CVE-2026-6657: CORS Origin Validation Bypass in jupyter-server

Severity: high (CVSS 8.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 5.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-06-03 · updated 2026-07-21
Weakness: CWE-346

Description

A vulnerability in jupyter-server versions 1.12.0 through 2.17.0 allows an attacker to bypass CORS origin validation when the `allow_origin_pat` configuration is used. The issue arises from the use of `re.match()` for validating the `Origin` header, which only anchors at the start of the string. This allows attacker-controlled domains such as `trusted.example.com.evil.com` to pass validation against patterns intended to match `trusted.example.com`. The vulnerability affects multiple locations in the codebase, including CORS headers, WebSocket connections, referer validation, and login redirects, potentially enabling phishing attacks, arbitrary code execution, and unauthorized access to sensitive API responses.

Affected products

jupyter-server jupyter_server: from 1.12.0 through 2.17.0

References

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