CVE-2026-40934
CVE-2026-40934 is a security vulnerability in jupyter-server. O3 Security confirms whether CVE-2026-40934 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
jupyter-server authentication cookies remain valid after password reset due to static cookie secret
Blast Radius
jupyter-serverReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Jupyter Server is the backend for Jupyter web applications. In versions 2.17.0 and earlier, the secret used to sign authentication cookies is persisted to a static file at ~/.local/share/jupyter/runtime/jupyter_cookie_secret and is never rotated when a user changes their password. After a password reset and server restart, any previously issued authentication cookie remains cryptographically valid because the signing key has not changed. An attacker who has captured a session cookie through any means retains full authenticated access to the server regardless of subsequent password changes. This affects deployments using password-based authentication, particularly shared or public-facing servers where credential rotation is expected to revoke existing sessions. This issue has been fixed in version 2.18.0.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | jupyter-server | all versions | 2.18.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for jupyter-server. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update jupyter-server to 2.18.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40934 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-40934 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-40934. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-40934 in your dependencies?
O3 detects CVE-2026-40934 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.