Security researchers at Microsoft have spotted a dangerous new version of MalLocker, a constantly evolving Android ransomware family that has been floating around in the wild since at least 2014.
The new version is notable for how it surfaces the ransom demand on infected devices and its integration of an open source machine-learning module for context-aware cropping of the ransom note, depending on screen size. The latest variant of MalLocker also uses a new obfuscation method to hinder code analysis and to evade detection by anti-malware tools.
In a report this week, Microsoft described MalLocker as being distributed via arbitrary websites and online forums, or hidden in popular apps and video players for mobile devices. Like many other Android ransomware variants, the new MalLocker does not actually encrypt data on infected devices. Instead, it attempts to prevent users from using an infected device by displaying a ransom note over every window. Regardless of what button the user clicks, the ransom note remains on top of all other windows.
What is different in the new MalLocker variant is the manner in which it achieves this persistence. Previous Android ransomware tools took advantage of a system alert feature in the OS to display the ransomware note. But that has become almost impossible to do now because of certain platform-level changes that Google has implemented to thwart the abuse, Microsoft said.
The new variant instead abuses two other functions that are present in recent versions of Android. "First, it sets its notification as a very important notification requiring immediate user attention," says Tanmay Ganacharya, partner director, security research, at Microsoft. "This notification is wired to pop up the ransom notice," he says.
Second, the malware is designed to ensure that this notification is always displayed when the user tries to do other activities or performs other functions. "It does this by using a callback, which is a way for functions to pass a piece of code to each other," Ganacharya says.
On Android, a callback is a way for one function to let another function know that an action — such as a user pressing the Home button — is completed, he notes. The new version of MalLocker is designed to take advantage of the callback method to know when a user might have completed a specific action so it can promptly display the ransom note. "This means that whatever the user does, the ransomware's notification is always displayed, effectively preventing the user from performing any other action," Ganacharya says.
In addition, the new version of MalLocker also incorporates an open source machine-learning module that lets it know an infected device's screen size so the ransom note can be automatically resized and cropped to fit it without distortion.
According to Microsoft, the new Android malware's obfuscation tactics are also noteworthy. The manner in which the malware authors have encrypted and hidden the payload, the decryption routine it uses and the presence of lots of deliberately introduced junk code all make the malware hard to analyze and detect, Microsoft said.
Users with infected devices can try rebooting the system in safe mode and then uninstalling the malware, Microsoft said.