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CVE Record

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

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.

CriticalCVSS 9.3Not KEV-listedUpdated
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Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

A malicious notebook can run attacker-controlled browser code when another user views its converted HTML in Jupyter Server. Because the code runs as the Jupyter site, it may access the victim’s session, use Jupyter APIs, and ultimately execute code through a kernel.

Executive priority

Treat remediation as immediate for shared or externally reachable Jupyter environments. Successful exploitation could cross user trust boundaries and lead to sensitive-data access or server-side code execution through kernels. Upgrade promptly, identify high-risk collaborative deployments first, and investigate suspicious notebook activity where exposure existed.

Technical view

Before version 2.20, NbconvertFileHandler and NbconvertPostHandler serve user-authored notebook HTML without a sandbox directive in Content-Security-Policy. Combined with HTMLExporter’s default non-sanitizing behavior, this enables stored cross-site scripting. Exploitation requires limited privileges to introduce content and user action to render it, but can compromise cookies, API authority, data, and kernels.

Likely exposure

Exposure exists wherever jupyter_server versions below 2.20 render notebooks from users or other untrusted sources. Shared, multi-user, research, education, and hosted notebook environments deserve particular attention because one user may influence content opened by another. The supplied sources do not establish how widely vulnerable versions are deployed.

Exploitation context

The CVSS 4.0 score is 9.3. The attack is network-accessible, low complexity, requires limited privileges, and needs a victim action. The source bundle reports that this CVE is not in KEV and provides no evidence of active exploitation. That absence should not be interpreted as proof that exploitation has never occurred.

Researcher notes

The core issue is an origin-trust failure: unsanitized notebook HTML is served under the privileged Jupyter origin without CSP sandboxing. CWE-79 and CWE-1021 are assigned. The documented impact chain reaches full /api/* authority and kernel code execution. The supplied evidence identifies the fixed version and commit, but does not document public exploitation or a proof of concept.

Mitigation direction

  • Upgrade jupyter_server to version 2.20 or later, following the upstream advisory and applicable distributor errata.
  • Prioritize shared Jupyter systems where users can upload notebooks or induce others to render them.
  • Until upgraded, limit ingestion and viewing of notebooks from untrusted or lower-trust users.
  • Apply relevant Red Hat errata when using affected Red Hat-distributed packages.

Validation and detection

  • Inventory every Jupyter Server deployment and confirm the installed jupyter_server version is 2.20 or later.
  • Verify rendered nbconvert HTML responses enforce the sandbox Content-Security-Policy behavior introduced by the fix.
  • Confirm package-level remediation against the applicable upstream advisory or distributor erratum.
  • Review notebook sharing, upload, and nbconvert access records for suspicious activity; clean logs cannot exclude prior compromise.
Prepared
Confidence
high
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-1021: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · medium confidence lookup

CWE-79: User-session and phishing behavior lookup

Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
description · low confidence lookup

Execution behavior lookup

The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2026-44727 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
Critical
CVSS
9.3 (4.0)
Known Exploited
No
Published

Vector: 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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
5Timeline events
2ADP providers
8Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: total

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
9.3CVSS 4.0CriticalCVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:A/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H——GitHub_M
9CVSS 3.1CriticalCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:H2.36redhat-SADP

Vulnerability scoring details

Base CVSS 4.0 score

9.3Critical
CVSS 4.0 vector shape for CVE-2026-44727Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: 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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. ADP timelineredhat-SADP

    Made public.

  3. CVE publishedCVE Program

    The CVE record was published.

  4. ADP timelineredhat-SADP

    Reported to Red Hat.

  5. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPjupyter-server: Jupyter Server: Remote Code Execution via stored Cross-Site Scripting in nbconvert handlers
other:Red Hat severity ratingcvssV3_1
  • 2026-06-22T21:00:50.127Z: Reported to Red Hat.
  • 2026-06-22T19:56:56.776Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
jupyter-serverjupyter_server< 2.20Listed
Weakness

CWE details

CWE-1021CWE-79

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.