Executive brief
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.
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
Timeline
- 2026-06-17: advisory: GitHub Security Advisory published
- 2026-06-22: disclosed: CVE published to NVD