What is CVE-2026-86049?

CVE-2026-86049 is Jupyter Server: 5xx request logging leaks token-bearing Referer header values. The supplied enrichment data classifies it as Info Disclosure mapped to CWE-532 CWE-532.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Active exploitation is not confirmed from current sources for CVE-2026-86049. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-86049 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

A public PoC is not confirmed from current sources for CVE-2026-86049. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-86049 using CVSS 7.1, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

Workarounds are not confirmed from current sources. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

highInfo Disclosure

CVE-2026-86049

CVE-2026-86049 — Jupyter Server: 5xx request logging leaks token-bearing Referer header values

Jupyter Server is the backend for Jupyter web applications. Prior to version 2.21.0, the 5xx request logging path in jupyter_server/log.py copies the Referer header into a JSON header block without applying the token scrubbing used for the request URI. A request that returns HTTP 500 while the Referer contains a token-bearing URL can therefore write that token to server logs in plaintext. An attacker who can read those logs can recover the token and use the affected user's Jupyter Server permissions. This issue is fixed in version 2.21.0.

Published Sep 17, 2026Updated Sep 17, 2026Sources: NVD, jupyter-server (CNA), GitHub, GitHub Security Advisory, SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Unreported

no source claims exploitation

EPSS

ahead of 0% of scored CVEs

Affects

jupyter-server

2 products listed

CVSS base

7.1

high

Remediation

The vendor's own words where we have them.

Upgrade jupyter_server to 2.21.0 or later (pip advisory GHSA-c3mw-737p-c7g2).

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
jupyter-server	jupyter_server	< 2.21.0	Vulnerable
pip	jupyter_server	<= 2.20.0	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Availability

High

Confidentiality

High

Integrity

High

Scope

Unchanged

Attack Complexity

High

Attack Vector

Network

Privileges Req

Low

User Interaction

Required

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
7.1	CVSS 3.1	high	1.2	5.9	GitHub (CNA), jupyter-server (CNA), GHSA

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

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-532 · Insertion of Sensitive Information into Log File

Read Application Data

Mitigation: Consider seriously the sensitivity of the information written into log files. Do not write secrets into the log files.

Weakness & attack patterns

CWE-532

Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.

CAPEC-215 · Fuzzing and observing application log data/errors for application mapping

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Search application, proxy, and WAF logs for requests touching /log.py.
Prioritize edge telemetry for network-reachable jupyter_server.

web/proxy/WAF logsapplication logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 7.1 (AV:N)
Sep 17, 2026 · NVD
Initial · CVE published
Sep 17, 2026 · NVD
Added · GHSA-c3mw-737p-c7g2 published (high)
Sep 17, 2026 · GitHub Security Advisory
CVE published by MITRE.
Sep 17, 2026 · SOCRadar CTI

References

5 on the record

github.com/jupyter-server/jupyter_server/commit/52513529af7096e622c00911b17627b266294717
Exploit, Third Party Advisory, Mitre
github.com/jupyter-server/jupyter_server/pull/1681
Exploit, Third Party Advisory, Mitre
github.com/jupyter-server/jupyter_server/releases/tag/v2.21.0
Exploit, Third Party Advisory, Mitre
github.com/jupyter-server/jupyter_server/security/advisories/GHSA-c3mw-737p-c7g2
Exploit, Third Party Advisory, Mitre
github.com/advisories/GHSA-c3mw-737p-c7g2
Exploit, Third Party Advisory

Elsewhere on this site

jupyter-serverevery CVE for this vendor
pipevery CVE for this vendor
Info Disclosuresame class
CWE-532other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-86049 is Jupyter Server: 5xx request logging leaks token-bearing Referer header values, a high vulnerability affecting jupyter_server from jupyter-server, pip. The current evidence does not list it in CISA KEV, and the exploit status is: Active exploitation is not confirmed from current sources for CVE-2026-86049. Public exploit evidence is: A public PoC is not confirmed from current sources for CVE-2026-86049. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 7.1 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.