CVE-2025-66296 is a vulnerability identified in the Grav content management system (CMS), specifically within its Admin plugin versions before 1.8.0-beta.27. Grav is a file-based web platform used for building websites without a database backend. The vulnerability stems from improper privilege assignment due to the absence of username uniqueness validation during user creation. A user who possesses the 'create user' permission can exploit this flaw by creating a new user account with the exact username as an existing administrator. By doing so, the attacker can set a new password and email address for this duplicated username and subsequently log in as the administrator, effectively escalating their privileges from a limited user-manager role to full administrator access. This bypasses intended access controls and compromises the system's security. The vulnerability is remotely exploitable over the network without requiring user interaction, and no authentication beyond the create user permission is needed. The CVSS v3.1 base score is 8.8, reflecting high severity with impacts on confidentiality, integrity, and availability. Although no public exploits have been reported, the flaw presents a critical risk to Grav installations that have not applied the fix introduced in version 1.8.0-beta.27. The root cause is classified under CWE-266 (Incorrect Privilege Assignment), highlighting a fundamental access control weakness. The vulnerability allows attackers to fully compromise affected systems, potentially leading to data breaches, website defacement, or further internal network compromise.

For European organizations, this vulnerability poses a significant threat to the security of websites and web applications built on Grav CMS. Successful exploitation results in full administrator control, enabling attackers to modify content, steal sensitive data, inject malicious code, or disrupt services. This can lead to reputational damage, regulatory non-compliance (e.g., GDPR violations due to data exposure), and financial losses. The file-based nature of Grav means that compromised administrator accounts can manipulate site files directly, increasing the risk of persistent backdoors or malware implants. Organizations relying on Grav for public-facing or internal portals are particularly vulnerable. The ease of exploitation and high impact on confidentiality, integrity, and availability make this a critical concern. Additionally, the lack of known exploits in the wild suggests that timely patching can prevent widespread attacks, but delayed response could lead to targeted or opportunistic exploitation. European entities with limited security resources or delayed patch management processes are at heightened risk.

1. Upgrade all Grav CMS installations to version 1.8.0-beta.27 or later immediately to apply the official fix that enforces username uniqueness validation. 2. Restrict the 'create user' permission to only highly trusted administrators to minimize the attack surface. 3. Implement monitoring and alerting on user account creation events, especially for duplicate or suspicious usernames. 4. Conduct regular audits of user accounts to detect and remove any unauthorized duplicates or suspicious accounts. 5. Employ web application firewalls (WAFs) with custom rules to detect and block attempts to create duplicate usernames if patching is delayed. 6. Educate administrators and users about the risks of privilege escalation and enforce strong password policies. 7. Consider isolating Grav admin interfaces behind VPNs or IP whitelisting to reduce exposure. 8. Maintain comprehensive backups of Grav site files and configurations to enable rapid recovery in case of compromise. 9. Review and tighten access controls and permissions within Grav and the hosting environment to limit lateral movement post-exploitation.