CVE-2026-18111
Received Received - Intake

Stored XSS in Concrete CMS Blocks

Publication date: 2026-09-15

Last updated on: 2026-09-15

Assigner: ConcreteCMS

Description
Concrete CMS 9 before 9.5.3 was vulnerable to stored cross-site scripting (XSS) in the Feature, Feature Link, Hero Image, and Image blocks and before Concrete 8.5.21 in the feature and Image blocks because the external link URL was insufficiently validated by the link filter and was rendered without output escaping. A user with page-editing permissions (such as Add Block combined with Edit Contents on a single page) could store a crafted external link value that broke out of the link markup and injected arbitrary JavaScript. The script executed in the browser session of any user who subsequently viewed, previewed, or edited the affected page, which could lead to session hijacking and escalation of privileges up to full administrative takeover. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 8.5 with vector CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N. Thanks to KhanMarshai for reporting this issue.

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Meta Information

Published
2026-09-15
Last Modified
2026-09-15
Generated
2026-09-15
AI Q&A
2026-09-15
EPSS Evaluated
N/A
NVD
CVE-2026-18111
EUVD
EUVD-2026-78839

Affected Vendors & Products

Showing 2 associated CPEs
Vendor Product Version / Range
concrete_cms concrete_cms to 9.5.3 (exc)
concrete_cms concrete to 8.5.21 (exc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-306 The product does not perform any authentication for functionality that requires a provable user identity or consumes a significant amount of resources.

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Executive Summary

Concrete CMS versions before 9.5.3 (and 8.5.21 for older versions) had a stored cross-site scripting (XSS) vulnerability in specific blocks like Feature, Feature Link, Hero Image, and Image. The issue occurred because external link URLs were not properly validated, allowing malicious JavaScript to be injected and stored. When users viewed or edited these pages, the script would execute in their browser, potentially leading to session hijacking or full administrative access.

Impact Analysis

If you are a user with page-editing permissions, an attacker could exploit this to inject malicious scripts into pages you edit. For regular users, visiting an affected page could lead to session hijacking, where an attacker takes control of your account. In severe cases, this could escalate to full administrative takeover of the Concrete CMS instance.

Compliance Impact

This vulnerability could lead to unauthorized access to user data, violating GDPR's data protection requirements and HIPAA's safeguards for protected health information. A breach could result in legal penalties, reputational damage, and loss of trust due to compromised user sessions and potential data exposure.

Mitigation Strategies

Update Concrete CMS to version 9.5.3 or later for Concrete 9, or to version 8.5.21 or later for Concrete 8. This addresses the stored XSS vulnerability in the Feature, Feature Link, Hero Image, and Image blocks by properly validating external link URLs and escaping output.

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