Patched code sample

import asyncio
from aiohttp import web, MultipartReader

# The vulnerability described (a hypothetical CVE-2026-34516) is analogous to
# the real-world CVE-2023-47627, which was fixed in aiohttp 3.9.0.
# The fix involves adding a limit to the number of multipart parts that can be
# processed, preventing resource exhaustion (DoS).
#
# This code demonstrates how such a fix is implemented and used.
# It sets up two endpoints:
# 1. A conceptual "/vulnerable" endpoint that does not limit parts.
# 2. A "/fixed" endpoint that uses the `max_parts` parameter to prevent DoS.
#
# To run this code, you need aiohttp installed (version >= 3.9.0 for the fix):
# pip install aiohttp

async def handle_vulnerable(request):
    """
    This handler represents the vulnerable behavior. It does not limit the
    number of multipart parts, allowing a client to send a request with an
    excessive number of parts, consuming significant memory and CPU.
    """
    try:
        # In vulnerable versions, MultipartReader had no limit on the number of parts.
        reader = MultipartReader.from_request(request)
        part_count = 0
        while True:
            part = await reader.next()
            if part is None:
                break
            # In a real application, processing would happen here.
            # We just discard the part to demonstrate the loop.
            await part.release()
            part_count += 1
        return web.Response(
            text=f"Processed {part_count} parts without any limit."
        )
    except Exception as e:
        return web.Response(
            text=f"An error occurred: {e}", status=500
        )


async def handle_fixed(request):
    """
    This handler demonstrates the fix. It uses the `max_parts` parameter,
    which was introduced to mitigate the vulnerability. If a client sends
    more than `max_parts` parts, a ValueError is raised, terminating the
    request processing early and preventing resource exhaustion.
    """
    MAX_ALLOWED_PARTS = 100  # Set a reasonable limit
    part_count = 0
    try:
        # The fix: The `max_parts` argument is passed to the reader.
        # This requires aiohttp >= 3.9.0.
        reader = MultipartReader.from_request(request, max_parts=MAX_ALLOWED_PARTS)

        print(f"Processing request with a limit of {MAX_ALLOWED_PARTS} parts...")

        while True:
            # When the number of parts exceeds max_parts, `await reader.next()`
            # will raise a ValueError.
            part = await reader.next()
            if part is None:
                break
            part_count += 1
            # In a real app, you would process the part here.
            # e.g., print(f"Processing part {part.name}")
            await part.release()

        return web.Response(
            text=f"Successfully processed {part_count} parts."
        )
    except ValueError as e:
        # This is the expected outcome when the part limit is exceeded.
        return web.Response(
            text=f"Request rejected: {e}",
            status=413, # Payload Too Large
        )
    except Exception as e:
        return web.Response(
            text=f"An unexpected error occurred: {e}", status=500
        )

async def main():
    app = web.Application()
    app.router.add_post("/vulnerable", handle_vulnerable)
    app.router.add_post("/fixed", handle_fixed)

    runner = web.AppRunner(app)
    await runner.setup()
    site = web.TCPSite(runner, "localhost", 8080)
    await site.start()

    print("Server running on http://localhost:8080")
    print("Endpoints:")
    print("  POST /vulnerable  (processes unlimited multipart parts)")
    print("  POST /fixed       (processes a maximum of 100 parts)")
    print("\nTo test, use a command like curl:")
    print("Create a file with many boundaries, e.g., 'many_parts.txt'")
    print("curl -X POST -H \"Content-Type: multipart/form-data; boundary=boundary\" --data-binary \"@many_parts.txt\" http://localhost:8080/fixed")

    # Keep the server running until manually stopped
    await asyncio.Event().wait()

if __name__ == "__main__":
    try:
        asyncio.run(main())
    except KeyboardInterrupt:
        print("\nServer shutting down.")