“UnCrackable App for Android Level 1. This app holds a secret inside. Can you find it?”
This post details several ways of solving the level 1 of the Android crackmes released by the OWASP guys (Bernhard Mueller). To begin with, a hardened APK is provided and the main goal is to extract a hidden secret from the app.
Requirements: What do we need?
- Android phone or emulator to run the crackme APK
- Android decompiler of your preference to obtain Java code. (BytecodeViewer, Jadx-gui, JEB, JD-GUI,…)
- Dynamic binary instrumentation framework of your preference (Xposed or Frida)
- Time and a bit of thinking
Assumptions and highlights:
- Anti-debugging and anti-rooting checks are in place at the Java level. We do not need to bypass all of them but get the flag
- Dynamic binary instrumentation is the approach chosen to obtain the secret inside the application
- Repackaging the application and patching out the security checks might be also possible but it is not covered in this write-up
My Solution:
This challenge can be solved in so many different ways. Though, I decided to approach it in a dynamic way by performing dynamic binary instrumentation (DBI) with Frida.
Java.perform(function () {
send("Starting hooks OWASP uncrackable1...");
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 :)");
};
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 : " + var_0);
send("Encrypted : " + var_1);
var ret = this.a.overload("[B","[B").call(this,var_0,var_1);
send("Decrypted : " + ret);
flag = "";
for (var i=0; i < ret.length; i++){
flag += String.fromCharCode(ret[i]);
}
send("Decrypted flag: " + flag);
return ret; //[B
};
var mainactivity = Java.use("sg.vantagepoint.uncrackable1.MainActivity");
mainactivity.onStart.overload().implementation = function() {
send("MainActivity.onStart() HIT!!!");
var ret = this.onStart.overload().call(this);
};
//var mainactivity = Java.use("sg.vantagepoint.uncrackable1.MainActivity");
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);
};
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);
};
/* 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 0;
};
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 0;
};
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 0;
};
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 0;
};
*/
send("Hooks installed.");
});
Flag1
References: