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HIGH severity

Jupyter Server: 5xx request logging leaks token-bearing Referer header valuesGHSA-c3mw-737p-c7g2

HIGHFix: jupyter-server/jupyter_server#1681

GHSA-c3mw-737p-c7g2 is a high-severity (CVSS 7.1) CWE-532 vulnerability in jupyter-server. A fix is available for jupyter-server — see the affected versions and patch details below.

Also known asCVE-2026-86049PYSEC-2026-4054
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 1, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • A successful exploit gives an attacker total control of the affected component, not partial access.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-c3mw-737p-c7g2.

EPSS Exploitation Probability

via FIRST.org ↗
0.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs36th percentile — riskier than 36% of all scored CVEsHighest risk
0.00%0.31%0.61%0.92%0.4%0.4%Oct 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐍jupyter-server

Real-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

Summary

When a request returns a 500, jupyter_server/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:

curl -i -X POST \
  -H 'Content-Type: application/json' \
  -H 'Referer: http://127.0.0.1:8899/tree?token=REFERTOKEN' \
  --data '{"name":123}' \
  'http://127.0.0.1:8899/api/kernels?token=VALIDTOKEN'

The log shows the token twice, once raw and once redacted:

"Referer": "http://127.0.0.1:8899/tree?token=REFERTOKEN",
[E ... ServerApp] 500 POST /api/kernels?token=[secret] (...) referer=http://127.0.0.1:8899/tree?token=[secret]

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.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIjupyter-serverall versions2.21.0pip install --upgrade 'jupyter-server==2.21.0'

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for jupyter-server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update jupyter-server to 2.21.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-c3mw-737p-c7g2 is resolved across your whole dependency graph.

  3. 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.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This vulnerability is rated as Moderate impact because exploiting it requires an unhandled HTTP 500 error condition and direct access to the server's logging output. In Red Hat OpenShift AI, notebook workloads execute within managed container environments where log inspection is restricted to authorized cluster…

Workaround published by Red Hat
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Source: Red Hat security advisory for GHSA-c3mw-737p-c7g2 (CC BY 4.0)
ProductFixed inAdvisory
Red Hat OpenShift AI 2.25rhoai/odh-feature-server-rhel9:1792148523RHSA-2026:87410
Red Hat OpenShift AI 3.4rhoai/odh-feature-server-rhel9:1791863220RHSA-2026:84734

Frequently Asked Questions

### Summary When a request returns a 500, `jupyter_server/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: ```bash curl -i -X POST \ -H 'Content-Type: application/json' \ -H 'Referer: http://127.0.0.1:8899/tree?
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