What is the CVSS score of CVE-2026-6657?

CVE-2026-6657 has a CVSS 3.1 base score of 8.8 (High). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H. EPSS exploitation probability: 0.0%.

Which versions of Jupyter Server are affected by CVE-2026-6657?

A CORS origin validation bypass in Jupyter Server 1.12.0-2.17.0 allows remote attackers to defeat cross-origin security boundaries when the allow_origin_pat configuration is enabled, potentially…

How to fix or mitigate CVE-2026-6657?

Within 24 hours: Disable the allow_origin_pat configuration option in all Jupyter Server instances; replace with explicit allow_origin whitelist of trusted domains only. Within 7 days: Conduct security audit of Jupyter deployments to identify exposed instances; implement network segmentation and firewall rules restricting Jupyter access; review access logs for suspicious cross-origin requests. Within 30 days: Develop migration timeline for patched version when released; deploy Web Application Firewall rules to restrict CORS requests at perimeter; evaluate moving Jupyter instances behind authentication proxy or API gateway.

Jupyter Server CVE-2026-6657

EUVD-2026-34104 HIGH

Origin Validation Error (CWE-346)

2026-06-03 @huntr_ai

GHSA-v42x-x7jp-845h

Authentication Bypass RCE Jupyter Jupyter

8.8

CVSS 3.1 · NVD

Severity by source

NVD PRIMARY

8.8 HIGH

AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

vuln.today AI

7.5 HIGH

AC:H because exploitation needs the non-default allow_origin_pat config plus an attacker-controlled lookalike domain; UI:R for the lured victim; PR:N since no server auth is needed and kernel access yields high C/I/A.

3.1 AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H

4.0 AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

SUSE

6.1 MEDIUM

AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Red Hat

6.1 MEDIUM

qualitative

Primary rating from NVD.

CVSS VectorNVD

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Lifecycle Timeline

Analysis Updated

Jun 30, 2026 - 19:15 vuln.today

v3 (cvss_changed)

Analysis Updated

Jun 30, 2026 - 19:14 vuln.today

v2 (cvss_changed)

Re-analysis Queued

Jun 30, 2026 - 19:07 vuln.today

cvss_changed

Severity Changed

Jun 30, 2026 - 19:07 NVD

MEDIUM HIGH

CVSS changed

Jun 30, 2026 - 19:07 NVD

6.1 (MEDIUM) 8.8 (HIGH)

Analysis Generated

Jun 03, 2026 - 16:21 vuln.today

DescriptionNVD

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.

AnalysisAI

Origin validation bypass in Jupyter Server 1.12.0 through 2.17.0 lets remote attackers defeat CORS origin checks whenever the optional allow_origin_pat configuration is enabled. Because origin matching uses re.match() (anchored only at the start of the string), an attacker-controlled lookalike domain such as trusted.example.com.evil.com satisfies a pattern meant to allow only trusted.example.com, exposing CORS-protected responses, WebSocket kernels, referer checks, and login redirects. Publicly available exploit code exists via the huntr report, but EPSS is very low (0.02%) and SSVC rates exploitation as POC-only and not automatable, so there is no evidence of widespread active exploitation.

Technical ContextAI

Jupyter Server is the backend web/REST and WebSocket service that powers JupyterLab, Notebook 7, and many data-science IDEs, exposing kernels capable of executing arbitrary code. The flaw is a classic CWE-346 (Origin Validation Error): the affected versions validate the inbound Origin (and referer) header against the operator-supplied allow_origin_pat regular expression using Python's re.match(), which anchors the pattern at the start but not the end of the string. Without a trailing anchor ($) or fullmatch(), any value that merely begins with the intended host passes, so an attacker-owned suffix-extension domain is accepted. The provided CPE (cpe:2.3:a:jupyter:jupyter/jupyter) is generic; the concretely affected component is the jupyter-server package, and the bug recurs across several call sites - CORS response headers, WebSocket connection acceptance, referer validation, and login redirect handling.

RemediationAI

No vendor-released fixed version is identified in the provided data, and the description indicates all releases through 2.17.0 are vulnerable, so upgrade to the first jupyter-server release published after 2.17.0 once it is confirmed against the project's release notes and the huntr advisory (https://huntr.com/bounties/18f642db-3569-43b3-b58d-ff97be4b09d7); do not assume a version number not stated in the source data. As an immediate compensating control, stop relying on allow_origin_pat and instead use an exact allow_origin allowlist of full origins, which removes the vulnerable regex path entirely (trade-off: you lose wildcard/pattern flexibility and must enumerate each trusted origin). If a pattern is unavoidable, rewrite it to anchor the end of the host and escape the dots (for example ^https://trusted\.example\.com$) so suffix-extension domains are rejected (trade-off: legitimate subdomains must be listed explicitly). Additionally restrict network exposure of the server to trusted users or a VPN and keep token/password authentication enabled to limit the blast radius of any successful cross-origin request.