Skip to content

python-engineio has possible denial of service due to maximum payload size sometimes not being enforced

High severity GitHub Reviewed Published May 23, 2026 in miguelgrinberg/python-engineio • Updated Jun 26, 2026

Package

python-engineio (pip)

Affected versions

<= 4.13.1

Patched versions

4.13.2

Description

Impact

There are 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
  • WebSocket messages, when using Aiohttp with the WebSocket transport

Patches

Version 4.13.2 addresses this issue as follows:

  • 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.

References

@miguelgrinberg miguelgrinberg published to miguelgrinberg/python-engineio May 23, 2026
Published to the GitHub Advisory Database Jun 26, 2026
Reviewed Jun 26, 2026
Last updated Jun 26, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS score

Weaknesses

Allocation of Resources Without Limits or Throttling

The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated. Learn more on MITRE.

CVE ID

CVE-2026-48809

GHSA ID

GHSA-m9gh-vj53-gvh9

Source code

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.