The authors of a particularly persistent Android malware family called "Joker" have once again found a way to sneak their product into Google's official Play mobile app store.
The malware (aka "Bread") is known for subscribing mobile users to premium content without their knowledge and has been floating around since at least early 2017.
Google security researchers previously described Joker as malware that was originally designed for SMS fraud but is now being used for large-scale billing fraud. According to the company, the creators of Joker "have at some point used just about every cloaking and obfuscation technique under the sun in an attempt to go undetected."
The latest gambit to sneak Joker into the Play store actually involves an old technique used in the conventional PC threat landscape, according to researchers from Check Point.
Aviran Hazum, team leader of Check Point's Mobile Malware and Threat Intelligence Team, says the authors of Joker beat Google's security controls this time by hiding the malicious payload in a file called the "Android Manifest" file.
"Without this file, an [Android] application cannot be installed or executed," he says.
Instead of having the Joker dropper downloading the malicious payload from a remote command-and-control server, the newest version just reads developer-inserted fields in the Manifest file, he says. The payload was not executed — or decoded — while the app went through Google's security inspection process when being uploaded to the Play Store.
"So the malware was able to bypass Google's inspection," Hazum says. "In general, Joker is not an easy malware to detect, and on top of that the actor is spending constant efforts to bypass those protections."
Other tricks that Joker has employed to evade detection include geolocation checks to target or avoid specific countries and implementation of malicious behavior in native code.
According to Google, as of January 2020, the company's Play mobile app store security controls had detected and removed at least 1,700 unique Android apps containing Joker.
In the past, the creators of Joker have hidden the malware in seemingly legitimate apps, such as filters, animations, and other camera utility apps. This time around, the malware was hidden in software posing as messaging apps for Android.
"The apps themselves are not legitimate — they are actor-created," Hazum says. "But they do provide some sort of functionality."
Once the fake app is installed on a system, it uses code downloaded from a command-and-control server to register users to premium services. It then takes advantage of an Android feature called "Notification Listener" to quietly intercept and kill any registration confirmation notifications that might be sent to the unsuspecting user's device.
"Notification Listener is a service that is called by the operating system whenever a notification event occurs," Hazum says.
By using the service, Joker is able to read the content of all incoming notifications, including SMS notifications. This allows the malware to intercept and remove any registration verification codes that might be sent to the Android user, thereby keeping them in the dark about what had just happened, he noted.
Both Google and Apple have spent considerable efforts deploying layered security controls to prevent developers from uploading malware-laden applications to their respective mobile app stores. Security researchers generally agree the app review efforts have made the stores, particularly the Apple App Store, considerably more secure in recent years. The number of malicious apps — as a proportion of the overall number of application on these app stores — still remains very small.
Even so, bad actors have been able to continue uploading malicious software — mainly to Google Play — relatively frequently. In 2019, for instance, RiskIQ detected 25,647 apps on the Google Play Store as being malicious. Though the number represented a more than 76% decline from the 108,770 malicious applications detected in 2018, it still presented a risk to users who trusted the store to be safe.
"Google and Apple invest a lot in security research, but that's not enough," Hazum says. "As we have shown time after time, malware is still able to bypass market security," he says. "Security vulnerabilities are discovered in a constant basis, and if your device is not patched, you are vulnerable."