Introduction

If you're working with Frida, a powerful dynamic instrumentation toolkit, you might find yourself needing to pass data between JavaScript and Python environments. One common scenario is when you want to pass an array of bytes, possibly from an image bitmap, from your JavaScript context back to Python. In this article, we'll explore how to effectively send a byteArray from a Frida script back to a Python session, resolving the common error: Error: expected a pointer.

Understanding the Frida Environment

Frida allows you to run JavaScript code within the context of a target application, enabling you to interact with its components. This interaction can involve manipulating data types, such as bitmaps, and transferring them back to your Python script.

In your case, you've encountered an error when trying to send a byteArray through the send() function. The root of the issue is that send() requires a pointer to the data, so if you attempt to send the byteArray directly, it results in an error.

Why the Error Occurs

The byteArray obtained from a bitmap's output stream is a native array in the Frida JavaScript context, and it is not directly compatible with the send() function without a proper conversion. Frida needs data to be in the form of a serializable format, typically a string or an object. Therefore, we need to convert our byte array into a format that send() can handle, such as a base64 string or a raw buffer.

Step-by-Step Solution

Step 1: Convert the ByteArray to Base64

Instead of sending the byte array directly, you can convert it to a Base64 string, which is safer and ensures compatibility. Modify your existing script in the JavaScript section:

const byteArray = stream.toByteArray();
const base64String = Java.use('android.util.Base64').encodeToString(byteArray, 0);
// Send the base64 string instead
send({ type: "bitmap", page: pageNum, data: base64String });

Step 2: Modify the Python Handler

You will also need to update your Python on_message function to handle the incoming Base64 string and convert it back to bytes:

def on_message(message, data):
    if message['type'] == 'send' and message['payload'].get('type') == 'bitmap':
        page = message['payload'].get('page')
        base64_data = message['payload'].get('data')

        # Convert Base64 string back to byte array
        import base64
        byte_data = base64.b64decode(base64_data)

        with open(OUTPUT_FILENAME, "wb") as f:
            f.write(byte_data)
        print(f"[+] Saved page {page} as {OUTPUT_FILENAME}")
    else:
        print(f"[?] Unknown message: {message}")

Summary of Changes

  1. JavaScript: Convert the byte array to a Base64 string before sending it back to the Python script.
  2. Python: Decode the Base64 string back into a byte array for further processing.

Conclusion

Passing data between Frida’s JavaScript context and Python can be challenging, especially when dealing with byte arrays. By converting your byte array to a Base64 string before sending it, you can easily avoid type mismatch issues. This solution not only resolves the Error: expected a pointer issue but also demonstrates effective data handling practices in hybrid environments.

Frequently Asked Questions

1. What is Frida?

Frida is a dynamic instrumentation toolkit that allows developers and security researchers to inject JavaScript code into native apps, making it easier to analyze how they work.

2. Why use Base64 for sending data?

Base64 encoding is an effective way to convert binary data (like byte arrays) into text format, making it safe to transmit over systems that handle formatted text.

3. Can I send other data types using Frida's send()?

Yes, you can send various types of data such as strings, objects, and arrays, as long as they are serializable in JavaScript.

4. Is there an alternative to Base64?

You can also consider other serialization methods, but Base64 is one of the most widely supported for binary data transfer across different languages and platforms.