What’s Vulnerable › Vulnerabilities

CVE-2026-44727: Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP

• Severity: medium (CVSS 5.4 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-06-22 · updated 2026-10-25
• Weakness: CWE-79

Description

Jupyter Server is the backend for Jupyter web applications. Prior to 2.20, the nbconvert HTTP handlers in jupyter_server render user-authored notebook HTML under the Jupyter origin without a sandbox directive in their Content-Security-Policy. Combined with nbconvert.HTMLExporter's default non-sanitizing behavior, a notebook carrying an HTML payload in a display_data output triggers stored XSS with cookie access, full /api/* authority, and kernel RCE. This vulnerability is fixed in 2.20.

Affected products

• jupyter-server jupyter_server: before 2.20.0 — fixed in 2.20.0

How to fix

Upgrade to a fixed version: jupyter-server jupyter_server 2.20.0. Check the vendor advisory for mitigations.

References

• https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd
• https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp
• https://access.redhat.com/errata/RHSA-2026:43038
• https://access.redhat.com/errata/RHSA-2026:60520
• https://access.redhat.com/security/cve/CVE-2026-44727
• https://bugzilla.redhat.com/show_bug.cgi?id=2491516
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-44727.json

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