Researchers at Symantec say a massive botnet impacting Google Android users in China is built on a new, sophisticated variant of the Backscript Trojan.

Last week, researchers at Kingsoft Security identified the MDK botnet, which they stated has infected up to one million devices. According to Symantec, an analysis of the code of MDK has shown strong similarities to Android.Backscript, and they use the same certificate to sign APKs (Android Application Packages). Unlike previous versions of the malware however, this new variant uses an Advanced Encryption Standard (AES) algorithm to encrypt data in a new file.

"Once installed, the Trojan enables the attacker to remotely control users’ devices, consequently allowing the attacker to harvest user data, download additional APKs, and generate nuisance adware," blogged Symantec researcher Flora Liu.

The server app.looking3g.com is used to download scripts and additional APKs, according to Liu.

"The Trojan has been repackaged into legitimate apps, including popular games such as Temple Run and Fishing Joy, to lull users into installing the malware," the researcher blogged. "The Trojan also uses dynamic loading, data encryption, and code obfuscation to evade detection."

So far, Symantec has caught more than 11,000 malicious apps infected by Backscript. The infections appear to be confined to China, as the Trojanized apps are mostly found on Chinese third-party app markets.

In a recent report, mobile security firm TrustGo Mobile analyzed 2.27 million applications found on 187 Android marketplaces worldwide and discovered that more than 1 in 5 applications (21.1 percent) have high-risk code that can compromise users' personal data. Overall, the number of apps categorized by TrustGo as high-risk – not necessarily malicious - increased to 511,043 from 356,675 between the third and fourth quarters of last year.

"High-risk code is most often the result of unsafe and aggressive adware and ad networks," Jeff Becker, head of marketing at TrustGo, told SecurityWeek at the time. "These networks may collect private user data such as phone number and device ID and transmit it to third parties who use it for unscrupulous notifications - e.g. those that look like system updates - or leverage unnecessary permissions that allow aggressive tactics such as modifying browser homepage, or put unwanted icons and apps on the device."

According to Trend Micro, the amount of Android malware increased to 350,000 by the end of 2012. Much of that growth was driven by adware and premium service abusers, the company found.