How severe is CVE-2026-86049?

CVE-2026-86049 has a CVSS score of 7.1/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-86049?

CVE-2026-86049 affects the following packages: jupyter-server (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-86049?

Update jupyter-server to 2.21.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-86049 is resolved across your whole dependency graph.

How do I detect CVE-2026-86049 in my PyPI dependencies?

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.

How do I mitigate CVE-2026-86049 if there is no patch (or I can't update yet)?

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.

Is CVE-2026-86049 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-86049 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-86049?

CVE-2026-86049 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 36th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-86049?

CVE-2026-86049 is classified as CWE-532 (CWE-532). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-86049 published, and has it been updated?

CVE-2026-86049 was published on September 17, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

HIGH severity

CVE-2026-86049 — jupyter-server

HIGHFix: jupyter-server/jupyter_server@5251352

CVE-2026-86049 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.

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

Also known asGHSA-c3mw-737p-c7g2PYSEC-2026-4054

Published

Sep 17, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 31, 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 CVE-2026-86049.

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

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

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`jupyter-server`	all versions	2.21.0`pip install --upgrade 'jupyter-server==2.21.0'`

Detection & mitigation playbook

Open-source dependency

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.
Fix
Update jupyter-server to 2.21.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-86049 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.

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 CVE-2026-86049 (CC BY 4.0)

Product	Fixed in	Advisory
Red Hat OpenShift AI 2.25	`rhoai/odh-feature-server-rhel9:1792148523`	RHSA-2026:87410
Red Hat OpenShift AI 3.4	`rhoai/odh-feature-server-rhel9:1791863220`	RHSA-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?