CVE-2025-66515 is an improper authentication vulnerability (CWE-287) found in the Nextcloud Approval app, which facilitates file approval workflows within the Nextcloud environment. The flaw exists in versions prior to 1.3.1 and 2.5.0, where an authenticated user designated as a requester in a workflow can exploit the numeric file ID to set another user's file into a 'pending approval' state without having access rights to that file. This occurs because the application does not properly verify the requester's authorization to modify the approval status of files they do not own or have access to. The vulnerability impacts the integrity of the approval process, potentially allowing unauthorized manipulation of file states, which could disrupt workflow accuracy and trustworthiness. The CVSS v3.1 score is 2.7 (low severity), reflecting that the attack vector is network-based, requires high privileges (authenticated requester), no user interaction is needed, and the impact is limited to integrity without affecting confidentiality or availability. No known exploits have been reported in the wild, and the issue has been addressed in Nextcloud Approval app versions 1.3.1 and 2.5.0 by enforcing proper authorization checks on file approval actions.

For European organizations, this vulnerability primarily threatens the integrity of document approval workflows within Nextcloud environments. While it does not expose confidential data or cause denial of service, unauthorized changes to file approval statuses can lead to workflow disruptions, miscommunication, and potential compliance issues, especially in regulated sectors relying on accurate audit trails and approval processes. Organizations using affected versions may face challenges in maintaining trust in their document management systems, which could indirectly affect operational efficiency and regulatory adherence. The impact is more pronounced in environments where file approval workflows are critical to business processes, such as legal, financial, or governmental institutions. However, since exploitation requires authenticated access with requester privileges, the risk is limited to insiders or compromised accounts.

European organizations should promptly upgrade the Nextcloud Approval app to version 1.3.1 or 2.5.0 or later to ensure the vulnerability is patched. Additionally, organizations should audit user roles and permissions to ensure that only trusted users have requester privileges in workflows. Implementing strict access controls and monitoring for unusual approval activity can help detect potential exploitation attempts. Employing multi-factor authentication (MFA) for Nextcloud accounts reduces the risk of account compromise that could lead to exploitation. Regularly reviewing and updating workflow configurations to minimize unnecessary requester privileges can further reduce attack surface. Finally, organizations should maintain an inventory of Nextcloud app versions in use and subscribe to vendor security advisories to stay informed about future vulnerabilities.