CVE-2022-24741 is a medium-severity vulnerability affecting Nextcloud Server, an open-source, self-hosted cloud services platform widely used for file sharing and collaboration. The vulnerability is classified under CWE-400: Uncontrolled Resource Consumption. Specifically, in affected versions of Nextcloud Server (versions prior to 21.0.8, versions from 22.0.0 up to but not including 22.2.4, and versions from 23.0.0 up to but not including 23.0.1), an attacker can upload specially crafted files that trigger excessive memory and CPU usage during preview generation. This can lead to a denial of service (DoS) condition, where the server becomes unresponsive or crashes due to resource exhaustion. The root cause lies in the preview generation feature, which processes uploaded files to create thumbnails or previews. Maliciously crafted files can exploit this processing to consume disproportionate system resources. To remediate this, Nextcloud recommends upgrading to fixed versions 21.0.8, 22.2.4, or 23.0.1. For users unable to upgrade immediately, disabling the preview generation feature by setting the 'enable_previews' configuration flag to false is advised to mitigate the risk. There are no known exploits in the wild at this time, but the vulnerability's nature means it could be exploited by unauthenticated attackers simply by uploading files, making it a significant risk for publicly accessible Nextcloud instances. The vulnerability does not require authentication or user interaction beyond file upload, which is a common feature of Nextcloud deployments. This vulnerability impacts confidentiality minimally but has a direct impact on availability and potentially integrity if service disruption affects data access or processing.

For European organizations using Nextcloud Server, this vulnerability poses a risk of denial of service attacks that can disrupt business operations reliant on file sharing and collaboration. Organizations in sectors such as government, finance, healthcare, and education, which often use Nextcloud for secure document management, could experience service outages leading to operational delays and potential data access issues. The DoS condition could also be leveraged as a distraction or part of a multi-stage attack. Since Nextcloud is popular among privacy-conscious organizations and those preferring self-hosted solutions, the impact on availability could undermine trust and compliance with data protection regulations like GDPR if service disruptions prevent timely access to personal data. Additionally, organizations with public-facing Nextcloud instances are at higher risk, as attackers do not need authentication to exploit the vulnerability. The resource exhaustion could also increase operational costs due to the need for emergency response and potential hardware scaling to mitigate attacks.

1. Immediate upgrade to Nextcloud Server versions 21.0.8, 22.2.4, or 23.0.1 to apply the official patch addressing the vulnerability. 2. For environments where immediate upgrade is not feasible, disable the preview generation feature by setting 'enable_previews' to false in the Nextcloud configuration file to prevent resource-intensive processing of uploaded files. 3. Implement strict file upload controls, including file type whitelisting and size limits, to reduce the risk of malicious files triggering resource exhaustion. 4. Deploy web application firewalls (WAFs) with custom rules to detect and block anomalous file uploads or excessive resource usage patterns. 5. Monitor server resource utilization closely to detect unusual spikes in CPU or memory usage that could indicate exploitation attempts. 6. Restrict Nextcloud upload access to authenticated users where possible, or limit upload permissions to trusted users to reduce exposure. 7. Regularly audit and review Nextcloud logs for signs of attempted exploitation or abnormal file upload activity. 8. Consider network segmentation and rate limiting to contain potential DoS impacts and prevent lateral movement in case of exploitation.