Apk.Sh : Automating Repetitive Tasks Pulling, Decoding, Rebuilding And Patching An APK

Apk.sh makes reverse engineering Android apps easier, automating some repetitive tasks like pulling, decoding, rebuilding and patching an APK. apk.sh is a Bash script that makes reverse engineering Android apps easier, automating some repetitive tasks like pulling, decoding, rebuilding and patching an APK.

Features

apk.sh basically uses apktool to disassemble, decode and rebuild resources and some bash to automate the frida gadget injection process. It also supports app bundles/split APKs.

• Patching APKs to load frida-gadget.so on start.
• Support for app bundles/split APKs.
• Disassembling resources to nearly original form with apktool.
• Rebuilding decoded resources back to binary APK/JAR with apktool.
• Code signing the apk with apksigner.
• Multiple arch support (arm, arm64, x86, x86_64).
• No rooted Android device needed.

Getting started

Pulling an APK from a device is simple as running ./apk.sh pull <package_name>

Decoding an APK is simple as running ./apk.sh decode <apk_name>

Rebuilding an APK is simple as running ./apk.sh build <apk_dir>

apk.sh pull

apk.sh pull pull an APK from a device. It supports app bundles/split APKs, which means that split APKs will be joined in a single APK (this is useful for patching). If the package is an app bundle/split APK, apk.sh will combine the APKs into a single APK, fixing all public resource identifiers.

apk.sh patch

apk.sh patch patch an APK to load frida-gadget.so on start.

frida-gadget.so is a Frida’s shared library meant to be loaded by programs to be instrumented (when the Injected mode of operation isn’t suitable). By simply loading the library it will allow you to interact with it using existing Frida-based tools like frida-trace. It also supports a fully autonomous approach where it can run scripts off the filesystem without any outside communication.

Patching an APK is simple as running ./apk.sh patch <apk_name> --arch arm.

You can calso specify a Frida gadget configuration in a json ./apk.sh patch <apk_name> --arch arm --gadget-conf <config.json>

Frida’s Gadget configurations

In the default interaction, Frida Gadget exposes a frida-server compatible interface, listening on localhost:27042 by default. In order to achieve early instrumentation Frida let Gadget’s constructor function block until you either attach() to the process, or call resume() after going through the usual spawn() -> attach() -> ...apply instrumentation... steps.

If you don’t want this blocking behavior and want to let the program boot right up, or you’d prefer it listening on a different interface or port, you can customize this through a json configuration file.

The default configuration is:

{
  "interaction": {
    "type": "listen",
    "address": "127.0.0.1",
    "port": 27042,
    "on_port_conflict": "fail",
    "on_load": "wait"
  }
}

You can pass the gadget configuration file to apk.sh with the --gadget-conf option.

Script interaction

A typically suggested configuration might be:

{
  "interaction": {
    "type": "script",
    "path": "/data/local/tmp/script.js",
    "on_change":"reload"
  }
}

script.js could be something like:

var android_log_write = new NativeFunction(
    Module.getExportByName(null, '__android_log_write'),
    'int',
    ['int', 'pointer', 'pointer']
);

var tag = Memory.allocUtf8String("[frida-script][ax]");

var work = function() {
    setTimeout(function() {
        android_log_write(3, tag, Memory.allocUtf8String("ping @ " + Date.now()));
        work();
    }, 1000);
}

work();

android_log_write(3, tag, Memory.allocUtf8String(">--(O.o)-<"));

adb push script.js /data/local/tmp

./apk.sh patch <apk_name> --arch arm --gadget-conf <config.json>

adb install file.gadget.apk

Note

Add the following code to print to logcat the console.log output of any script from the frida codeshare when using the Script interaction type.

// print to logcat the console.log output
// see: https://github.com/frida/frida/issues/382
var android_log_write = new NativeFunction(
    Module.getExportByName(null, '__android_log_write'),
    'int',
    ['int', 'pointer', 'pointer']
);
var tag = Memory.allocUtf8String("[frida-script][ax]");
console.log = function(str) {
    android_log_write(3, tag, Memory.allocUtf8String(str));
}

Requirements

• apktool
• apksigner
• unxz
• zipalign
• aapt
• adb

Usage

SYNOPSIS

apk.sh [SUBCOMMAND] [APK FILE|APK DIR|PKG NAME] [FLAGS]
apk.sh pull [PKG NAME] [FLAGS]
apk.sh decode [APK FILE] [FLAGS]
apk.sh build [APK DIR] [FLAGS]
apk.sh patch [APK FILE] [FLAGS]
apk.sh rename [APK FILE] [PKG NAME] [FLAGS]

SUBCOMMANDS

pull Pull an apk from device/emulator.
decode Decode an apk.
build Re-build an apk.
patch Patch an apk.
rename Rename the apk package.

FLAGS

-a, --arch <arch> Specify the target architecture, mandatory when patching.

-g, --gadget-conf <json_file> Specify a frida-gadget configuration file, optional when patching.

-n, --net Add a permissive network security config when building, optional. It can be used with patch, pull and rename also.

-s, --safe Do not decode resources when decoding (i.e. apktool -r). Cannot be used when patching.

-d, --no-dis Do not disassemble dex, optional when decoding (i.e. apktool -s). Cannot be used when patching.

Links of Interest

• https://frida.re/docs/gadget/
• https://lief-project.github.io/doc/latest/tutorials/09_frida_lief.html
• https://koz.io/using-frida-on-android-without-root/
• https://github.com/sensepost/objection/
• https://github.com/NickstaDB/patch-apk/
• https://neo-geo2.gitbook.io/adventures-on-security/frida-scripting-guide/frida-scripting-guide

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