Vulnerabilities in pkg:pypi/jupyter_server

PACKAGE
Advisory Identifier: GHSA-c3mw-737p-c7g2
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Advisory Summary

CVE-2026-86049 is a HIGH (CVSS 7.1) 5xx request logging leaks token-bearing vulnerability (CWE-532: Insertion of Sensitive Information into Log File) affecting Jupyter Server libraries in the PyPI ecosystem. The issue involves 5xx request logging leaks token-bearing referer header values. Remediation is available by upgrading affected packages to version 2.21.0.

2 Known Advisory Record(s)
Package Vulnerability Intelligence

pkg:pypi/jupyter_server

2 Known Advisory Record(s)

Vulnerability advisories affecting package pkg:pypi/jupyter_server. Select an advisory below to view technical details or fixes.

Advisory ID CVE / NVD Severity Summary
GHSA-c3mw-737p-c7g2 CVE-2026-86049 HIGH Jupyter Server: 5xx request logging leaks token-bearing Referer header values
GHSA-hrw6-wg82-cm62 CVE-2024-35178 HIGH Jupyter server on Windows discloses Windows user password hash

CVE-2026-86049 at a Glance

CVE Identifier CVE-2026-86049
GHSA Identifier GHSA-c3mw-737p-c7g2
Severity HIGH
CVSS v3.1 Base Score 7.1
CWE Classification CWE-532: Insertion of Sensitive Information into Log File
Affected Product Jupyter Server
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H).

Attack Vector Network (AV:N)
Attack Complexity High (AC:H)
Privileges Required Low (PR:L)
User Interaction Required (UI:R)
Scope Unchanged (S:U)
Confidentiality High (C:H)
Integrity High (I:H)
Availability High (A:H)

How to Fix CVE-2026-86049

To remediate this vulnerability, upgrade all affected Jupyter Server dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

jupyter_server fixed: 2.21.0 
pip install jupyter_server>=2.21.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show jupyter_server

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-86049, inspect your installed packages using pip:

pip show jupyter_server

Analyze the terminal output to locate any version of Jupyter Server in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.21.0), your application is affected.

Note: The pip show jupyter_server command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-532: Insertion of Sensitive Information into Log File, where the product writes sensitive information to a log file. In Jupyter Server, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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?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.

CVE-2026-86049 Affected Jupyter Server Versions

1 component(s)
Fixed Versions:
2.21.0
Product / Package Ecosystem Affected Versions Fixed Version
jupyter_server
pypi/jupyter_server
PyPI
< 2.21.0
2.21.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-c3mw-737p-c7g2
Jupyter Server Source Repository REPOSITORY
https://github.com/jupyter-server/jupyter_server/pull/1681
Security Fix Commit FIX COMMIT
https://github.com/jupyter-server/jupyter_server/commit/52513529af7096e622c00911b17627b266294717
Jupyter Server Source Repository REPOSITORY
https://github.com/jupyter-server/jupyter_server
Jupyter Server Release RELEASE
https://github.com/jupyter-server/jupyter_server/releases/tag/v2.21.0

Frequently Asked Questions

What is CVE-2026-86049?
CVE-2026-86049 is a security vulnerability in Jupyter Server involving 5xx request logging leaks token-bearing Referer header values under CWE-532: Insertion of Sensitive Information into Log File.
Which Jupyter Server versions are affected?
Affected versions include releases prior to 2.21.0 across affected packages: jupyter_server.
What version fixes CVE-2026-86049?
Upgrading to 2.21.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is jupyter_server affected?
Yes, jupyter_server is affected in releases prior to 2.21.0. Upgrade your dependency to 2.21.0 or later.
What is the CVSS score of CVE-2026-86049?
CVE-2026-86049 has a CVSS v3.1 base score of 7.1 (HIGH) with vector string CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H.
What CWE is associated with CVE-2026-86049?
CVE-2026-86049 is classified under CWE-532: Insertion of Sensitive Information into Log File (The product writes sensitive information to a log file.).
How can I check PyPI dependencies for CVE-2026-86049?
Run the dependency tree command: 'pip show jupyter_server'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-c3mw-737p-c7g2",
  "modified" : "2026-09-17T20:28:55Z",
  "published" : "2026-09-17T20:28:55Z",
  "aliases" : [ "CVE-2026-86049" ],
  "summary" : "Jupyter Server: 5xx request logging leaks token-bearing Referer header values",
  "details" : "### Summary\n\nWhen a request returns a 500, `jupyter_server/log.py` logs a small JSON block of request headers. \n\nThe Referer header was copied into it as-is, so a token in the Referer URL ended up in the logs in plain text.\n\n### Impact\n\nAnyone 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.\n\nAffected: all versions before 2.21.0.\n\n### PoC\n\nAny malformed request that returns a 500 works:\n\n```bash\ncurl -i -X POST \\\n  -H 'Content-Type: application/json' \\\n  -H 'Referer: http://127.0.0.1:8899/tree?token=REFERTOKEN' \\\n  --data '{\"name\":123}' \\\n  'http://127.0.0.1:8899/api/kernels?token=VALIDTOKEN'\n```\n\nThe log shows the token twice, once raw and once redacted:\n\n```\n\"Referer\": \"http://127.0.0.1:8899/tree?token=REFERTOKEN\",\n[E ... ServerApp] 500 POST /api/kernels?token=[secret] (...) referer=http://127.0.0.1:8899/tree?token=[secret]\n```\n\n### Patches\n\nFixed in 2.21.0 by 5251352. Header values are now scrubbed before the block is logged. Upgrade to 2.21.0 or later.\n\n### Workarounds\n\nLimit who can read the server logs. If you can, avoid flows that put the token in the URL.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "jupyter_server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.21.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.20.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-c3mw-737p-c7g2"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jupyter-server/jupyter_server/pull/1681"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jupyter-server/jupyter_server/commit/52513529af7096e622c00911b17627b266294717"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/jupyter-server/jupyter_server"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jupyter-server/jupyter_server/releases/tag/v2.21.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-532" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-17T20:28:55Z",
    "nvd_published_at" : null
  }
}