In This section we want to change the value of inputs and outputs in native library functions. It should be clear for you that we can’t just simply change the function arguments and return value because they may have different class types. For example if you want to change the input of a function that is jstring you can’t provide a JavaScript string.

Frida is awesome. It provide bridge between Frida to Java. Therefore we can create any Java class and Objects. You can access them with Java.vm.getEnv(). Also to see full abilities you have you can check there:

As you see we can create jstring by using newStringUtf.

Example:

Interceptor.attach( Module.findExportByName("libnativesecret.so", "Java_lab_seczone64_nativesecret_MainActivity_encryptDecrypt"), {
                    onEnter: (args) => {
                        args[2] = Java.vm.getEnv().newStringUtf("Seczone64")
                    },
                    onLeave: (ret) => {
                        ret.replace(Java.vm.getEnv().newStringUtf("Hooora. We did it. :)"))
                    }
                })
            }

<aside> 💡 You can’t directly change return value. You should use .replace function to change it.

</aside>

If you need C/C++ string type look at here:

Hooking strcmp Function