Frida 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

Python

Frida allows you to insert JavaScript code inside functions of a running application. But you can use python to call the hooks and even to interact with the hooks.

This is a easy python script that you can use with all the proposed examples in this tutorial:

#hooking.py
import frida, sys

with open(sys.argv[1], 'r') as f:
        jscode = f.read()
process = frida.get_usb_device().attach('infosecadventures.fridademo')
script = process.create_script(jscode)
print('[ * ] Running Frida Demo application')
script.load()
sys.stdin.read()

Call the script:

python hooking.py <hookN.js>

It is useful to know how to use python with frida, but for this examples you could also call directly Frida using command line frida tools:

frida -U --no-pause -l hookN.js -f infosecadventures.fridademo

Hook 1 - Boolean Bypass

Here you can see how to hook a boolean method (checkPin) from the class: infosecadventures.fridademo.utils.PinUtil

//hook1.js
Java.perform(function() {
 console.log("[ * ] Starting implementation override...")
 var MainActivity = Java.use("infosecadventures.fridademo.utils.PinUtil");
 MainActivity.checkPin.implementation = function(pin){
     console.log("[ + ] PIN check successfully bypassed!")
     return true;
 }
});

python hooking.py hook1.js

Mirar: La funcion recibe como parametro un String, no hace falta overload?

Hook 2 - Function Bruteforce

Non-Static Function

If you want to call a non-static function of a class, you first need a instance of that class. Then, you can use that instance to call the function.
To do so, you could find and existing instance and use it:

Java.perform(function() {
 console.log("[ * ] Starting PIN Brute-force, please wait...");
 Java.choose("infosecadventures.fridademo.utils.PinUtil", {
  onMatch: function(instance) {
   console.log("[ * ] Instance found in memory: " + instance);
   for(var i = 1000; i < 9999; i++){
    if(instance.checkPin(i + "") == true){
     console.log("[ + ] Found correct PIN: " + i);
     break;
    }
   }
  },
  onComplete: function() { }
 });
});

In this case this is not working as there isn't any instance and the function is Static

Static Function

If the function is static, you could just call it:

//hook2.js
Java.perform(function () {
    console.log("[ * ] Starting PIN Brute-force, please wait...")
    var PinUtil = Java.use("infosecadventures.fridademo.utils.PinUtil");
 
    for(var i=1000; i < 9999; i++)
    {
        if(PinUtil.checkPin(i+"") == true){
            console.log("[ + ] Found correct PIN: " + i);
        }
    }
});

Hook 3 - Retrieving arguments and return value

You could hook a function and make it print the value of the passed arguments and the value of the return value:

//hook3.js
Java.perform(function() {
 console.log("[ * ] Starting implementation override...")
  
 var EncryptionUtil = Java.use("infosecadventures.fridademo.utils.EncryptionUtil");
 EncryptionUtil.encrypt.implementation = function(key, value){
     console.log("Key: " + key);
     console.log("Value: " + value);
     var encrypted_ret = this.encrypt(key, value); //Call the original function
     console.log("Encrypted value: " + encrypted_ret);
     return encrypted_ret;
 }
});

Important

In this tutorial you have hooked methods using the name of the mathod and .implementation. But if there were** more than one method with the same name, you will need to specify the method that you want to hook indicating the type of the arguments**.

You can see that in the next tutorial.

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