CriticalCVE-2026-44727

    Jupyter Server: Cross-site Scripting (CVE-2026-44727)

    Published Last modified Sources: Red Hat, CVE.org, OSV, Hersteller

    What is affected, what to do

    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.

    Update Jupyter Server to version 2.20.

    Affected
    • Jupyter Server lt *..2.20
    Fixed in
    • Jupyter Server 2.20

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.3

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v4

    5.4

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: Red Hat, CVE.org, OSV, Hersteller

    0.4 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 37.9

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    77.44 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    9.3 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    5.4
    CVSS v4

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.3
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.4 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    37.9
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-79, CWE-1021, CWE-79, CWE-79

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    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.

    Source: redhat_csaf, NVD (NIST), OSV (Open Source Vulnerabilities), cvelistv5, CVE.org (MITRE), GitHub Security Advisories

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    jupyter-serverjupyter_server
    • < 2.20
    • 2.20
    jupyterjupyter_server
    • < 2.20.0
    • 2.20.0
    pypijupyter-server
    • 1.19.0
    • 1.12.0
    • 1.1.3
    • 2.12.3
    • 1.8.0
    • 2.11.1
    • 1.2.1
    • 2.12.5
    • 1.0.0rc12
    • 0.3.0
    • 0.0.1
    • 2.1.0
    • 2.0.6
    • 2.0.0rc6
    • 1.0.8
    • 2.7.3
    • 1.1.2
    • 1.0.5
    • 2.0.0rc5
    • 2.0.3
    • 2.6.0
    • 2.0.0rc4
    • 2.10.0
    • 2.5.0
    • 2.19.0
    • 1.0.7
    • 1.0.0rc10
    • 1.21.0
    • 1.23.3
    • 2.15.0
    • 2.0.0rc2
    • 2.0.0a0
    • 1.6.3
    • 1.18.1
    • 1.12.1
    • 1.0.0rc2
    • 1.6.2
    • 1.1.4
    • 1.2.2
    • 1.1.1
    • 2.14.1
    • 2.0.7
    • 1.17.1
    • 1.23.2
    • 1.15.6
    • 2.9.1
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    Vendors and products

    Weakness classes (CWE)

    • CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

      Base

      The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

      Entry at MITRE CWE-79

    • CWE-1021Improper Restriction of Rendered UI Layers or Frames

      Base

      The web application does not restrict or incorrectly restricts frame objects or UI layers that belong to another application or domain.

      Entry at MITRE CWE-1021

    • CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

      Base

      The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

      Entry at MITRE CWE-79

    • CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

      Base

      The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

      Entry at MITRE CWE-79

    References & sources

    • github.comjupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j…web, x_refsource_CONFIRM, Mitigation, Patch, Vendor advisory
    • NVD (NIST)vuln/detail/CVE-2026-44727advisory
    • github.comjupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fe…web, x_refsource_MISC, Patch
    • access.redhat.comerrata/RHSA-2026:43038web, Vendor advisory, x_refsource_REDHAT
    • access.redhat.comsecurity/cve/CVE-2026-44727web, Vulnerability database, x_refsource_REDHAT
    • bugzilla.redhat.comRHBZ#2491516web, Issue tracking, x_refsource_REDHAT
    • GitHub Security Advisoriesadvisories/GHSA-fcw5-x6j4-ccmpadvisory
    • github.comjupyter-server/jupyter_serverpackage
    • github.compypa/advisory-database/tree/main/vulns/jupyter-server/PYSEC-202…web
    • pypi.orgproject/jupyter-serverweb
    • security.access.redhat.comdata/csaf/v2/vex/2026/cve-2026-44727.jsonweb, x_sadp-csaf-vex
    • access.redhat.comerrata/RHSA-2026:60520Vendor advisory, x_refsource_REDHAT

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedGitHub

      Show 6 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd
      Referenceadded
      —
      https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp
      Referenceremoved
      https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd
      —
      Referenceremoved
      https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp
      —
      Reference typeremoved
      https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd Types: Patch
      —
      Reference typeremoved
      https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp Types: Mitigation, Patch, Vendor Advisory
      —

      Source identifier: security-advisories@github.com

    2. Record modifiedRed Hat ADP

      Show 10 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)added
      —
      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/44xxx/CVE-2026-44727.json">CVE-2026-44727</a>
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:43038
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:60520
      Referenceadded
      —
      https://access.redhat.com/security/cve/CVE-2026-44727
      Referenceadded
      —
      https://bugzilla.redhat.com/show_bug.cgi?id=2491516
      Referenceadded
      —
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-44727.json
      Referenceremoved
      https://access.redhat.com/errata/RHSA-2026:43038
      —
      Referenceremoved
      https://access.redhat.com/security/cve/CVE-2026-44727
      —
      Referenceremoved
      https://bugzilla.redhat.com/show_bug.cgi?id=2491516
      —
      Referenceremoved
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-44727.json
      —

      Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

    3. Record modifiedRed Hat ADP

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:43038
      Affected versions (publisher)changed
      Migration Toolkit for Applications 8, Red Hat OpenShift AI (RHOAI), Red Hat OpenShift AI (RHOAI) (+13)
      Red Hat Migration Toolkit for Applications 8.2, Red Hat OpenShift AI (RHOAI), Red Hat OpenShift AI (RHOAI) (+13)

      Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

    4. ReceivedGitHub

      CVSS V4.0: AV:N/AC:L/AT:N/PR:L/UI:A/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

      Weakness type: CWE-79, CWE-1021

      Description added or changed

      Show 7 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)added
      —
      jupyter_server
      Descriptionadded
      —
      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.
      CVSSadded
      —
      AV:N/AC:L/AT:N/PR:L/UI:A/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
      Weakness type (CWE)added
      —
      CWE-79
      Weakness type (CWE)added
      —
      CWE-1021
      Referenceadded
      —
      https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd
      Referenceadded
      —
      https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp

      Source identifier: security-advisories@github.com

    One NVD translation into another language is hidden.

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