EUVD-2026-80879Severity by source
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/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
Unauthenticated remote storage (PR:N, not vendor's PR:L); victim must open the file (UI:R); XSS executes in a different security context (S:C); admin-session takeover yields C:H/I:H, no availability impact (A:N).
Primary rating from Vendor (ConcreteCMS).
CVSS VectorVendor: ConcreteCMS
Lifecycle Timeline
2DescriptionCVE.org
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.
AnalysisAI
Stored cross-site scripting in Concrete CMS before 9.5.4 allows an attacker to upload a plain XML document through a publicly reachable Form Block file-upload question; because uploads were validated by extension only and stored as inline-served, same-origin files, the XML can carry an xml-stylesheet processing instruction pointing at an attacker-controlled XSLT stylesheet. When a victim opens the stored file's URL directly in a browser, the stylesheet transforms the document into HTML and runs attacker JavaScript in the Concrete CMS origin - and if that victim is an authenticated administrator, the script acts with the admin session (the reporter demonstrated creating a new user in the Administrators group). …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Requires a Concrete CMS instance older than 9.5.4 that exposes a publicly reachable Form Block containing a file-upload question, since XML uploads were validated by file extension only and stored as inline-served, same-origin, publicly accessible files. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | This is a genuine but conditional stored-XSS priority rather than an internet-wide emergency. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | Vendor-released patch: upgrade to Concrete CMS 9.5.4, the version that resolves this issue, per the release notes at https://documentation.concretecms.org/developers/introduction/version-history/954-release-notes. … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours, inventory every Concrete CMS instance and identify which ones are below version 9.5.4 and expose a publicly reachable Form Block with a file-upload question; treat those as the priority tier and confirm whether any existing uploads are plain XML documents. …
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External POC / Exploit Code
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