CVE-2025-66557 is an improper access control vulnerability (CWE-284) identified in Nextcloud Deck, a kanban-style project and personal planning tool integrated with Nextcloud. The flaw exists in the permission logic of versions prior to 1.14.6 and between 1.15.0-beta.1 and 1.15.2, where users granted the "Can share" permission can improperly modify the permissions of other recipients. This means that a user who should only be able to share boards or tasks can escalate their privileges by changing access rights of other users, potentially granting themselves or others higher privileges than intended. The vulnerability affects the integrity and availability of the system by allowing unauthorized permission changes that could disrupt workflows or enable further unauthorized actions. The CVSS 3.1 base score is 5.4 (medium), reflecting network exploitability with low complexity and no user interaction required, but requiring some privileges (PR:L). No confidentiality impact is noted. The issue was publicly disclosed on December 5, 2025, with fixes released in versions 1.14.6 and 1.15.2. No known exploits have been reported in the wild. This vulnerability highlights the importance of strict access control enforcement in collaborative tools to prevent privilege escalation and unauthorized modifications.

For European organizations, this vulnerability poses a risk primarily to the integrity and availability of collaborative project management data within Nextcloud Deck. Unauthorized permission modifications could lead to disruption of team workflows, unauthorized data manipulation, or further privilege escalation within the Nextcloud environment. Organizations relying on Nextcloud Deck for sensitive project planning or team coordination may face operational impacts if attackers exploit this flaw. Although confidentiality is not directly impacted, the ability to alter permissions could indirectly facilitate data exposure or unauthorized access if attackers escalate privileges further. The medium severity score reflects moderate risk, but the potential for cascading effects in complex organizational environments elevates the importance of timely remediation. Given Nextcloud's popularity in Europe, especially among privacy-conscious public sector and private enterprises, the impact could be significant if left unpatched.

European organizations using Nextcloud Deck should immediately verify their deployed versions and upgrade to 1.14.6 or 1.15.2 or later to remediate this vulnerability. Beyond patching, administrators should audit user permissions, especially those with "Can share" rights, to ensure they are granted only to trusted users. Implement strict role-based access controls and regularly review permission assignments to minimize the attack surface. Monitoring logs for unusual permission changes can help detect exploitation attempts early. Additionally, organizations should consider network segmentation and limiting external access to Nextcloud instances to reduce exposure. Employing multi-factor authentication and enforcing strong user authentication policies can further reduce risk. Finally, educating users about the risks of permission misuse and maintaining an incident response plan tailored to collaboration platform compromises will enhance resilience.