Security researchers from antivirus vendor Symantec identified two malware apps on Google Play that used a multistage payload delivery system in order to remain undetected.
The apps, which have since been removed by Google, masqueraded as two games -- "Super Mario Bros." and "GTA 3 - Moscow city."
"Both were posted to Google Play on June 24 and since then have generated in the range of 50,000 to 100,000 downloads," Symantec security researcher Irfan Asrar said Tuesday in a blog post.
Once installed, the apps downloaded an additional package called Activator.apk from a Dropbox account and prompted the device owners to install it.
This secondary Activator app sent SMS messages to a premium-rate number located in Eastern Europe, after which it asked to be uninstalled.
The fact that the malicious payload was delivered in multiple stages is probably why the apps managed to remain undetected for so long on Google Play, Asrar said.
Earlier this year, Google started using an automated scanner called Bouncer to detect malware on Google Play. Bouncer runs all published apps in an emulated Android environment and monitors them for suspicious activity.
However, downloading a secondary app from a developer's server and prompting the user to install it might not necessarily represent malicious behavior.
This is not the first time when Android malware developers have used multi-stage payloads. The Android.Lightdd and Android.Jsmshider threats discovered in 2011 both downloaded additional components after the installation of an initial app.
There are several advantages to spreading the payload across multiple apps, Asrar said about those threats at the time. For one, the initial malicious app no longer needs to display an extensive list of permissions that might attract the user's attention.
Secondly, if the initial app is downloaded from the official Android marketplace -- now called Google Play -- the user is likely to assume that the additional apps also originate from there.
Symantec detects the two newly found malware apps as Android.Dropdialer. The Android security team immediately removed the threat after being notified by Symantec, Asrar said.
