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Custom Native Library Loader for Android

If you read my co-worker Neal Hindocha's recent post "Debugging Android Libraries using IDA" you notice he mentioned using a "custom library loader". We had used this on a recent mobile penetration test to have complete control over some home grown custom native libraries the target application was using.

The biggest problem we were facing with the test was that the library in question was being used for anit-rooting and anti-debug functionality to protect the app, and part of our job was to bypass this and patch it out. Of course, attaching directly to the running Android app to get at the code in this library was problematic, since most of the protections were likely loaded before we could attach.

What we needed was something like dllloader for Olly or Immunity that we could use to load the target .so file independently of the process, so we could have total control and access.

Here is what we came up with. While obviously not as feature rich as dllloader, this quick and dirty loader did the job:

#include <stdio.h>#include <stdlib.h>#include <dlfcn.h>int main(){      printf("Loading libs\n");     int (*pt2Function)(void) = NULL;                        //pointer to a void function - change this to match method sig     void* sdl_library = dlopen("/system/libtarget.so", RTLD_LAZY);     if (sdl_library == NULL) {     // report error ...     printf("Unable to load library\n");     char *errstr;  errstr = dlerror();  if (errstr != NULL)  printf ("A dynamic linking error occurred: (%s)\n", errstr); } else {  printf("Lib loaded, getting dlysm\n");     void* initializer = dlsym(sdl_library,"JNI_OnLoad");  if (initializer == NULL) {     // report error ...   printf("Unable to get address of JNI_OnLoad\n");   char *errstr;   errstr = dlerror();   if (errstr != NULL)   printf ("A dynamic linking error occurred: (%s)\n", errstr);  } else {      // cast initializer to its proper type and use      printf("calling get process\n");      //asm("BKPT #0");      pt2Function = initializer;      printf("got get process, setting up\n");            printf("Ok, lets Calling the function");      int result=pt2Function();      printf("Result of call is %d", result);        //asm("BKPT #0");  } }     return 0;}

As you can see, our target was the 'JNI_Onload' function. In our case, we targeted this because it was in this function that the anti-debug and system monitoring functionality was set up. We could have set up the function pointer to point to ANY of the .so's exported functions and called them.

This code could easily have been compiled on run on any standard linux system. We used dlopen() to dynamically load the library and dlsym() to obtain a pointer to the exported function we wanted to test. We could then call the target function directly and pass in any parameters (if any) the function took in order to follow code execution or even to fuzz, looking for exploits. The only ARM specific code we had (which it turns out we really didn't need) are the commented out ARM breakpoints.

To compile this, you simply use the Android NDK and create an ndk-specific Android.mk file:

LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS) # give module nameLOCAL_MODULE    := loader  # list your C files to compileLOCAL_SRC_FILES := test.c# Build executables instead of a library for android.include $(BUILD_EXECUTABLE)

You place the files in the normal Android NDK file structure - code and the Android.mk file, including the C code for the lib goes under the jni directory and your executable will appear under libs/armabi/ directory. To build, you open a terminal in the root of your project directory (same level as jni and libs) and execute the following command:

Mike-Park:android-loader mpark$ /path-to/android-ndk-r8e/ndk-build

In my case, I created a directory called 'android-loader' that contained jni/ and libs/ and ran from there.

After a few warnings, you should get a perfectly useable ARM executable in the libs/armabi/ directory called loader. You can then use adb push to place this in the /system directory (after a remount of course) along with your target .so and run it from there. It works great in the emulator and on the devices we tried.

Again, this is a quick and dirty bit of code that works. We used it for debugging with IDA, but it could easily be adapted for fuzzing and to take the library to load from the commandline. I'd like to make it more robust, so any feedback or suggestions are welcome.

In the meantime, enjoy hacking android native code.