CVE-2026-40934

A persistent cookie secret vulnerability allows authenticated users to maintain indefinite access even after password changes.

The cookie secret used to sign authentication cookies is stored in a permanent file (~/.local/share/jupyter/runtime/jupyter_cookie_secret) that is never automatically rotated or cleared, allowing stolen or compromised cookies to remain valid indefinitely regardless of password resets.

PoC

• Start a Jupyter server with password authentication: jupyter server password, jupyter server
• Log in with the password and capture the authentication cookie (e.g., just login with a browser).
• Change the password to revoke access: jupyter server password
• Restart the server
• Use the old stolen cookie => remains valid and provides full authenticated access.

Jupyter Server 2.18+

Workaround

rm ~/.local/share/jupyter/runtime/jupyter_cookie_secret
# Then restart the server

• All jupyter-server deployments using password authentication where security incidents may occur
• Multi-user systems where one user's compromised session should be revocable by administrators
• Shared or public-facing Jupyter servers where credential rotation is a security requirement
• Any deployment where password changes are expected to revoke existing sessions

CVE-2026-40934 has a CVSS score of 6.8 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.18.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-40934? CVE-2026-40934 is a high-severity security vulnerability in jupyter-server (pip), affecting versions <= 2.17.0. It is fixed in 2.18.0.
2. How severe is CVE-2026-40934? CVE-2026-40934 has a CVSS score of 6.8 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of jupyter-server are affected by CVE-2026-40934? jupyter-server (pip) versions <= 2.17.0 is affected.
4. Is there a fix for CVE-2026-40934? Yes. CVE-2026-40934 is fixed in 2.18.0. Upgrade to this version or later.
5. Is CVE-2026-40934 exploitable, and should I be worried? Whether CVE-2026-40934 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-40934 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-40934? Upgrade jupyter-server to 2.18.0 or later.