# CVE-2026-44727: Jupyter Server stored XSS in nbconvert handlers

Source: https://junglewise.ai/threats/cve-2026-44727-jupyter-server-stored-xss-in-nbconvert-handlers  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-22. Updated: 2026-06-22  
License: CC BY 4.0 with a link back

CVE-2026-44727 · Severity: critical · CVSS 4 · Published 2026-06-22

Jupyter Server is the core engine that powers Jupyter web applications used for data science and research. A security flaw allows a malicious user to create a notebook that, when viewed by another user, can steal their login credentials or take full control of the server. This could lead to the theft of sensitive research data or unauthorized access to the underlying computing infrastructure.

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

## Quick answers

**Is CVE-2026-44727 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-44727 being exploited in the wild.

**How severe is CVE-2026-44727?** Critical severity, with a CVSS score of 4, published 22 June 2026.

**Which products and versions are affected?** jupyter-server (PyPI) ECOSYSTEM:introduced=0,fixed=2.20.0.

**Is there a fix for CVE-2026-44727?** Yes. jupyter-server (PyPI): upgrade to 2.20.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-44727 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-44727?** 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.

## How to fix

- jupyter-server (PyPI): upgrade to 2.20.0 or later

  ```sh
  pip install "jupyter-server>=2.20.0"
  ```


## Technical details

A stored cross-site scripting (XSS) vulnerability exists in the NbconvertFileHandler and NbconvertPostHandler components of jupyter_server. The root cause is the lack of a 'sandbox' directive in the Content-Security-Policy (CSP) when rendering user-authored notebook HTML, combined with nbconvert.HTMLExporter's default behavior of not sanitizing HTML payloads in display_data outputs. An authenticated attacker can craft a notebook with a malicious HTML payload; when a victim views this notebook via the /nbconvert/html/ path, the script executes within the Jupyter origin. This allows the attacker to access session cookies, interact with the full /api/* scope, and ultimately achieve remote code execution (RCE) via the Jupyter kernel. The issue is fixed in version 2.20.0.

## Affected products

- jupyter-server jupyter_server < 2.20.0

## Identifiers

- CVE-2026-44727
- GHSA-fcw5-x6j4-ccmp
- PYSEC-2026-366

## Timeline

- 2026-06-17: advisory: GitHub Security Advisory published
- 2026-06-22: disclosed: CVE published to NVD

## References

- [https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd](https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd)
- [https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp](https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp)

## Related threats

- [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 Jupyter Server CORS bypass via improper origin validation](https://junglewise.ai/threats/cve-2026-6657-jupyter-jupyter-server-cors-bypass-via-improper-origin-validation)
- [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)