CVE-2022-24889 is a medium-severity vulnerability affecting Nextcloud Server, a widely used self-hosted productivity and file sharing platform. The vulnerability is classified under CWE-345, which pertains to insufficient verification of data authenticity. Specifically, in Nextcloud versions prior to 21.0.8, 22.2.4, and 23.0.1, an attacker can trick administrators into enabling "recommended" apps that are not necessary for their environment. This manipulation occurs because the system does not adequately verify the authenticity of the data that suggests which apps should be enabled. As a result, administrators might inadvertently expand their attack surface by activating additional apps, potentially introducing new vulnerabilities or increasing the risk of compromise. The issue arises from the trust placed in the data source without sufficient validation, allowing an attacker to influence administrative decisions indirectly. Although no known exploits are currently reported in the wild, the vulnerability could be leveraged in targeted attacks where an adversary has some level of access or influence over the data presented to administrators. The flaw was addressed and fixed in Nextcloud versions 21.0.8, 22.2.4, and 23.0.1, which implement proper verification mechanisms to ensure that only legitimate recommended apps are enabled.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on Nextcloud for file sharing and collaboration. By tricking administrators into enabling unnecessary apps, attackers can increase the attack surface, potentially introducing new vectors for malware, data exfiltration, or privilege escalation. This is particularly concerning for organizations handling sensitive or regulated data, such as financial institutions, healthcare providers, and government agencies, where unauthorized access or data leakage could lead to regulatory penalties and reputational damage. The indirect nature of the attack means that it could be used as a stepping stone in a larger attack chain, making detection more difficult. Additionally, since Nextcloud is often self-hosted, organizations with less mature IT security practices may be more vulnerable to such manipulation. The vulnerability does not directly allow remote code execution or immediate data compromise but facilitates conditions that could lead to more severe breaches if combined with other weaknesses.

To mitigate this vulnerability, European organizations should immediately upgrade Nextcloud Server installations to versions 21.0.8, 22.2.4, or 23.0.1 or later, depending on their current version branch. Beyond patching, administrators should audit the list of enabled apps and disable any that are not explicitly required for business operations. Implement strict administrative policies to verify any recommended app activations manually rather than relying solely on automated recommendations. Organizations should also monitor logs and configuration changes for unusual app enablement activities. Employ network segmentation and access controls to limit the exposure of the Nextcloud server to trusted administrators only. Additionally, consider implementing application whitelisting and integrity monitoring on the Nextcloud server to detect unauthorized changes. Regular security awareness training for administrators about the risks of blindly trusting automated recommendations can further reduce the risk. Finally, organizations should keep abreast of Nextcloud security advisories and subscribe to vulnerability feeds to respond promptly to future issues.