Description
Concrete CMS before 9.5.4 did not sanitize XML and XSLT documents uploaded through a public Form Block file-upload question. Plain XML uploads were validated by file extension only and stored as publicly accessible files that were served inline from the application's own origin. An unauthenticated visitor could therefore store an XML document containing an xml-stylesheet processing instruction that referenced an attacker-supplied, same-origin XSLT stylesheet. When a victim opened the stored file directly in a browser, the browser fetched the stylesheet, transformed the document into HTML, and executed attacker-controlled JavaScript in the Concrete CMS origin (stored cross-site scripting). If the victim was an authenticated administrator, the script could act with that administrator's session, and the reporter demonstrated creation of a new user in the Administrators group. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 7.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N. Thanks Valentin SARRE (Independent security researcher) for reporting.
Published: 2026-09-16
Score: 7.3 High
EPSS: n/a
KEV: No
Impact: n/a
Action: n/a
AI Analysis

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Remediation

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History

Wed, 16 Sep 2026 16:45:00 +0000

Type Values Removed Values Added
Description Concrete CMS before 9.5.4 did not sanitize XML and XSLT documents uploaded through a public Form Block file-upload question. Plain XML uploads were validated by file extension only and stored as publicly accessible files that were served inline from the application's own origin. An unauthenticated visitor could therefore store an XML document containing an xml-stylesheet processing instruction that referenced an attacker-supplied, same-origin XSLT stylesheet. When a victim opened the stored file directly in a browser, the browser fetched the stylesheet, transformed the document into HTML, and executed attacker-controlled JavaScript in the Concrete CMS origin (stored cross-site scripting). If the victim was an authenticated administrator, the script could act with that administrator's session, and the reporter demonstrated creation of a new user in the Administrators group. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 7.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N. Thanks Valentin SARRE (Independent security researcher) for reporting.
Title Concrete CMS before 9.5.4 stored is vulneratble to cross-site scripting via unauthenticated XML/XSLT file upload in the Form Block
Weaknesses CWE-79
References
Metrics cvssV4_0

{'score': 7.3, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N'}

Subscriptions

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cve-icon MITRE

Status: PUBLISHED

Assigner: ConcreteCMS

Published:

Updated: 2026-09-16T16:33:53.412Z

Reserved: 2026-09-03T18:09:31.837Z

Link: CVE-2026-85386

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Awaiting Analysis

Published: 2026-09-16T17:18:15.710

Modified: 2026-09-16T19:16:15.097

Link: CVE-2026-85386

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

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Weaknesses
  • CWE-79

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