What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-17 · updated 2026-09-24
• Weakness: CWE-532

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.

Affected products

• jupyter-server jupyter_server: before 2.21.0

References

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

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