CVE-2026-6657 CORS Origin Validation Bypass in jupyter-server
Attackers can bypass CORS validation in Jupyter Server when using regex patterns, enabling phishing, code execution, and data theft.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-6657?
CVE-2026-6657 is a medium-severity vulnerability in jupyter/jupyter, published on 3 Jun 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official 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.
Weakness: CWE-346
Which versions are affected by CVE-2026-6657?
3 open source packages ship an affected version, including jupyter-server. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
jupyter-server PyPI | From 1.12.0 to 2.17.0 | None |
jupyter-server PyPI | From 1.12.0 to 2.17.0 | None |
jupyter-server PyPI | From 1.12.0 to 2.17.0 | None |
Products include Jupyter Jupyter Server.
How do I fix CVE-2026-6657?
No fix has been published for CVE-2026-6657 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-6657 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6657 exists.
Other vulnerabilities in jupyter/jupyter
- CVE-2026-44727 Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP
- CVE-2026-35397 jupyter-server path traversal allows access to sibling directories sharing root_dir name prefix
- CVE-2026-40110 jupyter-server CORS origin validation bypass via unanchored regex in allow_origin_pat
- CVE-2026-40934 jupyter-server authentication cookies remain valid after password reset due to static cookie secret
- CVE-2024-35178 Jupyter server on Windows discloses Windows user password hash
Sources
Also known as DEBIAN-CVE-2026-6657, GHSA-v42x-x7jp-845h, PYSEC-2026-3472, UBUNTU-CVE-2026-6657.