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

Should you act?

Critical, preconditions apply

Severe if exploited, but exploitation needs conditions that may not apply to you. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.4%, percentile 37: FIRST’s model estimates a 0.4% chance of exploitation activity in the next 30 days, higher than 37% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Needs an account (low (any account)). Exposure depends on who can log in.
• Only applies if your dependency tree includes jupyter-server (PyPI) <= 2.19.0, fixed in 2.20.0. Check your lockfile, including transitive dependencies.

What it is

Jupyter Server is the backend for Jupyter web applications. Prior to 2.20, the nbconvert HTTP handlers in jupyterserver 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 displaydata output triggers stored XSS with cookie access, full /api/ authority, and kernel RCE. This vulnerability is fixed in 2.20.

From the GitHub advisory

The nbconvert HTTP handlers in jupyterserver 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 displaydata output triggers stored XSS with cookie access, full /api/ authority, and kernel RCE.

Impact

An authenticated victim who navigates to /nbconvert/html/ containing attacker-authored output can have their token exfiltrated to another domain because it is executed in the Jupyter origin.

Patches

Weakness type: CWE-79, CWE-1021.

Product: jupyter-server (PyPI) · Assigned by: GitHub_M · Published: 2026-06-22

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: low (any account)
• User interaction: active

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.44% probability of exploitation activity in the next 30 days; percentile 37.

Timeline

• 2026-06-18 — GitHub-reviewed advisory GHSA-fcw5-x6j4-ccmp published (critical)
• 2026-06-22 — CVE record published by GitHub_M
• 2026-08-28 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. access.redhat.com access.redhat.com
2. access.redhat.com access.redhat.com

Vulnerability records

1. GitHub Advisory GHSA-fcw5-x6j4-ccmp github.com
2. CVE.org record CVE-2026-44727 cve.org
3. NVD entry CVE-2026-44727 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. access.redhat.com access.redhat.com
4. RHBZ#2491516 bugzilla.redhat.com
5. security.access.redhat.com security.access.redhat.com
6. github.com github.com
7. github.com github.com
8. pypi.org pypi.org

Critical PyPI advisories · Today’s Security Digest