Jupyter Server: 5xx request logging leaks token-bearing Referer header values
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 software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | jupyter-server | < 2.21.0 | 2.21.0 |
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
- https://github.com/jupyter-server/jupyter_server/releases/tag/v2.21.0
- https://pypi.org/project/jupyter-server
- https://github.com/advisories/GHSA-c3mw-737p-c7g2
- https://nvd.nist.gov/vuln/detail/CVE-2026-86049
- CVSS vector
CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H- Weakness (CWE)
CWE-532- Platform
- Python (PyPI)
- Package
jupyter-server- Disclosed
- 17.09.2026
- Updated
- 11.10.2026 15:18
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-4054 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-c3mw-737p-c7g2 - CC BY 4.0
Vulnerability history: jupyter-server
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 14
- First → latest
- 11.2020 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 7
- Medium 6
Disclosures per year
- 2020: 2
- 2021: 0
- 2022: 2
- 2023: 3
- 2024: 1
- 2025: 0
- 2026: 6
-
Jupyter Server: 5xx request logging leaks token-bearing Referer header values this entry
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