Schneier - xHelper Malware for Android

xHelper is not interesting because of its infection mechanism; the user has to side-load an app onto his phone. It's not interesting because of its payload; it seems to do nothing more than show unwanted ads. it's interesting because of its persistence:

Furthermore, even if users spot the xHelper service in the Android operating system's Apps section, removing it doesn't work, as the trojan reinstalls itself every time, even after users perform a factory reset of the entire device.

How xHelper survives factory resets is still a mystery; however, both Malwarebytes and Symantec said xHelper doesn't tamper with system services system apps. In addition, Symantec also said that it was "unlikely that Xhelper comes preinstalled on devices."

In some cases, users said that even when they removed the xHelper service and then disabled the "Install apps from unknown sources" option, the setting kept turning itself back on, and the device was reinfected in a matter of minutes after being cleaned.

From Symantec:

We first began seeing Xhelper apps in March 2019. Back then, the malware's code was relatively simple, and its main function was visiting advertisement pages for monetization purposes. The code has changed over time. Initially, the malware's ability to connect to a C&C server was written directly into the malware itself, but later this functionality was moved to an encrypted payload, in an attempt to evade signature detection. Some older variants included empty classes that were not implemented at the time, but the functionality is now fully enabled. As described previously, Xhelper's functionality has expanded drastically in recent times.

We strongly believe that the malware's source code is still a work in progress.

It's a weird piece of malware. That level of persistence speaks to a nation-state actor. The continuous evolution of the malware implies an organized actor. But sending unwanted ads is far too noisy for any serious use. And the infection mechanism is pretty random. I just don't know.

from Schneier on Security https://www.schneier.com/blog/archives/2019/11/xhelper_malware.html