Jupyter Server: 5xx request logging leaks token-bearing Referer header values

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.1 · CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H

EPSS

• CVE-2026-86049: 0.42% estimated exploitation activity in the next 30 days; percentile 35.93 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary When a request returns a 500, jupyterserver/log.py logs a small JSON block of request headers. The Referer header was copied into it as-is, so a token in the Referer URL ended up in the logs in plain text. Impact Anyone who can read the server logs can pick tokens out of these 500 entries. Tokens end up in the Referer during normal token-based login and launch flows. Affected: all versions before 2.21.0. PoC Any malformed request that returns a 500 works: The log shows the token twice, once raw and once redacted: Patches Fixed in 2.21.0 by 5251352. Header values are now scrubbed before the block is logged. Upgrade to 2.21.0 or later. Workarounds Limit who can read the server logs. If you can, avoid flows that put the token in the URL.

Published by the source

17 Sep 2026, 20:28 UTC

Last modified by the source

1 Oct 2026, 17:55 UTC

Source last checked

26 Oct 2026, 13:52 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-c3mw-737p-c7g2

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-c3mw-737p-c7g2

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-c3mw-737p-c7g2

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# Patch Next advisory brief: CVE-2026-86049

Advisory: Jupyter Server: 5xx request logging leaks token-bearing Referer header values
IDs: CVE-2026-86049, GHSA-c3mw-737p-c7g2
Record: https://patchnext.com/advisories/cve-2026-86049 (revision 3, source checked 26 Oct 2026, 13:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: jupyter-server 2.21.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI jupyter-server: affected <= 2.20.0; fixed in 2.21.0

## Suggested inspection (read-only)
- `python -m pip show jupyter-server`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on jupyter-server.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-c3mw-737p-c7g2 (GitHub-reviewed advisory GHSA-c3mw-737p-c7g2 lists affected ranges: PyPI jupyter-server >=0 <=2.20.0)
- Verified: https://osv.dev/vulnerability/GHSA-c3mw-737p-c7g2 (OSV record GHSA-c3mw-737p-c7g2 lists affected ranges: PyPI jupyter-server >=0 <2.21.0 (fixed))

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