Android Apk reverse engineering using Apktool and Frida

On By RussellIn Android, Hooking, Reverse Engineering

Android Apk reverse engineering using Apktool and Frida

Table of Content

  1. Environment Setup (macOS mojave)
  2. Tamper Smali Code
  3. Hooking Android Java Methods
  4. Hooking Android C Functions
  5. References

Environment Setup (macOS mojave)

All the steps mentioned below are on macOS Mojave

Install Tools on PC

  1. Install frida-tools with pip3

    Run command $ pip --version and $ pip3 --version to check which pip is from at Python 3x. E.g. you should see version information like below:

    $ pip 19.0.3 from /usr/local/lib/python3.7/site-packages/pip (python 3.7)

    Then run the right pip command to install frida-tools, e.g. pip3

    $ pip3 install frida-tools

    Success outputs:

    ....
Successfully built frida-tools frida

Installing collected packages: colorama, frida, six, wcwidth, prompt-toolkit, pygments, frida-tools

Successfully installed colorama-0.4.1 frida-12.4.7 frida-tools-1.3.2 prompt-toolkit-2.0.9 pygments-2.3.1 six-1.12.0 wcwidth-0.1.7
  2. Testing your installation

    Copy the cat binary to a temporary folder, e.g., /tmp/cat then run cat from that directory:

    $ mkdir ~/frida
$ cp /bin/cat /frida/cat
$ /frida/cat

    In another terminal, make a file example.py with the following contents:

    import frida

def on_message(message, data):
    print("[on_message] message:", message, "data:", data)

session = frida.attach("cat")

script = session.create_script("""'use strict';

rpc.exports.enumerateModules = function () {
return Process.enumerateModulesSync();
};
""")
script.on("message", on_message)
script.load()

print([m["name"] for m in script.exports.enumerate_modules()])

    Then run the example.py script with below command

    $ python3 example.py

    The output should be something similar to this (depending on your platform and library versions):

    [u'cat', …, u'ld-2.15.so']
  3. Android adb command

    Make sure adb can see your device:

    $ adb devices -l

    This will also ensure that the adb daemon is running on your desktop, which allows Frida to discover and communicate with your device regardless of whether you’ve got it hooked up through USB or WiFi.

Install frida-server on Emulator (or Real Device)

Below steps are based on Android Emulator Nexus 6P (x86) api 23 on macOS Majave.

First off, download the latest frida-server for Android from frida-server releases page, e.g. for Android x86 emulator, you should download frida-server-12.4.7-android-x86.xz, and get it run on your emulator:

$ adb root # might be required
$ adb push frida-server /data/local/tmp/
$ adb shell "chmod 755 /data/local/tmp/frida-server"
$ adb shell "/data/local/tmp/frida-server &"

For the last step, if you see below error:

Unable to load SELinux policy from the kernel: Failed to open file “/sys/fs/selinux/policy”: Permission denied

Then you need to run frida-server using the root shell, e.g.

$ adb shell
angler:/ $ su
angler:/ # /data/local/tmp/frida-server &
[1] 12089
angler:/ #

[1] 12089 is the process id of frida-server.

A quick smoke-test

Now, on your desktop it’s time to make sure the basics are working. Run:

frida-ps -U

This should give you a process list along the lines of:

  PID  Name
-----  ---------------------------------------------------
  721  ATFWD-daemon
 4450  adbd
  730  android.hardware.biometrics.fingerprint@2.1-service
  407  android.hardware.configstore@1.0-service
  408  android.hardware.graphics.allocator@2.0-service
  409  android.hardware.usb@1.0-service
  410  android.hardware.wifi@1.0-service
  406  android.hidl.allocator@1.0-service

Tamper Smali Code

To tamper a Boolean value inside Java source code, i.e. the Boolean bTamperingSucces = false;, or some other code you have interest in.

Tools required

apktool

apktool can be feteched from Apktool website. Just follow the steps inside this page to install apktool.

adb

adb is shipped with Android SDK, it can be found from directory <your-some-path>/Android/sdk/platform-tools/adb

apksigner

apksigner is to sign your apk with a keystore file. This tool can be found at directory <ANDROID_HOME>/Android/sdk/build-tools/28.0.3/apksigner, and the usage is documented at command-line apksigner.

Steps

  1. Clone the example project from DecompileApk.
  2. Find the already compiled apk file DecompileApk/app/release/app-release.apk.
  3. Decompile it using apktool. 
    $ cd <your-path>/DecompileApk/app/release/
$ apktool d --no-res -f app-release.apk

    You will see below outputs

    I: Using Apktool 2.4.0 on app-release.apk
I: Copying raw resources...
I: Baksmaling classes.dex...
I: Copying assets and libs...
I: Copying unknown files...
I: Copying original files...
  4. Look for DecompileApk/app/release/app-release/smali/com/arophix/decompileapk/MainActivity.smali under the smali code directory and find below code 
    const/4 p1, 0x0
  5. Just change 0x0 (meaning false) to 0x1 (meaning true) and save the file.
  6. Using apktool to build the tampered apk. 
    apktool b app-release

    You should see below outputs

    I: Using Apktool 2.4.0
I: Checking whether sources has changed...
I: Smaling smali folder into classes.dex...
I: Checking whether resources has changed...
I: Copying raw resources...
I: Copying libs... (/lib)
I: Building apk file...
I: Copying unknown files/dir...
I: Built apk...
  7. Find the newly built apk from dist directory DecompileApk/app/release/app-release/dist/app-release.apk
  8. Sign the apk using apksigner and keystore located at DecompileApk/app/decompileapk.jks (please modify the paths for keystore and apk per your own case accordingly), 
    $ <ANDROID_HOME>/sdk/build-tools/28.0.3/apksigner sign --ks ../decompileapk.jks app-release.apk

    You should see below outputs and enter the password 123456

    Keystore password for signer #1:
  9. Install the signed apk using adb command. 
    $ adb install <path-to-the-tampered-apk>/app-release.apk
  10. Instead of seeing "Hello from C++" from the screen, you should now see "Hello, Android reverse engineer!".

Hooking Android Java Methods

Hooking Android Java source code using Frida.

Find the script from hook_java_methods.py and run it using below command, and then click on the button of your started Android app.

$ python3 hook_java_methods.py

[*] Running CTF
[*] onClick
[*] Called - isPhoneRooted()
[*] onClick
[*] Called - isPhoneRooted()

If you see an error like below:

$ frida.ServerNotRunningError: unable to connect to remote frida-server: closed

Remember that you have started frida-server on your emulator.

Meanwhile, on your emulator screen, you shoud see the toast message changed to Device not rooted if success.

Hooking Android C Functions

Hooking Android C source code using Frida.

  1. Decompile APK

    Firstly, decompile the apk using apktool to extract the shared library, i.e. libnative-lib.so.

    $ cd DecompileApk/app/release
$ apktool d --no-res app-release.apk
  2. Find the target JNI method

    Secondly, use below command to find the JNI function to hook.

    $ nm --demangle --dynamic app-release/lib/x86/libnative-lib.so

    You should see below outputs:

    00004d80 T Java_com_arophix_decompileapk_MainActivity_stringFromJNI
000090b0 T std::bad_typeid::what() const
00005cf0 T std::bad_exception::what() const
00005e70 T std::bad_array_length::what() const
00005df0 T std::bad_array_new_length::what() const
00008ff0 T std::bad_cast::what() const
...
  3. Hook C function by name

    Find the script from hooknative-by-function-name.py and run it using below command, and then click on the button of your started Android app.

    $ python3 hooknative-by-function-name.py

[*] Running Arophix Hook Test ...
Java_com_arophix_decompileapk_MainActivity_stringFromJNI called with:
ret: 0x200019

    If you see an error like below:

    $ frida.ServerNotRunningError: unable to connect to remote frida-server: closed

    Remember that you have started frida-server on your emulator.

    Meanwhile, on your emulator screen, you shoud see the text changed to Frida is hooking this displayed text from Native layer by function name. if success.

  4. Hook C function by address

    Find the script from hooknative-by-function-address.py and run it using below command, and then click on the button of your started Android app.

    $ python3 hooknative-by-function-address.py

[*] Running Arophix Hook Test ...
[+] membase: 0xb2acd000
[+] addressOfStringFromJni: 0xb2ad1d80
[++] addressOfStringFromJni: 0xb2ad1d80
ret: 0x19

    If you see an error like below:

    $ frida.ServerNotRunningError: unable to connect to remote frida-server: closed

    Remember that you have started frida-server on your emulator.

    Meanwhile, on your emulator screen, you shoud see the text changed to Frida is hooking this displayed text from Native layer by function address. if success.

References

 

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