How severe is CVE-2026-6657?

CVE-2026-6657 has a CVSS 3.x base score of 8.8, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2026-6657 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (10th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-6657?

CVE-2026-6657 affects Jupyter Jupyter Server. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-6657?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-6657 have an EU (EUVD) identifier?

Yes. CVE-2026-6657 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-34104.

When was CVE-2026-6657 published?

CVE-2026-6657 was published on 2026-06-03 and last updated on 2026-06-30.

CVE-2026-6657

CVE-2026-6657 is a high-severity vulnerability in Jupyter Jupyter Server with a CVSS 3.x base score of 8.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-346.

Key facts

Severity: High (CVSS 3.x base score 8.8)
EPSS exploit prediction: 0% (10th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-34104
Weakness: CWE-346
Affected product: Jupyter Jupyter Server
Published: 2026-06-03
Last modified: 2026-06-30

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.

Frequently asked questions

What is CVE-2026-6657?: 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.
How severe is CVE-2026-6657?: CVE-2026-6657 has a CVSS 3.x base score of 8.8, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2026-6657 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (10th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-6657?: CVE-2026-6657 affects Jupyter Jupyter Server. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-6657?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-6657 have an EU (EUVD) identifier?: Yes. CVE-2026-6657 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-34104.
When was CVE-2026-6657 published?: CVE-2026-6657 was published on 2026-06-03 and last updated on 2026-06-30.

References

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

Affected products (1)

cpe:2.3:a:jupyter:jupyter_server:*:*:*:*:*:*:*:*

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