CVE-2026-40864

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.

How to fix CVE-2026-40864

To remediate CVE-2026-40864, upgrade the affected package to a fixed version below.

• —upgrade to 5.4.5 or later
• —no fix listed
• —upgrade to 5.4.5 or later

Is CVE-2026-40864 being exploited?

Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.

Affected packages (3)

• >= 4.1.0, < 5.4.5
• from 0
• >= 4.1.0, < 5.4.5

CVSS scores

References (5)

• ADVISORYsecurity-tracker.debian.org/tracker/CVE-2026-40864
• PATCHgithub.com/jupyterhub/jupyterhub
• WEBgithub.com/jupyterhub/jupyterhub/commit/9c5ec277d3cda5a59de2d8c8117efa77bd941127
• WEBgithub.com/jupyterhub/jupyterhub/security/advisories/GHSA-m68r-v472-jgq9