A sneaky new Android Trojan family employs four payload stages in its attack rather than the more typical two stages, researchers say.
The Android/TrojanDropper.Agent.BKY family was found in at least eight apps in Google Play so far, but the damage has been limited. Each of the apps only had a few hundred downloads before Google pulled them from the store, according to ESET, which discovered the malware family and notified Google.
The attack's use of four payload stages before delivering its final nastiness, a mobile banking Trojan, is rare. "Two-stage payloads are really common in the Android ecosystem. Four-stage malware on Google Play isn't so common," observes Lukas Stefanko, an ESET malware researcher.
With more payload stages, attackers are able to deeply hide the true intent of their payload.
"This one added some extra obfuscating layers - dropper and decryptor, plus a downloader - to hide its malicious purpose," Stefanko explains.
Once a user launches the app, it initially behaves like a legitimate app by mimicking its advertised functions and withholding suspicious permission requests. The first stage calls for the malicious app to decrypt and execute the second-stage payload. Both steps are invisible to users.
Inside the second-stage payload is a hardcoded URL, which then downloads another malicious app or third payload. App users are prompted to install this bogus but legitimate-looking app. In some cases, it's disguised as an Adobe Flash Player or Android update.
"Once they see a request to install the third stage payload, it should become a bit suspicious for users," Stefanko notes.
After the third payload, or app, has all its requested permissions granted, it will decrypt and execute the fourth and final payload – a mobile banking Trojan. The malicious app will take users to a bogus login form to steal their credentials or credit card details, according to ESET's report.
One of the malicious apps ESET reviewed had 3,000 downloaded links, with the vast majority of the victims coming from the Netherlands.
Although the Android/TrojanDropper.Agent.BKY samples ESET came across were banking Trojans or spyware, the downloader could be any nefarious piece of code that the attacker wants, the researchers say.
ESET came across the Android/TrojanDropper.Agent.BKY family in late September when its systems detected the apps dropping payloads in an unusual way. For now, it is not clear who is behind these attacks, Stefanko says.
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