My notes on injecting a frida gadget into an apk

fridanotes.md

Android RE using Frida

I figured that I would write down my findings somewhere since this is my first time using Frida. This won't cover installing frida, adb, apktool because these are well covered in other sources.

Tools

• https://github.com/frida/frida/
• https://github.com/sensepost/objection
• https://github.com/dweinstein/awesome-frida

Injecting Frida gadget into APKs

This is what has worked for me. Obviously this won't apply to all use cases but I have found that this is generally the process that I take.

Decompile the app using apktool.

apktool d appname.apk

Add the Frida gadget to the decompiled apk. You can find a gadget for your architecture here.

Put the gadget in lib/[arch]/libfrida-gadget.so

Open the AndroidManifest.xml and find the main activity path. It should look something like this:

<activity android:label="@string/app_name" android:name="com.packagename.path.to.MainActivity">

In MainActivity.smali, we need to inject libfrida-gadget.so. Ideally, we need to do it before anything else loads. We can load it using the following smali:

const-string v0, "frida-gadget"

invoke-static {v0}, Ljava/lang/System;->loadLibrary(Ljava/lang/String;)V

Which can be read as System.loadLibrary("frida-gadget"). It's important that this is done early in the app's lifecycle, so we can do it in the MainActivity static constructor. In the app that I am using, it looks like this:

.method static constructor <clinit>()V
    .locals 1 # this is the number of non-param registers
    ...

Insert the smali above in the beginning of the static constructor (after the .locals line if present).

Now we need to rebuild the app.

apktool b -o appname_patched.apk decompiledfolder

Sign the app

jarsigner -sigalg SHA1withRSA -digestalg SHA1 -keystore my.keystore appname_patched.apk keyname
jarsigner -verify appname_patched.apk

And zipalign.

zipalign 4 appname_patched.apk appname_patched_aligned.apk

Now we can install this on our target device and use your frida library of choice to poke around. :)

When I follow these instructions I end up with an app that crashes (exits to home screen) immediately on launch. Any tips on how to debug?

@dkbarn Have you tried logcat? Runtime errors should be there

Please i need help. My AndroidManifest.xml looks like this.
How can I find the MainActivity using this ?

@nzalik if you had using apktool to decode your apk, try using Jadx to decompile it.

If you encounter the [INSTALL_FAILED_INTERNAL_ERROR: Failed to extract native libraries, res=-2] try to modify the apktool.yml and add your library path to doNotCompress section before rebuilding app with apktool b folder. Or set the android:extractNativeLibs to true in your AndroidManifest.xml.