Security researchers have uncovered another instance of Android malware hiding in apps and signing up unsuspecting users for premium services without them knowing. Check Point researchers have discovered that the infamous Joker malware has found another trick to bypass Google Play Store protections : it hides the malicious DEX executable within the app as “Base64 encoded strings,” which are then decoded and loaded onto the compromised device.
Following the revelation by Check Point researchers, the 11 suspicious apps were removed from the Google Play Store on April 30, 2020.
“Joker malware is difficult to detect, despite the addition of additional layers of protection to the Google Play Store,” said Aviran Hazum of Check Point, who identified the new modus operandi of Joker malware. “Although Google has removed the malicious apps from the Play Store, we expect Joker to adapt again.”
Joker malware was discovered in 2017 and is one of the most widespread types of Android malware. It is known for committing fraud, spyware capabilities, and stealing SMS messages, contact lists, and device.
Joker infected many Android apps over the past year, discovered by CSIS Security Group, Trend Micro, Dr.Web, and Kaspersky. Researchers noted that the malware was constantly finding ways to exploit gaps in the Play Store's security controls.
The creators of Joker malware have used various methods to hide the true nature of the applications and avoid detection.
“As the Play Store introduced new policies and Google Play Protect strengthened its defenses, Joker apps began looking for new security,” the Android security and privacy team said. “They had used almost every obfuscation technique in an attempt to avoid detection.”
Since January 2020, Google has removed more than 1,700 apps that were uploaded to the Play Store in the last three years and were infected with malware.
The new variant of Joker malware detected by Check Point achieves its goals by exploiting the application's manifest file, which it uses to load a file Base64-encoded DEX
A second “intermediate” version uses a similar technique of hiding the .dex file.
"To achieve the ability to register users for premium services without their knowledge or consent, the Joker malware used two main components – the Notification Listener as part of the initial application and a dex file loaded by the C&C server to perform the registration," the researchers said.
The latest Joker variant reminds us that Android malware is constantly evolving, so we must always be on guard.
