CVE-2025-66303 is a Denial of Service vulnerability classified under CWE-400 (Uncontrolled Resource Consumption) affecting the Grav CMS, a file-based web platform. The flaw exists in versions prior to 1.8.0-beta.27 and is related to the handling of the scheduled_at parameter, which accepts cron expressions for scheduling tasks. The application fails to properly sanitize this input, allowing an attacker with administrative privileges to inject malformed cron expressions, such as including a single quote character. This injection corrupts the backup.yaml configuration file, which is critical for the admin panel’s scheduled tasks. As a result, the admin panel becomes non-functional, effectively denying administrative access and control over the platform. The only remediation without patching is manual access to the host server to correct the corrupted cron expression in backup.yaml. The vulnerability requires authenticated access with high privileges but no user interaction beyond the attacker’s own actions. The CVSS 3.1 score is 4.9 (medium), reflecting the limited scope and required privileges but significant impact on availability. No known exploits are currently in the wild, and the issue was publicly disclosed on December 1, 2025. The fix is included starting with Grav version 1.8.0-beta.27.

For European organizations using Grav CMS versions prior to 1.8.0-beta.27, this vulnerability poses a risk of administrative disruption. The inability to access or use the admin panel can delay or prevent critical content updates, security patching, and site management, potentially leading to extended downtime or degraded service quality. While the vulnerability does not expose sensitive data or allow unauthorized data modification, the loss of administrative availability can indirectly increase risk by delaying response to other threats. Organizations with strict uptime requirements or those managing multiple sites via Grav’s admin panel may experience operational and reputational impacts. Recovery requires manual server access, which may be challenging for organizations with limited technical staff or those relying on managed hosting. The vulnerability’s exploitation requires administrative credentials, so the risk is higher in environments with weak access controls or insider threats.

European organizations should immediately upgrade Grav CMS to version 1.8.0-beta.27 or later to apply the official fix. Until upgrading, restrict administrative access to trusted personnel and enforce strong authentication mechanisms to reduce the risk of exploitation. Regularly audit scheduled tasks and the backup.yaml file for signs of corruption or unauthorized changes. Implement monitoring and alerting on admin panel availability and configuration file integrity to detect potential exploitation attempts early. Maintain secure backups of configuration files to enable rapid recovery without manual correction. Consider isolating the Grav admin panel behind VPNs or IP whitelisting to limit exposure. Educate administrators on the risks of malformed cron expressions and the importance of input validation. Finally, coordinate with hosting providers to ensure rapid manual intervention capability if needed.