Detected by ESET products as Android/DoubleLocker, it is based on the foundations of a particular banking Trojan, renowned for misusing accessibility services of the Android operating system. However, DoubleLocker lacks the functions related to harvesting users’ banking credentials and wiping out their accounts. Instead, it has two other powerful tools for extorting money from its victims.

DoubleLocker can change the device’s PIN, preventing victims from accessing their devices, and also encrypts the data it finds in it - a combination that has not been seen to date in the Android ecosystem.

“Given its banking malware roots, DoubleLocker may well be turned into what can be called ransom-bankers. Two-stage malware that first tries to wipe your bank or PayPal account and subsequently locks your device and data to request a ransom… Speculation aside, we spotted a test version of such a ransom-banker in the wild as long ago as May, 2017,” comments Lukáš Štefanko, the ESET malware researcher who discovered DoubleLocker.

Distribution

DoubleLocker spreads in the very same way as its banking parent does. It is distributed mostly as a fake Adobe Flash Player through compromised websites.

Once launched, the app requests activation of the malware’s accessibility service, named “Google Play Service”. After the malware obtains the accessibility permissions, it uses them to activate device administrator rights and set itself as the default Home application, in both cases without the user’s consent.

“Setting itself as a default home app – a launcher – is a trick that improves the malware’s persistence. Whenever the user clicks on the Home button, the ransomware gets activated and the device gets locked again. Thanks to using the accessibility service, the user doesn’t know that they launched malware by hitting Home,” explains Lukáš Štefanko.

Locking both device and data

DoubleLocker, once executed on the device, creates two reasons for the victims to pay.

First, it changes the device’s PIN, effectively blocking the victim from using it.  The new PIN is set to a random value which is neither stored on the device nor sent anywhere, so it’s impossible for the user or a security expert to recover it. After the ransom is paid, the attacker can remotely reset the PIN and unlock the device.

Second, DoubleLocker encrypts all files from the device’s primary storage directory. It utilizes the AES encryption algorithm, appending the filename extension “.cryeye”. The ransom has been set to 0.0130 BTC (approximately USD 54 at time of writing) and the message highlights that it must be paid within 24 hours. However, if the ransom is not paid, the data will remain encrypted and will not be deleted.