Frida Tutorial

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Installation

Install frida tools:

pip install frida-tools
pip install frida

Download and install in the android the frida server (Download the latest release).
One-liner to restart adb in root mode, connect to it, upload frida-server, give exec permissions and run it in backgroud:

adb root; adb connect localhost:6000; sleep 1; adb push frida-server /data/local/tmp/; adb shell "chmod 755 /data/local/tmp/frida-server"; adb shell "/data/local/tmp/frida-server &"

Check if it is working:

frida-ps -U #List packages and processes
frida-ps -U | grep -i <part_of_the_package_name> #Get all the package name

Tutorials

Tutorial 1

From: https://medium.com/infosec-adventures/introduction-to-frida-5a3f51595ca1
APK: https://github.com/t0thkr1s/frida-demo/releases
Source Code: https://github.com/t0thkr1s/frida-demo

Follow the link to read it.

Tutorial 2

From: https://11x256.github.io/Frida-hooking-android-part-2/ (Parts 2, 3 & 4)
APKs and Source code: https://github.com/11x256/frida-android-examples

Follow the link to read it.

Tutorial 3

From: https://joshspicer.com/android-frida-1
APK: https://github.com/OWASP/owasp-mstg/blob/master/Crackmes/Android/Level_01/UnCrackable-Level1.apk

Follow the link to read it.

You can find more Awesome Frida scripts here: https://codeshare.frida.re/

Quick Examples

Calling Frida from command line

frida-ps -U

#Basic frida hooking
frida -l disableRoot.js -f owasp.mstg.uncrackable1

#Hooking before starting the app
frida -U --no-pause -l disableRoot.js -f owasp.mstg.uncrackable1
#The --no-pause and -f options allow the app to be spawned automatically,
#frozen so that the instrumentation can occur, and the automatically
#continue execution with our modified code.

Basic Python Script

import frida, sys

jscode = open(sys.argv[0]).read()
process = frida.get_usb_device().attach('infosecadventures.fridademo')
script = process.create_script(jscode)
print('[ * ] Running Frida Demo application')
script.load()
sys.stdin.read()

Hooking functions without parameters

Hook the function a() of the class sg.vantagepoint.a.c

Java.perform(function () {
;  rootcheck1.a.overload().implementation = function() {
  rootcheck1.a.overload().implementation = function() {
    send("sg.vantagepoint.a.c.a()Z   Root check 1 HIT!  su.exists()");
    return false;
  };
});

Hook java exit()

var sysexit = Java.use("java.lang.System")
sysexit.exit.overload("int").implementation = function (var_0) {
  send("java.lang.System.exit(I)V  // We avoid exiting the application  :)")
}

Hook MainActivity .onStart() & .onCreate()

var mainactivity = Java.use("sg.vantagepoint.uncrackable1.MainActivity")
mainactivity.onStart.overload().implementation = function () {
  send("MainActivity.onStart() HIT!!!")
  var ret = this.onStart.overload().call(this)
}
mainactivity.onCreate.overload("android.os.Bundle").implementation = function (
  var_0
) {
  send("MainActivity.onCreate() HIT!!!")
  var ret = this.onCreate.overload("android.os.Bundle").call(this, var_0)
}

Hook android .onCreate()

var activity = Java.use("android.app.Activity")
activity.onCreate.overload("android.os.Bundle").implementation = function (
  var_0
) {
  send("Activity HIT!!!")
  var ret = this.onCreate.overload("android.os.Bundle").call(this, var_0)
}

Hooking functions with parameters and retrieving the value

Hooking a decryption function. Print the input, call the original function decrypt the input and finally, print the plain data:

function getString(data) {
  var ret = ""
  for (var i = 0; i < data.length; i++) {
    ret += data[i].toString()
  }
  return ret
}
var aes_decrypt = Java.use("sg.vantagepoint.a.a")
aes_decrypt.a.overload("[B", "[B").implementation = function (var_0, var_1) {
  send("sg.vantagepoint.a.a.a([B[B)[B   doFinal(enc)  // AES/ECB/PKCS7Padding")
  send("Key       : " + getString(var_0))
  send("Encrypted : " + getString(var_1))
  var ret = this.a.overload("[B", "[B").call(this, var_0, var_1)
  send("Decrypted : " + ret)

  var flag = ""
  for (var i = 0; i < ret.length; i++) {
    flag += String.fromCharCode(ret[i])
  }
  send("Decrypted flag: " + flag)
  return ret //[B
}

Hooking functions and calling them with our input

Hook a function that receives a string and call it with other string (from here)

var string_class = Java.use("java.lang.String") // get a JS wrapper for java's String class

my_class.fun.overload("java.lang.String").implementation = function (x) {
  //hooking the new function
  var my_string = string_class.$new("My TeSt String#####") //creating a new String by using `new` operator
  console.log("Original arg: " + x)
  var ret = this.fun(my_string) // calling the original function with the new String, and putting its return value in ret variable
  console.log("Return value: " + ret)
  return ret
}

Getting an already created object of a class

If you want to extract some attribute of a created object you can use this.

In this example you are going to see how to get the object of the class my_activity and how to call the function .secret() that will print a private attribute of the object:

Java.choose("com.example.a11x256.frida_test.my_activity", {
  onMatch: function (instance) {
    //This function will be called for every instance found by frida
    console.log("Found instance: " + instance)
    console.log("Result of secret func: " + instance.secret())
  },
  onComplete: function () {},
})

Other Frida tutorials

• https://github.com/DERE-ad2001/Frida-Labs
• Part 1 of Advanced Frida Usage blog series: IOS Encryption Libraries