CVE-2026-44727Network attack vector

Kernel code execution via unsandboxed notebook HTML

Published Jun 22, 2026 · Updated Jun 22, 2026

Stored XSS in Jupyter Server before 2.20 allows remote authenticated users to execute code through crafted notebook output. NbconvertFileHandler and NbconvertPostHandler serve HTMLExporter output under the Jupyter origin without a sandbox Content-Security-Policy directive, so embedded scripts retain cookie and API access. Exploitation requires an authenticated victim to open the attacker-authored conversion URL, after which the script can steal the token and use kernel APIs to run code.

CVSS severity9.3
Critical
EPSS probability0.44%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in Jupyter Server before 2.20 allows remote authenticated users to execute code through crafted notebook output. NbconvertFileHandler and NbconvertPostHandler serve HTMLExporter output under the Jupyter origin without a sandbox Content-Security-Policy directive, so embedded scripts retain cookie and API access. Exploitation requires an authenticated victim to open the attacker-authored conversion URL, after which the script can steal the token and use kernel APIs to run code.

The record

CVE
CVE-2026-44727
Published
Jun 22, 2026
Updated
Jun 22, 2026
Vendor
Project Jupyter
Product
Jupyter Server
Classifications
CWE-79, CWE-1021, T1204.001
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jun 22, 2026CVE publishedPublication date reported by the CVE source.
  2. Jun 22, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2.20

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Project Jupyter · Jupyter Serverversion=< 2.20

Published CVSS scores

9.3GitHub, Inc.CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:A/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H
5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
9.0Red HatCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-1021Improper Restriction of Rendered UI Layers or Frames
T1204.001Malicious Link

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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