CVE-2025-66551 is an authorization bypass vulnerability classified under CWE-639 (Authorization Bypass Through User-Controlled Key) affecting Nextcloud Tables, a component that allows users to create custom tables with individual columns. In versions prior to 0.8.6 and 0.9.3, a malicious authenticated user could exploit this flaw by creating their own table and then moving a column into another user's table without proper authorization checks. This unauthorized column movement compromises data integrity by allowing attackers to manipulate or corrupt data belonging to other users. The vulnerability requires the attacker to have some level of privileges (PR:L) and user interaction (UI:R), but no elevated privileges beyond that. The CVSS v3.1 score is 6.3 (medium), reflecting network attack vector (AV:N), low attack complexity (AC:L), partial impact on integrity (I:H), and low impact on availability (A:L), with no confidentiality impact (C:N). The flaw was fixed in versions 0.8.6 and 0.9.3, but no patches are linked in the advisory. No known exploits have been reported in the wild as of the publication date (December 5, 2025). The vulnerability poses a risk primarily to data integrity and availability within collaborative environments using Nextcloud Tables, potentially allowing attackers to alter or disrupt data belonging to other users without proper authorization.

For European organizations, this vulnerability threatens the integrity and availability of data managed within Nextcloud Tables, which is widely used for collaborative document and data management. Unauthorized column movement could lead to data corruption, loss of trust in data accuracy, and disruption of business processes relying on Nextcloud for data collaboration. Organizations handling sensitive or regulated data (e.g., financial, healthcare, or governmental data) may face compliance risks if data integrity is compromised. The vulnerability does not directly expose confidential data but could indirectly affect confidentiality if data corruption leads to improper data exposure or system instability. The ease of exploitation by authenticated users means insider threats or compromised accounts could leverage this flaw. Given Nextcloud's popularity in Europe, especially in Germany, France, and the Netherlands, the impact could be significant in sectors relying on secure data collaboration platforms.

1. Upgrade Nextcloud Tables to version 0.8.6 or later, or 0.9.3 or later, as these versions contain the fix for this vulnerability. 2. Conduct an audit of user permissions and roles within Nextcloud to ensure that only trusted users have the ability to create tables or modify table structures. 3. Implement strict access controls and monitoring on Nextcloud instances to detect unusual table or column modifications. 4. Use logging and alerting mechanisms to track changes to tables and columns, enabling rapid detection of unauthorized activities. 5. Educate users about the risks of privilege misuse and enforce strong authentication mechanisms to reduce the risk of account compromise. 6. If upgrading is not immediately possible, consider restricting the ability to create or modify tables to a minimal set of trusted administrators. 7. Regularly review Nextcloud security advisories and apply patches promptly to mitigate emerging vulnerabilities.