Backdoor:Android/Hummingbad apps are mostly distributed on third-party app repositories. According to reports, the apps are also spread by drive-by downloads, when a compromised or malicious site is able to exploit a vulnerability that allows it to silently install the app on a visitor's device.
According to reports, the majority of users affected by Hummingbad malware are located in China and India, though users in other regions have also been noted.
Rooting the device
Once installed on the device, the app tries to get root access by targeting multiple vulnerabilities; if one is successfully exploited, the app gains control of the entire system and essentially functions as a backdoor, allowing a remote attacker full control of the device.
If a vulnerability-based attack is not successful, the app tries to trick the user into granting it root access by displaying a message that looks like system notification. If the user grants system-wide privileges to the app, again it essentially gives a remote attacker control of the device.
Ads and app downloads
Whether or not root access is gained, the app will silently download and install other components onto the device. These components have various functions, usually either displaying advertisements or downloading still more apps.
The ad-displaying component displays advertising content sourced from third-party advertising networks on the device. It also interferes with attempts by the user to dismiss any ads shown (for example, by clicking back or returning to the home screen) in order to force them to click on the ad. This forced-clicking is used to generate revenue from the ad networks, a practice known as 'clickfraud'.
Another downloaded component is able to mimic the behavior of a user in order to download and install apps onto the affected device.