CVE-2025-66547 is an authorization bypass vulnerability identified in Nextcloud Server and Enterprise Server versions prior to 31.0.1. Nextcloud is a widely used self-hosted cloud storage and collaboration platform. The vulnerability stems from improper authorization checks when handling bulk tagging operations, allowing non-privileged users to modify tags on files they should not have access to. This is categorized under CWE-639, which involves authorization bypass through user-controlled keys or parameters. The flaw enables an attacker with low privileges to alter file metadata (tags) without having legitimate access rights to those files, potentially impacting the integrity of file organization and management. The vulnerability does not affect confidentiality or availability directly, as it does not allow reading or deleting files, nor does it require user interaction to exploit. The CVSS v3.1 score is 4.3 (medium severity), reflecting network attack vector, low complexity, required privileges, and no user interaction. No public exploits or active exploitation have been reported to date. The issue was fixed in Nextcloud version 31.0.1, and users are advised to upgrade promptly. The vulnerability could be leveraged in environments where tagging controls access or triggers workflows, potentially leading to indirect information disclosure or privilege escalation chains. The root cause is insufficient validation of user permissions during bulk tagging operations, allowing unauthorized metadata modifications.

For European organizations, the impact primarily concerns the integrity of file metadata within Nextcloud deployments. Unauthorized tag modifications can disrupt file classification, searchability, and automated processes relying on tags, potentially leading to operational inefficiencies or accidental exposure of sensitive information through mislabeling. While confidentiality and availability are not directly compromised, the integrity breach could facilitate further attacks if combined with other vulnerabilities or insider threats. Organizations using Nextcloud for regulated data or critical collaboration may face compliance risks if unauthorized metadata changes lead to improper data handling. The medium severity indicates moderate risk, but the widespread use of Nextcloud in European public and private sectors elevates the importance of timely remediation. The lack of known exploits reduces immediate risk but does not eliminate the threat, especially in targeted attacks or insider misuse scenarios.

1. Upgrade all Nextcloud Server and Enterprise Server instances to version 31.0.1 or later immediately to apply the official patch. 2. Restrict bulk tagging permissions to trusted users only, minimizing the attack surface by enforcing the principle of least privilege. 3. Implement monitoring and alerting on tagging activities, especially bulk operations, to detect unusual or unauthorized changes promptly. 4. Review and harden access control policies related to file tagging and metadata management within Nextcloud configurations. 5. Conduct regular audits of file tags and metadata to identify inconsistencies or unauthorized modifications. 6. Educate administrators and users about the risks of improper tagging and the importance of following security best practices. 7. Consider network segmentation and access controls to limit exposure of Nextcloud servers to untrusted networks or users. 8. Stay informed on Nextcloud security advisories for any further updates or related vulnerabilities.