Android banking malware SlemBunk continues to evolve

Pierluigi Paganini January 15, 2016

Experts at FireEye noticed a significant evolution of SlemBunk, a powerful Android Trojan is targeting bank users worldwide.

In December security experts at FireEye first spotted the Android banking Trojan SlemBunk, a threat specifically designed to target mobile banking users.

The trojan first variants were distributed as copies of legitimate bank apps through third-party websites.

The SlemBunk Trojan is able to monitor the processing running on the mobile devices for the execution of a mobile banking app. When the user launches the mobile app the malware displays him a fake user interface to trick users into providing their credentials.

Now the SlemBunk Android Trojan has also become more sophisticated, recently the experts noticed that the threat is even more persistent and hard-to-detect.

“As SlemBunk expands its coverage of banks, its code has also become more sophisticated. Notably, later samples utilize different techniques to obscure potential reverse engineering. Figure 4 shows an obscured string that is Base64 encoded. In a few cases, SlemBunk authors took advantage of a commercial packer, DexProtector, which was designed to protect apps from being pirated. However, when used by a malicious application, it raises the difficulty for the analysis process.” states a recent post published by FireEye.

slembunk banking trojan

The experts speculate that the rapid evolution of the SlemBunk threat requested a significant effort likely made by organized crime.

“The rise and evolution of the SlemBunk trojan clearly indicates that mobile malware has become more sophisticated and targeted, and involves more organized efforts.” continues FireEye.

The organization behind the Android Trojan have improved the application to target at least 31 different banks worldwide and two mobile payment service providers.

The latest variants detected by FireEye are distributed through drive-by download, threat actors targeted users visiting porn websites.

“We have not observed any instances of SlemBunk on Google Play, so users will only get infected if the malware is sideloaded or downloaded from a malicious website. Newer versions of SlemBunk were observed being distributed via porn websites. Users who visit these sites are incessantly prompted to download an Adobe Flash update to view the porn, and doing so downloads the malware.”

With this trick, attackers convince unwary users to install the bogus app that claims to be a Flash update.

The SlemBunk is downloaded to the machine through a stealth multi-stage mechanism, the APK downloaded during a first stage does not contain any malicious feature to avoid trigger security solutions installed on the device.

The “dropper” app hides some functions that generate code on the fly and save it to another temporary APK locally. The second APK is loaded into memory dynamically by the dropper that then deleted it from the file system. The second APK works as downloader for the final malicious payload.

“Even if the malicious action of the SlemBunk payload were detected and removed, the more surreptitious downloader could periodically attempt to re-download the payload to the device.” states FireEye.

To protect your device from such kind of threat, FireEye suggests that you:

  • Do not install mobile apps from third-party app stores.
  • Keep Android devices updated.

Pierluigi Paganini

(Security Affairs – SlemBunk, banking trojan)


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