CVE-2025-66545 is a vulnerability identified in the Nextcloud Groupfolders application, which manages admin-configured shared folders for groups or teams. The flaw arises from improper neutralization of inputs (CWE-707), allowing users with read-only permissions to restore files from the trash bin, an action that should be restricted to users with higher privileges. This vulnerability affects multiple Nextcloud versions prior to 14.0.11, 15.3.12, 16.0.15, 17.0.14, 18.1.8, 19.1.8, and 20.1.2. The issue is rooted in the access control logic within the Groupfolders app, where the permission checks do not adequately prevent read-only users from performing restore operations. The CVSS 3.1 base score is 3.5, reflecting a low severity due to the limited impact on confidentiality and availability, and the requirement for authenticated user interaction. Exploiting this vulnerability could allow an attacker with read-only access to restore deleted files, potentially leading to unauthorized data integrity changes within shared folders. No known exploits have been reported in the wild, and the vendor has released patches in the specified versions to address the issue. Organizations using affected versions should prioritize updating to mitigate the risk.

For European organizations, the impact of CVE-2025-66545 is primarily related to data integrity within collaborative environments using Nextcloud Groupfolders. Unauthorized restoration of deleted files by users with only read-only permissions could lead to confusion, potential reintroduction of outdated or unwanted files, and disruption of document management workflows. While this does not directly compromise confidentiality or availability, it could affect compliance with data governance policies and audit trails, especially in regulated sectors such as finance, healthcare, and government. The risk is heightened in organizations with large teams relying on Nextcloud for file sharing and collaboration, where improper restoration could undermine data accuracy and trust. However, the low CVSS score and absence of known exploits suggest the threat is limited in scope and complexity. Nonetheless, failure to address this vulnerability could expose organizations to insider threats or accidental misuse by legitimate users.

To mitigate CVE-2025-66545, European organizations should: 1) Immediately upgrade Nextcloud Groupfolders to the patched versions (14.0.11, 15.3.12, 16.0.15, 17.0.14, 18.1.8, 19.1.8, or 20.1.2) as applicable. 2) Review and tighten access control policies within Nextcloud, ensuring that permission assignments are strictly enforced and regularly audited. 3) Implement monitoring and alerting on file restoration activities within shared folders to detect unusual or unauthorized actions. 4) Educate users about proper file management practices and the importance of adhering to permission boundaries. 5) Consider deploying additional logging and forensic capabilities to track changes in shared folders for compliance and incident response. 6) If immediate patching is not feasible, temporarily restrict read-only users from accessing trash bin functionalities through configuration or custom access controls. These steps go beyond generic patching by emphasizing operational controls and user awareness to reduce risk.