CVE-2026-6658Public exploitNetwork attack vector

Cross-site scripting via unescaped Mermaid output

Published Jun 26, 2026 · Updated Jun 26, 2026

Cross-site scripting in Project Jupyter nbconvert 7.17.0 and earlier allows attackers to execute JavaScript through crafted Mermaid output. The data_mermaid block in share/templates/lab/base.html.j2 places text/vnd.mermaid content inside a pre element without escaping, so a closing tag can inject HTML and script. A victim must view an HTML export produced from attacker-controlled notebook content; execution is confined to that document's browser context.

CVSS severity5.4
Medium
EPSS probability0.23%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site scripting in Project Jupyter nbconvert 7.17.0 and earlier allows attackers to execute JavaScript through crafted Mermaid output. The data_mermaid block in share/templates/lab/base.html.j2 places text/vnd.mermaid content inside a pre element without escaping, so a closing tag can inject HTML and script. A victim must view an HTML export produced from attacker-controlled notebook content; execution is confined to that document's browser context.

The record

CVE
CVE-2026-6658
Published
Jun 26, 2026
Updated
Jun 26, 2026
Vendor
Project Jupyter
Product
nbconvert
Classifications
CWE-79, T1203
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jun 26, 2026CVE publishedPublication date reported by the CVE source.
  2. Jun 26, 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=unspecified <=latest

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Project Jupyter · nbconvertversion=unspecified <=latest

Published CVSS scores

5.4huntr.devCVSS:3.0/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

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.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1203Exploitation for Client Execution

Public exploit references

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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