---
id: "GHSA-xgw8-8ccx-rmqh"
page: "https://infrastack.ai/live/GHSA-xgw8-8ccx-rmqh"
kind: "vulnerability"
aliases: ["GHSA-xgw8-8ccx-rmqh"]
title: "Too many URL-encoded form fields block the event loop"
severity: "medium"
cvss: null
epss: null
exploited: false
status: "open"
category: "package"
products: [{"name": "aiohttp", "fixed": ["3.14.4"], "affected": "<= 3.14.3", "ecosystem": "pypi"}]
ecosystems: ["pypi"]
tags: ["aiohttp", "package", "python", "self-hosted"]
not_affected_if: ["multipart-only", "no-form-post-handlers"]
published_at: "2026-11-08T18:30:40+00:00"
first_seen_at: "2026-11-09T22:53:22+00:00"
updated_at: "2026-11-09T22:58:34+00:00"
sources: [{"source": "ghsa-repo", "url": "https://github.com/aio-libs/aiohttp/security/advisories/GHSA-xgw8-8ccx-rmqh", "first_seen_at": "2026-11-09T22:53:22+00:00"}]
exploit_sources: []
---

# Too many URL-encoded form fields block the event loop

Link for people: https://infrastack.ai/live/GHSA-xgw8-8ccx-rmqh (this .md is for agents)

aiohttp's Request.post() accepted any number of fields in a URL-encoded form body, and inserting many repeated keys into a MultiDict takes quadratic time. An attacker can send one form POST within the default client_max_size and block the event loop, stalling the whole server.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- pypi:aiohttp affected <= 3.14.3 fixed 3.14.4

## How to check

- Check the installed version with `pip show aiohttp`
- Look for handlers that call `Request.post()` on a URL-encoded form body
- Check the pinned version in requirements or lock files

## Fix

- aiohttp: move to `3.14.4`

## Timeline

- 2026-11-09T22:53:22+00:00 ghsa-repo https://github.com/aio-libs/aiohttp/security/advisories/GHSA-xgw8-8ccx-rmqh

## Documents

- [advisory] Maintainer's advisory: github.com https://github.com/aio-libs/aiohttp/security/advisories/GHSA-xgw8-8ccx-rmqh

## From the sources

### Summary

`Request.post()` had no limit on the number of fields in a URL-encoded form body. Inserting a large number of repeated keys into a `MultiDict` took quadratic time on the event loop.

### Impact

An attacker could block the event loop with a form POST within the default `client_max_size` to any handler that calls `Request.post()`.

---

Patch: https://github.com/aio-libs/aiohttp/commit/615fec3d4f088f30d2ee72cad760152e92aa1c53

---

_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_