How to Decompile and Recompile APKs for Penetration Testing

 How to Decompile and Recompile APKs for Penetration Testing

When conducting penetration testing on Android applications, understanding the internals of an APK file is essential. This blog covers the process of decompiling and recompiling APK files using APKTool, a powerful tool for reverse engineering Android apps. By the end, you'll be equipped to modify APKs, analyze vulnerabilities, and recompile apps for testing.

What is APKTool?

APKTool is an open-source tool designed for decoding and rebuilding Android APK files. It enables testers to-

• Access app resources like XML files, images, and strings
• Modify the app logic in smali code
• Rebuild APKs after making changes
• Perform deeper analysis of app behaviors   
  

  Prerequisites

  1. Java Development Kit (JDK) installed on your system.

    java -version 

   

  2. An APK file you want to analyze. 

  3. Update the files in Linux by the command:

  sudo apt update

  

• Step 1: Decompiling an APK

  Decompiling allows you to extract the resources and small code from an APK. Follow these steps:

• Install APKTool:

  Download the apktool file by the command:

  sudo apt install apktool

   
• Now type the below command for testing if apktool is working or not:
•  apktool 
• Decompile the APK: Use the following command to extract the APK's content:
• apktool d -f -r flappybird.apk   
• The extracted files will be stored in a folder where you downloaded the apk initially.
  • 

    Explore the Decompiled Files:

    1. AndroidManifest.xml: Contains app metadata like permissions and components.

    2. res/: Holds resources like layouts, strings, and images.

    3. smali/: Decompiled bytecode representing app logic.   

    Step 2: Modifying the APK

    Once you’ve decompiled the APK, you can make changes. Here are common modifications:

    1. Analyze Permissions

    Inspect AndroidManifest.xml for excessive or insecure permissions:

    xml
    <uses-permission android:name="android.permission.INTERNET"/>
    

    Modify permissions if necessary.

    2. Disable SSL Pinning

    Search for checkServerTrusted in smali files and comment out or bypass its implementation.

    3. Modify Hardcoded Values

    Look for sensitive data in the res/values or smali folders and replace or obfuscate them.

    Step 3: Recompiling the APK

    After modifications, the APK must be rebuilt:

     apktool b flappybird -o flappybird.apk 

      
    

    Conclusion

    By mastering APKTool, you gain a significant advantage in Android penetration testing. You can reverse-engineer apps, analyze their behavior, and modify them for testing purposes. This process is a critical skill for identifying security flaws in mobile applications.