A piece of sophisticated Android ransomware with novel techniques and behavior ended up discovered that is showing the evolution of mobile threats, researchers said.
The mobile ransomware, detected by Microsoft Defender for Endpoint as AndroidOS/MalLocker.B, is the latest variant of a ransomware family that’s been in the wild for a while but has been evolving non-stop, said researchers at Microsoft.

This ransomware family is known for being hosted on arbitrary websites and circulated on online forums using various social engineering lures, including masquerading as popular apps, cracked games, or video players, the researchers said in a post.

The variant caught researchers’ attention because it’s an advanced malware with unmistakable malicious characteristic and behavior and yet manages to evade many available protections, registering a low detection rate against security solutions.

As with most Android ransomware, this new threat doesn’t actually block access to files by encrypting them. Instead, it blocks access to devices by displaying a screen that appears over every other window, such that the user can’t do anything else. The screen is the ransom note, which contains threats and instructions to pay the ransom.

What’s innovative about this ransomware is how it displays its ransom note.

Attackers are using “innovative ways in which this ransomware surfaces its ransom note using Android features we haven’t seen leveraged by malware before, as well as incorporating an open-source machine learning module designed for context-aware cropping of its ransom note,” the researchers said.

In the past, Android ransomware used a special permission called “SYSTEM_ALERT_WINDOW” to display their ransom note. Apps that have this permission can draw a window that belongs to the system group and can’t be dismissed, the researchers said.

No matter what button is pressed, the window stays on top of all other windows. The notification meant to see use in system alerts or errors, but Android threats misused it to force the attacker-controlled UI to fully occupy the screen, blocking access to the device. Attackers create this scenario to persuade users to pay the ransom so they can gain back access to the device.

To catch these threats, security solutions used heuristics that focused on detecting this behavior. Google later implemented platform-level changes that practically eliminated this attack surface.

These changes include:

  1. Removing the SYSTEM_ALERT_WINDOW error and alert window types, and introducing a few other types as replacement
  2. Elevating the permission status of SYSTEM_ALERT_WINDOW to special permission by putting it into the “above dangerous” category, which means that users have to go through many screens to approve apps that ask for permission, instead of just one click.
  3. Introducing an overlay kill switch on Android 8.0 and later that users can activate anytime to deactivate a system alert window

To adapt, Android malware evolved to misusing other features, but these aren’t as effective, the researchers said. For example, some strains of ransomware abuse accessibility features, a method that could easily alarm users because accessibility is a special permission that requires users to go through several screens and accept a warning that the app will be able to monitor activity via accessibility services. Other ransomware families use infinite loops of drawing non-system windows, but in between drawing and redrawing, it’s possible for users to go to settings and uninstall the offending app.