CVE-2026-18119 in Concrete CMS
Summary
by MITRE • 09/14/2026
Concrete CMS below 9.5.3 did not sanitize custom style values in the Block Design dialog before writing them into page CSS via a DOM sink, permitting stored cross-site scripting. An editor-level user could execute script in an administrator's session and escalate privileges. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 7.0 with vector CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:P/VC:L/VI:H/VA:H/SC:N/SI:N/SA:N. Thanks Nguyen Manh Thuan for reporting.
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Analysis
by VulDB Data Team • 09/14/2026
The vulnerability identified in Concrete CMS versions prior to 9.5.3 represents a significant security flaw rooted in insufficient input validation and sanitization within the Block Design dialog interface. This specific component allows users to define custom style values that are subsequently written directly into page CSS files via a Document Object Model sink. The core technical failure lies in the application's inability to properly filter or encode user-supplied data before it is persisted, thereby creating an avenue for stored cross-site scripting attacks. By failing to sanitize these inputs, the system inadvertently allows malicious scripts to be embedded within legitimate-looking style definitions, which are then executed when other users view the affected page content.
From a technical perspective, this flaw aligns with CWE-79, commonly known as Improper Neutralization of Input During Web Page Generation or Cross-site Scripting. The attack vector is particularly dangerous because it involves stored persistence rather than reflected execution. An attacker does not need to trick an administrator into clicking a malicious link in real-time; instead, the payload remains on the server side within the CSS data structure. When any user with sufficient privileges accesses the page containing the compromised block, their browser interprets and executes the embedded script as part of the legitimate page rendering process. This mechanism bypasses many client-side security controls that might otherwise mitigate reflected XSS attempts.
The operational impact of this vulnerability is severe due to its potential for privilege escalation. Although the initial exploitation requires editor-level access or higher privileges to create such blocks, the resulting execution context allows scripts to run with the permissions of any administrator who views the page. This capability enables an attacker to hijack administrative sessions, potentially leading to full system compromise, data exfiltration, defacement, or further lateral movement within the network infrastructure. The ability to execute arbitrary JavaScript in an admin's session effectively neutralizes many traditional boundary defenses, as the malicious code operates from a trusted domain context that browsers typically allow unrestricted access to sensitive cookies and local storage.
In terms of threat modeling, this vulnerability maps closely to MITRE ATT&CK technique T1059.007, which covers JavaScript execution within web applications. The attack path involves initial access through valid credentials at the editor level, followed by persistence via stored script injection, and finally privilege escalation by exploiting the trust relationship between the browser and the CMS domain. This sequence highlights a critical weakness in how content management systems handle user-generated styling configurations, emphasizing that even non-executable data formats like CSS can serve as vectors for code execution if sinks are not properly secured against malicious payloads such as event handlers or expression functions.
To mitigate this risk, organizations running Concrete CMS must immediately upgrade to version 9.5.3 or later, where the security team has addressed these sanitization gaps. In addition to patching, administrators should enforce strict role-based access controls to limit who can create and edit blocks with custom styling options. Implementing a Content Security Policy header that restricts inline script execution can also provide an additional layer of defense by preventing the browser from running unauthorized JavaScript even if it is injected into the DOM. Regular security audits focusing on input validation across all user-facing forms are essential to prevent similar vulnerabilities in other components of the application stack.
The responsible disclosure and subsequent patching effort led by Nguyen Manh Thuan underscores the importance of community-driven vulnerability reporting in maintaining web platform integrity. The assigned CVSS v4.0 score of 7.0 reflects a high severity rating, driven primarily by the impact on confidentiality and integrity due to the potential for session hijacking and privilege escalation. While availability is not directly impacted, the loss of trust in administrative functions constitutes a critical breach of security principles. Maintaining up-to-date software versions remains the most effective strategy against such exploitation vectors, as developers continuously refine input handling mechanisms based on emerging attack patterns and industry best practices.