Frida Tutorial 1

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This is a summary of the post: 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.