The detection Trojan:Android/Zsone.A identifies a number of trojanized applications that were found on the Google Android Market in May 2011.
At the time of writing, these applications were already removed from the official Android Market, but could still be found in various forums and unofficial Android application distribution sites, most commonly catering to users in mainland China.
On installation, the affected applications display the following permissions requests:
Trojan:Android/Zsone.A's malicious routine starts when the application itself is launched. On the fifth click/action performed in the app, Zsone.A's SMS-sending payload is triggered.
The trojan will silently send SMS messages to the following hard-coded premium-rate phone numbers:
The SMS messages observed so far contain the following text strings:
These strings are used within Chinese mobile networks for subscribing to various paid SMS services. The trojan's behavior may result in registering the user's device to a service, thereby incurring unsolicited charges.
The trojan hides its SMS-sending activity from the user by aborting SMS messages received from the following numbers:
Zsone.A maintains a record of its SMS-sending status in an XML file in the shared preferences folder. When an SMS message is sent, the trojan saves a 'Y' ('Yes') in the XML file.