Patched code sample

import asyncio

# The vulnerability in older aiohttp versions was that for multipart form fields,
# the entire field was read into memory before checking `client_max_size`.
# This could lead to a Denial of Service (DoS) if a malicious client sent a
# very large field.

# The fix involves checking the size of the incoming data *as it is being read*
# in chunks, rather than after the entire field is buffered in memory.

# This code provides a conceptual representation of the FIX. It does not use
# aiohttp directly but models the corrected logic: checking size limits
# incrementally during the read process.

CHUNK_SIZE = 8192  # 8 KB, a typical chunk size for reading network data

class FixedMultipartReader:
    """
    A simplified reader that represents the fixed (non-vulnerable) logic.
    It checks the size limit while reading data chunk by chunk.
    """
    def __init__(self):
        self.total_read = 0
        self.part_data = bytearray()

    async def read_part(self, stream, max_size):
        """
        Reads a "part" from a stream, enforcing `max_size` incrementally.
        """
        print(f"Starting to read part with a max_size of {max_size} bytes.")
        while True:
            # In a real scenario, this would read from a network socket.
            chunk = await stream.read(CHUNK_SIZE)
            if not chunk:
                # End of the part/stream
                break

            chunk_len = len(chunk)
            print(f"Read a chunk of {chunk_len} bytes.")

            # --- THE FIX ---
            # Check if adding the new chunk would exceed the max_size.
            # This check happens *before* appending the chunk to the main buffer.
            if self.total_read + chunk_len > max_size:
                raise ValueError(
                    f"Part size limit of {max_size} bytes exceeded. "
                    f"Aborting read operation."
                )
            # --- END OF FIX ---

            # If the check passes, append the data and update the total.
            self.part_data.extend(chunk)
            self.total_read += chunk_len

        print(f"Finished reading part. Total size: {self.total_read} bytes.")
        return bytes(self.part_data)


async def main():
    """
    Demonstrates the fixed logic by trying to read a payload larger
    than the allowed maximum size.
    """
    # Set a small max size for demonstration purposes.
    client_max_size = 1024 * 100  # 100 KB

    # Create a payload that is intentionally larger than the max size.
    large_payload_size = client_max_size + 50000  # 150 KB
    large_payload = b'\x00' * large_payload_size
    print(f"Created a simulated request part of size: {large_payload_size} bytes.")
    print("-" * 20)

    # Use asyncio.StreamReader to simulate a network stream
    stream_reader = asyncio.StreamReader()
    stream_reader.feed_data(large_payload)
    stream_reader.feed_eof()

    # Instantiate the reader with the fixed logic
    reader = FixedMultipartReader()

    try:
        # Attempt to read the oversized part
        await reader.read_part(stream_reader, max_size=client_max_size)
        print("\n[DEMO FAILED]: The oversized part was read completely without error.")

    except ValueError as e:
        print(f"\n[DEMO SUCCESSFUL]: Caught expected error: {e}")
        print(
            "The reader stopped processing before allocating memory for the "
            f"entire {large_payload_size}-byte payload."
        )
        print(f"Total data read into memory before stopping: {reader.total_read} bytes.")


if __name__ == "__main__":
    asyncio.run(main())