CVE-2025-66305 is a vulnerability in the Grav CMS platform, specifically affecting versions prior to 1.8.0-beta.27. Grav is a file-based web platform used for building websites without a traditional database backend. The vulnerability exists in the 'Languages' submenu of the Grav admin configuration panel (/admin/config/system), where the 'Supported' parameter fails to properly validate user input. When an attacker inputs a malformed value—such as a single forward slash (/) or an XSS test string—this input is passed to the preg_match() PHP function with an improperly constructed regular expression. This causes a fatal regular expression parsing error, which is an uncaught exception (CWE-248). The result is a denial of service, as the entire Grav application becomes unavailable to all users due to the crash. The vulnerability requires administrative privileges to access the configuration panel, but no user interaction beyond that is needed. The CVSS 4.0 vector indicates network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:H - high privileges required), no user interaction (UI:N), and high impact on availability (VA:H). The vulnerability does not affect confidentiality or integrity directly. No known exploits have been reported in the wild, and the issue is resolved in Grav version 1.8.0-beta.27.

For European organizations, the primary impact of CVE-2025-66305 is the potential for denial of service on websites or web applications built using vulnerable versions of Grav CMS. This can lead to complete unavailability of affected sites, disrupting business operations, customer access, and potentially damaging reputation. Organizations relying on Grav for public-facing or internal web services may experience downtime until the vulnerability is remediated. Since the vulnerability requires administrative access to trigger, the risk is elevated if internal or external threat actors have compromised admin credentials or access. The disruption could affect sectors such as e-commerce, government portals, educational institutions, and SMEs that use Grav for content management. Given Grav’s popularity in Europe among small to medium enterprises and developers valuing lightweight CMS solutions, the impact could be widespread if unpatched. However, the lack of known exploits in the wild reduces immediate risk, though proactive patching is critical to prevent future exploitation.

European organizations should immediately upgrade Grav installations to version 1.8.0-beta.27 or later, where the vulnerability is fixed. If upgrading is not immediately feasible, restrict access to the Grav admin configuration panel to trusted administrators only, using network segmentation, VPNs, or IP whitelisting. Implement strong authentication and monitor admin access logs for suspicious activity. Validate and sanitize all user inputs in custom plugins or extensions interacting with the 'Languages' submenu to prevent malformed data reaching the preg_match() function. Employ web application firewalls (WAFs) with rules to detect and block suspicious payloads targeting the admin panel. Regularly audit Grav CMS versions across the organization’s web infrastructure and maintain an inventory to ensure timely patch management. Additionally, conduct penetration testing focusing on admin panel input validation to identify similar weaknesses.