CVE-2026-18111 in Concrete CMS
Summary
by MITRE • 09/15/2026
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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Analysis
by VulDB Data Team • 09/15/2026
Concrete CMS versions prior to 9.5.3 and version 8.5.21 contain a critical stored cross-site scripting vulnerability affecting multiple content block types including Feature, Feature Link, Hero Image, and Image blocks. The root cause of this flaw lies in the insufficient validation performed by the link filter when processing external URLs provided during page editing operations. When an administrator or user with specific permissions inputs data into these fields, the system fails to adequately sanitize the input before storing it in the database. This lack of rigorous sanitization allows malicious actors to inject script payloads directly into the stored content rather than merely displaying them transiently as seen in reflected XSS attacks.
The technical mechanism of exploitation involves crafting a specially formatted external link value that breaks out of the expected HTML anchor tag structure. Because the rendered output does not apply proper escaping, the injected JavaScript code is executed within the context of any user who subsequently views, previews, or edits the affected page. This execution occurs in the victim's browser session, granting the attacker control over the client-side environment. The vulnerability specifically targets users with page-editing permissions such as Add Block combined with Edit Contents on a single page, indicating that even lower-privileged roles can trigger this code execution if they have write access to specific content areas.
The operational impact of this vulnerability is severe due to its stored nature and the high privileges often associated with CMS administrators who frequently view edited pages. Successful exploitation leads to session hijacking where an attacker can steal authentication cookies or tokens, effectively impersonating legitimate users. This capability facilitates privilege escalation up to full administrative takeover, allowing complete control over the website content and potentially leading further into server-side compromise if combined with other vulnerabilities. The severity is underscored by a CVSS v4.0 score of 8.5 reflecting high confidentiality, integrity, and availability impacts within the affected scope while maintaining low attack complexity and no required user interaction beyond viewing the page.
This vulnerability aligns with CWE-79 Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting specifically under the stored variant where malicious scripts are permanently stored in target databases or content management systems. From a threat intelligence perspective, this behavior corresponds to ATT&CK technique T1059 Command and Control using JavaScript for initial access and potential lateral movement within web application environments. The persistence of the payload means that every page load serves as an attack vector without requiring additional social engineering efforts beyond the initial injection by a privileged user.
Mitigation strategies require immediate upgrading to Concrete CMS version 9.5.3 or later if running the newer branch, or version 8.5.21 for legacy systems. These updates include enhanced input validation and output escaping mechanisms that properly neutralize script tags within link attributes before rendering them in HTML contexts. Administrators should also implement strict Content Security Policy headers to mitigate potential impacts from any remaining client-side execution risks. Regular security audits of custom blocks or third-party extensions are recommended as similar flaws may exist outside the core feature set if they follow analogous patterns for handling external URLs without adequate sanitization protocols.