CVE-2021-41241 is an authorization vulnerability affecting the Nextcloud server's Groupfolders application, which is designed to facilitate shared folder access among groups of users. Nextcloud is a widely used self-hosted cloud service platform that enables organizations to manage and share files internally. The Groupfolders app allows administrators to share folders with groups and set granular permissions on subfolders, restricting access to certain users even if they have access to the parent group folder. However, due to an incorrect authorization check (classified as CWE-863), users with access to a group folder could bypass subfolder restrictions by copying the entire group folder to another location within Nextcloud. This flaw effectively allows unauthorized access to subfolders that should be restricted, undermining the intended access control policies. The vulnerability affects Nextcloud versions prior to 20.0.14, versions from 21.0.0 up to but not including 21.0.6, and versions from 22.2.0 up to but not including 22.2.1. The issue was publicly disclosed in March 2022, and while no known exploits have been reported in the wild, the risk remains significant due to the potential for unauthorized data exposure. The recommended remediation is to upgrade Nextcloud to versions 20.0.14, 21.0.6, or 22.2.1 and above. For users unable to upgrade immediately, disabling the Groupfolders application is advised to mitigate the risk. This vulnerability specifically impacts the integrity of access controls within Nextcloud, potentially leading to unauthorized data disclosure within an organization's file sharing environment.

For European organizations, the impact of this vulnerability can be considerable, especially for entities relying on Nextcloud for secure file sharing and collaboration, such as government agencies, financial institutions, healthcare providers, and enterprises with sensitive data. Unauthorized access to restricted subfolders could lead to exposure of confidential information, intellectual property, or personal data protected under GDPR. This could result in regulatory penalties, reputational damage, and loss of trust. Additionally, the breach of access controls may facilitate insider threats or lateral movement within the network. Since Nextcloud is often deployed on-premises or in private clouds, the vulnerability could be exploited by authenticated users with legitimate access to group folders, making insider misuse or compromised accounts a realistic threat vector. The absence of known exploits in the wild suggests limited active exploitation, but the medium severity rating and the nature of the flaw warrant prompt attention to prevent potential data breaches.

1. Immediate upgrade of Nextcloud servers to the patched versions: 20.0.14, 21.0.6, or 22.2.1 or later. This is the most effective mitigation to ensure the authorization checks are correctly enforced. 2. For organizations unable to upgrade promptly, disable the Groupfolders application via the Nextcloud admin settings to prevent exploitation of the vulnerability. 3. Conduct an audit of existing group folder permissions and access logs to identify any suspicious activity or unauthorized access attempts to subfolders. 4. Implement strict user access management policies, including the principle of least privilege, to limit the number of users with group folder access. 5. Monitor Nextcloud server logs for unusual copy or move operations involving group folders that could indicate attempts to exploit this vulnerability. 6. Educate users about the risks of unauthorized data access and enforce strong authentication mechanisms to reduce the risk of compromised accounts. 7. Consider network segmentation and additional access controls around Nextcloud servers to limit exposure to internal threats. 8. Regularly review and update Nextcloud and its applications to stay current with security patches and advisories.