It seems that every few weeks a new crop of malicious Android apps turns up in the market. Sometimes Google just removes them from the market; other times it uses the “kill switch” to disable already-downloaded apps from Android devices around the world.

Typically these threats are perfectly ordinary-looking apps. Like the Trojan Horse of legend, they enter your device freely, with your permission. Once installed they do something nasty. Some users are shocked that Google can remove stuff from your Android phone remotely. I’m more alarmed at the ridiculous ease with which malicious coders can create Trojans for Android.

Here are the five simple steps :

One. Start by downloading a free app. You can choose any app at all, but of course you’ll want to pick something that will draw plenty of downloads.

Two. The language compilers that create applications on your PC take textual source code and convert it into assembly language that the CPU can read and process. It’s a one-way translation; there’s no way to go from the final executable file back to the source code. Android apps are written in Java, though, and that means that you can decompile them back to the original source code using simple, easily-available tools. For the next step, decompile your target app.

Three. The third step is a little tricky. You’ll need to obtain Java source code that does something nasty, like sending personal information from the device to a third party. For the demonstration, Chien used a known threat called Android.Geinimi.

Four. Adding the Trojan code is absurdly simple. You copy it into the folder containing the existing source code, make a small change in the manifest to run the Trojan code before the rest of the app, and edit the permissions to give the Trojanized app free access to the entire device. While you’re at it, tweak the app’s name. We added “FREE!” to the name for his demo.

Five. Compile the modified app and upload it to the market. You’re done!

Of course, malicious apps don’t last long in the Android Market. If you really want to spread a dangerous program, you’re better off uploading it in China, where there is no official Android market. I

Not planning to do this yourself? Good! But I’m sure that like me you’re shocked at how easily someone with bad intentions can create a brand new Android Trojan. It’s time to look into mobile security for your Android device.