5.4

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-40864

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

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.

(GitHub Advisory)

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CVE-2026-40864

CVE-2026-40864:

5.4

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation NoneAutomatable NoTech Impact Partial

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
jupyterhub	pip	>= 4.1.0, < 5.4.5	5.4.5

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability, CVE-2026-40864, is a Cross-Site Request Forgery (CSRF) bypass in JupyterHub. The root cause was an error in the logic that determines whether an XSRF check is necessary. The analysis of the provided patches, specifically commit 9c5ec277d3cda5a59de2d8c8117efa77bd941127, pinpointed the vulnerable function as _needs_check_xsrf located in jupyterhub/_xsrf_utils.py. The patch explicitly removes the string 'no-cors' from a condition that checks the 'Sec-Fetch-Mode' request header. In the vulnerable versions, the presence of Sec-Fetch-Mode: no-cors caused the XSRF check to be skipped. Since cross-origin form POSTs are sent with this header, it created a CSRF vulnerability. An attacker could host a malicious form on an external website that, when submitted by a logged-in JupyterHub user, would execute actions on their behalf, such as spawning a new server. The fix ensures that requests with Sec-Fetch-Mode: no-cors are no longer exempt from XSRF checks, thereby mitigating the vulnerability.

Vulnerable functions

_needs_check_xsrf

jupyterhub/_xsrf_utils.py

This function determines whether an XSRF check is required for a given request. The vulnerable version of the code incorrectly included 'no-cors' in a set of 'Sec-Fetch-Mode' header values that would cause the XSRF check to be skipped. This allowed attackers to craft cross-origin form POSTs that would bypass XSRF protection, as browsers send 'no-cors' for such requests. The patch removes 'no-cors' from this condition, ensuring that these requests are properly validated.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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