Bangle Android App Packer: Unpacking & Analysis

Trustlook Labs has identified a malicious app which is most likely using social engineering attacks to trick users to install it. The app (MD5: eb9d394c1277372f01e36168a8587016) is packed by Bangle packer. The main activity triggering installation of the app is “com.goplaycn.googleinstall.activity.SplashActivity.” However, that activity is not found anywhere in the decompiled code:

A closer look at what is happening in the code
From class SecAppWrapper, there is a “System.loadLibrary” call to load “secShell.” The native layer code in the module is responsible for decrypting and loading the app’s primary payload from “assets\secData0.jar,” which is a zipped DEX file after it’s decrypted.

Most method names in the “secShell” module are obfuscated, and their strings are decrypted when in use.

The app detects most hooking and patching frameworks, such as Xposed. Xposed is a framework for manipulating Android applications’ flow at runtime.

The app forks a child process and calls “ptrace” to attach to the parent to prevent any attaching attempts by debuggers. The multiple processes trace one another to make sure the children stay alive.

The app also monitors values in the /proc files system to check the status of the process.

The JNI_OnLoad function in the “secShell” module has switch branches. One branch is responsible for anti-debugging, the other (located at 0x7543EAE4 below) will lead to the main DEX module for decrypting.

The following is the decrypting function:

After the anti-debugging is bypassed, the function “p34D946B85C4E13BE6E95110517F61C41” decrypts the data. Register R0 contains the file location, as identified by the header bytes “PK\x03\x04.” R1 stores the size of the file.

We can dump the memory:

After unzipping the file, we get the DEX file which can be viewed normally:

Summary
Android packers are valuable tools used to protect the intellectual property of legitimate mobile application developers. However, they can be also used for nefarious purposes, and make analyzing malicious apps more difficult. Trustlook Labs continues to work on identifying malicious applications to protect our customers and the mobile ecosystem.

#Research