High severity · published Sep 17, 2026
CVE-2026-86049
Jupyter Server: 5xx request logging leaks token-bearing Referer header values
7.1
CVSS
0.4%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
Attack vector
Network
Complexity
High
Privileges
Low
User interaction
Required
Confidentiality
High
Integrity
High
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.4% · higher than 36% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
JU
jupyter_server · jupyter-server
< 2.21.0 · source: vendor
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-86049
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-86049
github.com
https://github.com/jupyter-server/jupyter_server/commit/52513529af7096e622c00911b17627b266294717
github.com
https://github.com/jupyter-server/jupyter_server/pull/1681
github.com
https://github.com/jupyter-server/jupyter_server/releases/tag/v2.21.0
github.com
https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-c3mw-737p-c7g2
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