it very accessible to developers - and unfortunately hackers too.

Android Police, a blog unconnected with Google, said it took Google just five minutes to pull the 21 infected apps, uploaded by a publisher called Myournet, once they told Google of the problem on Tuesday.

Ballano recommended that Android users only download apps from Google's regulated Android marketplaces and that they turn on a feature in Android that prevents the installation of non-market applications. A safe and virus-free version of Steamy Window, built by the developers Swiss Codemonkeys, is available for download in the Android Marketplace.