Google recently discovered and blocked a sophisticated fraud botnet that was being distributed through multiple channels and which employed several methods to avoid detection.
Dubbed Chamois, the botnet is was one of the largest Potentially Harmful Application (PHA) families seen on Android to date, and could remain persistent on infected devices by not showing in the application list at all. The malicious program was also capable of generating revenue by engaging into numerous activities, Google says.
The malicious apps based on Chamois that Google analyzed could generate invalid traffic through ad pop-ups by displaying deceptive graphics inside the ads; could perform artificial app promotion by automatically installing apps in the background; could perform telephony fraud by sending premium text messages; and could also download and execute additional plugins on the compromised devices.
The malicious apps didn’t appear in the device’s app list, which prevented users from removing them. Furthermore, the deceptive graphics used to trick users into clicking ads could sometimes result in additional malicious applications being downloaded onto the device, such as SMS fraud programs.
In addition to staying well hidden on Android devices, Chamois had other features that made it unusual as well, such as a multi-staged payload, with its code being executed in 4 distinct stages using different file formats.
“This multi-stage process makes it more complicated to immediately identify apps in this family as a PHA because the layers have to be peeled first to reach the malicious part. However, Google’s pipelines weren’t tricked as they are designed to tackle these scenarios properly,” Security Software Engineers Bernhard Grill, Megan Ruthven, and Xin Zhao explain.
The PHA attempted to evade detection with the help of obfuscation and anti-analysis techniques, while also using a custom, encrypted file storage for its configuration files, along with additional code that required deeper analysis. Chamois also featured a great deal of (over 100,000 lines of) “sophisticated code written by seemingly professional developers,” Google’s engineers say.
To block the threat, Google used Verify Apps, in addition to kicking out “bad actors who were trying to game our ad systems.” With the help of Verify Apps, users are automatically warned when downloading apps that are considered PHAs, and they can also find and remove such threats if they have been already installed, even if they don’t appear in the application list, as was the case with Chamois.
According to Google, Verify Apps was also meant to monitor the state of the Android ecosystem for anomalies, as well as to investigate the ones that it finds, and leverages behavior analysis on devices to discover PHAs. Many of the applications that Chamois downloaded were highly ranked by the Dead or Insecure (DOI) scorer (apps that have a high chance of being downloaded by devices that are no longer checking up with Verify Apps are considered DOI apps).