# CVE-2026-6657: Jupyter Jupyter Server CORS bypass via improper origin validation

Source: https://junglewise.ai/threats/cve-2026-6657-jupyter-jupyter-server-cors-bypass-via-improper-origin-validation  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-03. Updated: 2026-07-13  
License: CC BY 4.0 with a link back

CVE-2026-6657 · Severity: medium · CVSS 6.1 · Published 2026-06-03

Jupyter Server, a core component for running Jupyter interactive notebooks, contains a security flaw in how it validates web requests. An attacker can trick the server into accepting malicious requests by using a domain name that starts with a trusted name but ends in a malicious one (e.g., trusted.example.com.evil.com). If exploited, this could allow an attacker to steal sensitive data, perform unauthorized actions, or potentially execute malicious code on the user's system.

Technologies: [jupyter-server (PyPI)](https://junglewise.ai/threats/technologies/pypi-jupyter-server). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-6657 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-6657 being exploited in the wild.

**How severe is CVE-2026-6657?** Medium severity, with a CVSS score of 6.1, published 3 June 2026.

**Which products and versions are affected?** jupyter-server (PyPI) ECOSYSTEM:introduced=1.12.0,last_affected=2.17.0.

**Is there a fix for CVE-2026-6657?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-6657 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-6657?** Check whether you run jupyter-server (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A vulnerability exists in jupyter-server (versions 1.12.0 through 2.17.0) due to improper validation of the Origin header when the `allow_origin_pat` configuration is active. The server uses Python's `re.match()` function, which only anchors the regular expression at the beginning of the string. This allows an attacker to bypass security checks by using a subdomain they control that begins with a legitimate, trusted domain string (e.g., `trusted.example.com.attacker.com`). The flaw impacts CORS headers, WebSocket connections, referer validation, and login redirects. Successful exploitation requires a user to visit a malicious site (User Interaction) and can result in sensitive data exposure or arbitrary code execution via compromised WebSocket communication.

## Affected products

- Jupyter jupyter-server 1.12.0 - 2.17.0

## Identifiers

- CVE-2026-6657
- GHSA-v42x-x7jp-845h
- PYSEC-2026-3472

## Timeline

- 2026-06-03: disclosed
- 2026-06-03: advisory

## References

- [https://huntr.com/bounties/18f642db-3569-43b3-b58d-ff97be4b09d7](https://huntr.com/bounties/18f642db-3569-43b3-b58d-ff97be4b09d7)

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- [Jupyter Server authorization bypass in kernel gateway handlers](https://junglewise.ai/threats/cve-2026-114135-jupyter-server-is-the-backend-for-jupyter-web-applications-from)
- [Jupyter Server token leakage in 5xx error logs](https://junglewise.ai/threats/cve-2026-86049-jupyter-server-token-leakage-in-5xx-error-logs)
- [Jupyter Server stored XSS in nbconvert handlers](https://junglewise.ai/threats/cve-2026-44727-jupyter-server-stored-xss-in-nbconvert-handlers)
- [Jupyter jupyter-server path traversal in _get_os_path](https://junglewise.ai/threats/cve-2026-5422-jupyter-jupyter-server-path-traversal-in-get-os-path)
- [Jupyter Server insufficient session expiration after password reset](https://junglewise.ai/threats/cve-2026-40934-jupyter-server-insufficient-session-expiration-after-password)