Frida Tutorial 3

Reading time: 3 minutes

This is a summary of the post: https://joshspicer.com/android-frida-1
APK: https://github.com/OWASP/owasp-mstg/blob/master/Crackmes/Android/Level_01/UnCrackable-Level1.apk

Solution 1

Based in https://joshspicer.com/android-frida-1

Hook the _exit()_ function and decrypt function so it print the flag in frida console when you press verify:

Java.perform(function () {
  send("Starting hooks OWASP uncrackable1...")

  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 : " + getString(ret))

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

  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  :)")
  }

  send("Hooks installed.")
})

Solution 2

Based in https://joshspicer.com/android-frida-1

Hook rootchecks and decrypt function so it print the flag in frida console when you press verify:

Java.perform(function () {
  send("Starting hooks OWASP uncrackable1...")

  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 : " + getString(ret))

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

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

  var rootcheck2 = Java.use("sg.vantagepoint.a.c")
  rootcheck2.b.overload().implementation = function () {
    send("sg.vantagepoint.a.c.b()Z  Root check 2 HIT!  test-keys")
    return false
  }

  var rootcheck3 = Java.use("sg.vantagepoint.a.c")
  rootcheck3.c.overload().implementation = function () {
    send("sg.vantagepoint.a.c.c()Z  Root check 3 HIT!  Root packages")
    return false
  }

  var debugcheck = Java.use("sg.vantagepoint.a.b")
  debugcheck.a.overload("android.content.Context").implementation = function (
    var_0
  ) {
    send("sg.vantagepoint.a.b.a(Landroid/content/Context;)Z  Debug check HIT! ")
    return false
  }

  send("Hooks installed.")
})