CVE-2025-66545 is a vulnerability identified in the Nextcloud Groupfolders app, which manages shared folders configured by administrators for groups or teams. The flaw exists in 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. It allows users who have only read-only permissions to restore files from the trash bin, an action that should be restricted to users with higher privileges. This improper neutralization of inputs (classified under CWE-707) leads to an integrity violation because unauthorized users can revert deletions, potentially reintroducing unwanted or malicious files into shared folders. The vulnerability requires user interaction and low privileges but does not affect confidentiality or availability. The CVSS 3.1 score is 3.5 (low severity), reflecting the limited impact and exploitation complexity. No known exploits have been reported in the wild, indicating it is not actively weaponized yet. The issue was publicly disclosed on December 5, 2025, and fixed in the specified versions. Nextcloud is widely used in Europe for secure file sharing and collaboration, making this vulnerability relevant for organizations relying on these versions. The flaw highlights the importance of strict permission enforcement in collaborative environments to maintain data integrity.

For European organizations, the primary impact of CVE-2025-66545 is on data integrity within shared group folders managed by Nextcloud. Unauthorized restoration of deleted files by read-only users could lead to reintroduction of outdated, corrupted, or malicious files, potentially disrupting workflows or causing confusion. Although confidentiality and availability are not directly affected, the integrity breach could undermine trust in shared data and complicate audit trails. Organizations in sectors with strict data governance, such as finance, healthcare, and government, may face compliance risks if unauthorized file restorations go undetected. The low CVSS score and absence of known exploits reduce immediate risk, but the widespread use of Nextcloud in Europe means many organizations could be exposed if patches are not applied. Attackers with limited privileges could exploit this vulnerability to bypass intended access controls, emphasizing the need for timely remediation.

1. Upgrade all Nextcloud Groupfolders installations to the fixed versions: 14.0.11, 15.3.12, 16.0.15, 17.0.14, 18.1.8, 19.1.8, or 20.1.2, depending on the version in use. 2. Conduct a thorough audit of groupfolder permissions to ensure that read-only users do not have unintended capabilities. 3. Implement monitoring and alerting on file restoration activities within group folders to detect unauthorized restorations promptly. 4. Educate administrators and users about the importance of permission hygiene and the risks of improper file restoration. 5. Consider deploying additional access controls or logging mechanisms to track changes in shared folders. 6. Review and update incident response plans to include scenarios involving unauthorized file restorations. 7. If upgrading immediately is not feasible, restrict access to the trash bin functionality or disable groupfolder features temporarily as a stopgap measure. 8. Regularly check for updates from Nextcloud security advisories to stay informed about related vulnerabilities.