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CVE-2026-44727 Jupyter CVE debrief

CVE-2026-44727 is a critical vulnerability in Jupyter Server, a backend for Jupyter web applications. Prior to version 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. This, combined with nbconvert.HTMLExporter's default non-sanitizing behavior, allows a notebook carrying an HTML payload in a display_data output to trigger stored XSS with cookie access, full /api/* authority, and kernel RCE. The vulnerability has a CVSS score of 9.3 and is considered critical. It was published on June 22, 2026, and modified on June 26, 2026.

Vendor
Jupyter
Product
Jupyter Server
CVSS
CRITICAL 9.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-22
Original CVE updated
2026-08-28
Advisory published
2026-06-22
Advisory updated
2026-08-28

Who should care

Defenders of Jupyter Server installations should prioritize patching to version 2.20 or later. This vulnerability allows for stored XSS with significant impact, including cookie access and kernel RCE. Jupyter Server users, especially those with high-security requirements, should take immediate action.

Technical summary

The nbconvert HTTP handlers in jupyter_server prior to 2.20 render user-authored notebook HTML under the Jupyter origin without a sandbox directive in their Content-Security-Policy. This allows a malicious notebook to execute arbitrary JavaScript, leading to stored XSS. The vulnerability is exacerbated by nbconvert.HTMLExporter's default non-sanitizing behavior, which enables an attacker to inject HTML payloads in display_data outputs. Successful exploitation grants the attacker cookie access, full /api/* authority, and kernel RCE.

Defensive priority

Patching to version 2.20 or later is the primary mitigation. Defenders should also review their Jupyter Server installations for any suspicious activity and ensure that notebook content is properly sanitized.

Recommended defensive actions

  • Patch Jupyter Server to version 2.20 or later
  • Review Jupyter Server installations for suspicious activity
  • Ensure proper sanitization of notebook content
  • Monitor /api/* authority usage
  • Implement additional security measures to prevent kernel RCE

Evidence notes

The CVE record and NVD detail provide official information about the vulnerability. The source item URL provides additional context from the NVD database. Vendor references include patches and advisories from the Jupyter Server project.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-44727 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-44727

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-44727 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-44727

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.