n this laboratory, we are going to add a toast message on the target APK. This approach can be also used to backdoor the APK with something including the frida gadget.
💉 Frida's Gadget Injection on Android: No Root, 2 Methods
Alexander Fadeev's Blog
Releases · frida/frida
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1. apktool - decode and compile APK
The first task is the decoding of the target APK. We can do this using the tool: apktool.
Apktool - A tool for reverse engineering 3rd party, closed, binary Android apps.
First, we need to access the folder and we have installed the apktool.jar, in my case: C:\Tools\android.
Tip: First time, we can remove the -r option to decode all the resource and analyze the Android-Manifest.xml file.
After adding the target payload, we need to build the APK with the changes. Inside the created folder, we can seethe apktool.yml file. It is necessary to proceed. So, we need to execute the akptool build process from this path.
After sign an APK, if you got some erros such as "Failed to extract native libraries, res=-2]", probably you need to align the APK.
Before aligning it, you can try to install it:
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.\adb.exe install C:\tmp\backdored.apk
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Performing Streamed Install
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adb: failed to install C:\tmp\backdored.apk: Failure [INSTALL_FAILED_INVALID_APK: Failed to extract native libraries, res=-2]
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Attention: You must use zipalign at one of two specific points in the app building process, depending on the app signing tool you use:
If you use apksigner, only run zipalign before signing the apk file. If you sign your APK using apksigner and make other changes to the APK, the signature will be invalidated.
If you use jarsigner, only run zipalign after signing the APK file.
zipalign  | Android Developers
Android Developers
As I used jarsigner, my last step is align the APK before installing it.