CVE-2026-44727

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Jupyter Server is the backend for Jupyter web applications. Prior to 2.20, the nbconvert HTTP handlers in jupyter_server

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.

Monitor

⚠ NVD has not scored this CVE yet - manual triage required (common for recent CVEs)

No Sigma yet — build one → YARA rules0

Look this up elsewhere - one-click external pivots

NVD CVE.org CISA Exploit-DB GitHub PoC VulnCheck KEV GreyNoise

▸ How to read a CVE - triage first, then detect and patch

This page is every public fact about CVE-2026-44727, cross-linked. Its job is to answer one question fast - does this need my attention now? - and then hand you the two things you do about it. Here is how an analyst reads it.

Triage: should I act now? Four signals, and they are not interchangeable:

CVSSseverity - how bad it is IF exploited, 0-10. A high CVSS alone is not urgency; a flaw can be a perfect 10 and never actually be attacked. EPSSprobability - a model’s estimate of the chance it is exploited in the next 30 days, 0-1. This is the “will it actually happen” signal. CISA KEVconfirmed - it is being exploited in the wild right now. The strongest signal on the page; KEV beats any score. Weaponisedavailability - public exploits / PoCs, and especially Metasploit modules rated Excellent / Great. Reliable, packaged exploit code means low-skill attackers can use it today.

How they combine: KEV, or a dependable Metasploit module, means patch now regardless of CVSS. High CVSS + low EPSS + no exploit is real but not an emergency - schedule it. Low CVSS but KEV-listed still gets patched now. The verdict above already weighed these for you; this is how it got there.

Then what - two workflows:

Detectwhen you cannot patch today, follow this CVE to the ATT&CK techniques it enables, then Build a SIEM detection (the green button) - author a rule, test it in Atomic, deploy it. That buys visibility while the patch waits. PatchAffected products / packages tell you if you are exposed; Fixed versions by distribution and Vendor advisories give the exact version that closes it.

Reading order for the panels below: verdict + badges, then Public exploits / Metasploit (is it weaponised), then ATT&CK techniques + Sigma / IDS rules (can I detect it), then Affected products / packages + Fixed versions (am I exposed, what patches it), then Threat actors / IOCs (who uses it), then Scoring & timeline / references (the evidence).

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ATT&CK techniques

Techniques this CVE enables. Pills with a solid outline are high confidence - named directly in ATT&CK or Nuclei, or human-curated by CTID; the rest are inferred from the weakness type using MITRE's CVE Mapping Methodology and the CWE → CAPEC chain. Broad, generic-weakness guesses are filtered out. A small N× marks a technique that N independent sources agree on.

T1036.004 · Masquerade Task or Service T1056 · Input Capture T1059.007 · JavaScript T1548.004 · Elevated Execution with Prompt

▤ Build a SIEM detection for these techniques

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CAPEC attack patterns

Attack patterns this CVE enables - the bridge from weakness to ATT&CK technique.

CAPEC-CAPEC-103 · Clickjacking CAPEC-CAPEC-181 · Flash File Overlay CAPEC-CAPEC-209 · XSS Using MIME Type Mismatch CAPEC-CAPEC-222 · iFrame Overlay CAPEC-CAPEC-504 · Task Impersonation CAPEC-CAPEC-506 · Tapjacking CAPEC-CAPEC-587 · Cross Frame Scripting (XFS) CAPEC-CAPEC-588 · DOM-Based XSS CAPEC-CAPEC-591 · Reflected XSS CAPEC-CAPEC-592 · Stored XSS CAPEC-CAPEC-63 · Cross-Site Scripting (XSS) CAPEC-CAPEC-654 · Credential Prompt Impersonation

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Weakness Classification

CWE-1021Improper Restriction of Rendered UI Layers or Frames

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

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Affected Packages

Language-ecosystem packages (from OSV) tied to this CVE, with the version that fixes it - the dependency-level detail NVD doesn’t carry.

PyPI jupyter-server CRITICAL fixed in 2.20.0

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Related CVEs

CVEs linked to this one by a shared weakness (CWE) or affected product - joins on data already in the engine, with the reason shown per row. Not a guess.

CVE-2005-2407 A design error in Opera 8.01 and earlier allows user-assisted attackers to execu... same CWE-1021 CVE-2008-2716 Unspecified vulnerability in Opera before 9.5 allows remote attackers to spoof t... same CWE-1021 CVE-2011-1244 Microsoft Internet Explorer 6, 7, and 8 does not enforce intended domain restric... same CWE-1021 CVE-2013-5614 Mozilla Firefox before 26.0 and SeaMonkey before 2.23 do not properly consider t... same CWE-1021 CVE-2000-1205 Cross site scripting vulnerabilities in Apache 1.3.0 through 1.3.11 allow remote... same CWE-79 CVE-2002-0270 Opera, when configured with the "Determine action by MIME type" option disabled,... same CWE-79 CVE-2002-1651 Cross-site scripting (XSS) vulnerability in Verity Search97 allows remote attack... same CWE-79 CVE-2002-1700 Cross-site scripting vulnerability (XSS) in the missing template handler in Macr... 1 same CWE-79

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References & Sources

Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.

https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd

https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp