CVE-2026-81920 in Concrete CMS
Summary
by MITRE • 09/15/2026
Concrete CMS below 9.5.3 was vulnerable to Cross-Site Request Forgery in the dashboard SEO Excluded Words page. The reset() controller action cleared the administrator-configured reserved-word list (concrete.seo.exclude_words) but did not validate the anti-CSRF token that the reset modal emitted, and it did not restrict the request to the POST method. A remote attacker who lured an authenticated user with SEO access to a crafted page could revert the reserved-word list to its default and silently alter future URL-slug generation for pages, files, topics, and other objects created through the Text urlify service, undoing the site's configured SEO slug policy. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 2.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N. Thanks riodrwn for reporting.
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Analysis
by VulDB Data Team • 09/15/2026
Concrete CMS versions prior to 9.5.3 contain a Cross-Site Request Forgery vulnerability within the dashboard SEO Excluded Words management interface, specifically affecting the reset functionality that clears administrator-configured reserved words from the concrete.seo.exclude_words configuration array. The core technical flaw lies in the implementation of the reset controller action, which fails to validate the anti-CSRF token generated by the frontend modal dialog and does not enforce strict HTTP method restrictions on incoming requests. This oversight allows an attacker to craft a malicious web page or email link that, when accessed by an authenticated administrator with SEO privileges, triggers the reset operation without requiring valid session-based CSRF protection. The vulnerability exploits the lack of state-changing request validation, enabling unauthorized modification of application configuration through simple browser-side interactions rather than complex exploitation techniques.
The operational impact of this flaw is significant for organizations relying on Concrete CMS for structured content management and search engine optimization strategies. By successfully exploiting this vulnerability, a remote attacker can silently revert the reserved-word list to its default state, effectively removing any custom exclusions that were previously configured by site administrators. This action directly undermines the integrity of the Text urlify service, which is responsible for generating URL slugs for pages, files, topics, and other content objects within the CMS environment. Consequently, future URLs generated after the reset will no longer adhere to the organization's specific SEO slug policies, potentially leading to broken links, loss of search engine rankings, or exposure of sensitive identifiers that were previously excluded from public-facing URLs due to their inclusion in the reserved word list.
From a threat modeling perspective, this vulnerability is classified under CWE-352: Cross-Site Request Forgery (CSRF), as it involves forcing an authenticated user to execute unwanted actions on a web application without their consent. The attack vector aligns with ATT&CK technique T1078: Valid Accounts, specifically leveraging existing administrative privileges through social engineering or lure-based delivery mechanisms such as phishing emails or malicious websites designed to trick the administrator into visiting a crafted URL. Although the CVSS v4.0 score assigned by the Concrete CMS security team is relatively low at 2.3 due to the requirement for user interaction and the absence of direct confidentiality, integrity, or availability impacts on underlying data storage, the indirect impact on SEO performance and content structure represents a meaningful business risk that warrants immediate attention.
Mitigation strategies should focus primarily on upgrading to Concrete CMS version 9.5.3 or later, where these security controls have been addressed by implementing proper CSRF token validation for all state-changing operations and enforcing strict HTTP method restrictions such as POST-only handling for configuration resets. In the interim, administrators can mitigate risk by ensuring that administrative interfaces are not accessible via untrusted networks, utilizing browser extensions that block third-party cookies if applicable, and monitoring audit logs for unexpected changes to SEO configurations or reserved word lists. Additionally, implementing SameSite cookie attributes on session tokens can provide an additional layer of defense against cross-origin request forgery attempts, reducing the likelihood of successful exploitation even in environments where immediate patching is not feasible.