CVE-2025-66294 is a critical Server-Side Template Injection (SSTI) vulnerability affecting Grav, a popular file-based web content management system. The vulnerability exists in versions prior to 1.8.0-beta.27 and arises from insufficient sanitization in the cleanDangerousTwig method, which uses weak regular expressions to filter dangerous Twig template syntax. This flaw enables attackers with editor-level authentication to inject and execute arbitrary code on the server, potentially leading to full system compromise. Under specific conditions, unauthenticated attackers may also exploit this vulnerability, increasing its threat surface. SSTI vulnerabilities allow malicious template code execution, which can bypass traditional input validation and lead to remote code execution (RCE). The CVSS 4.0 vector indicates network attack vector (AV:N), low attack complexity (AC:L), no user interaction (UI:N), and privileges required (PR:L), with high impact on confidentiality, integrity, and availability (VC:H/VI:H/VA:H). Although no exploits are publicly known yet, the severity and ease of exploitation make this a critical risk for Grav users. The vulnerability was publicly disclosed on December 1, 2025, and fixed in version 1.8.0-beta.27. Grav is widely used in Europe for lightweight CMS deployments, making this vulnerability relevant to many organizations. The weakness in regex validation highlights the importance of robust input sanitization in template engines. Attackers exploiting this flaw can execute arbitrary commands, access sensitive data, modify content, or disrupt services, severely impacting affected organizations.

For European organizations, this vulnerability poses a significant risk, especially for those relying on Grav CMS for public-facing websites, intranets, or e-commerce platforms. Successful exploitation can lead to unauthorized code execution, resulting in data breaches, defacement, service disruption, or full server takeover. This compromises the confidentiality, integrity, and availability of organizational data and services. Given the high CVSS score and potential for unauthenticated exploitation under certain conditions, attackers could leverage this vulnerability to pivot within networks or launch further attacks. Organizations in sectors such as government, finance, healthcare, and media, which often use CMS platforms and handle sensitive data, are particularly vulnerable. The file-based nature of Grav means that compromised servers could also be used to distribute malware or conduct phishing campaigns. Additionally, the lack of known exploits currently suggests a window of opportunity for defenders to patch and harden systems before widespread attacks occur. Failure to address this vulnerability promptly could lead to regulatory compliance issues under GDPR due to potential data breaches.

1. Upgrade Grav CMS installations immediately to version 1.8.0-beta.27 or later where the vulnerability is patched. 2. Restrict editor permissions strictly to trusted users and review user roles regularly to minimize the risk of insider exploitation. 3. Implement Web Application Firewalls (WAFs) with rules specifically designed to detect and block SSTI payloads and suspicious template syntax. 4. Conduct regular security audits and code reviews focusing on template usage and input sanitization mechanisms. 5. Monitor server logs and application behavior for unusual template rendering activities or command execution attempts. 6. Employ network segmentation to isolate CMS servers from critical backend systems to limit lateral movement in case of compromise. 7. Educate developers and administrators about secure template handling and the risks of SSTI vulnerabilities. 8. Consider deploying runtime application self-protection (RASP) solutions that can detect and prevent code injection at runtime. 9. Backup Grav CMS data and configurations regularly to enable rapid recovery if exploitation occurs. 10. Stay informed about updates from the Grav project and security advisories for any further patches or mitigations.