CVE-2022-41882 is a code injection vulnerability (CWE-94) affecting the Nextcloud Desktop Client version 3.6.0. Nextcloud Desktop Client is widely used to synchronize files between a Nextcloud Server and a user's local machine. The vulnerability arises when a user receives a malicious file share and either syncs it locally or has the virtual filesystem feature enabled. If the user then clicks on a specially crafted nc://open/ link associated with the shared file, the client opens the default editor for the file type. On Windows systems, this behavior can lead to the execution of malicious scripts or code, for example, Visual Basic Script (.vbs) files, effectively allowing remote code execution through code injection. This occurs because the client does not properly control or sanitize the generation and execution of code triggered by these file shares and links. The issue is specific to version 3.6.0 and was addressed in version 3.6.1. Workarounds include configuring the server to block clients running version 3.6.0 by setting the 'minimum.supported.desktop.version' to 3.6.1, preventing the download of new malicious files exploiting this vector. Another mitigation is to enforce share acceptance on the server side by enabling 'sharing.force_share_accept', which requires users to explicitly accept shares before syncing, reducing the risk of automatic download of malicious files. However, these workarounds do not prevent exploitation of already existing malicious shares or files. No known exploits in the wild have been reported to date. The vulnerability primarily affects Windows clients due to the way file types are handled and executed. The attack requires user interaction (clicking the malicious link) and the presence of a malicious shared file. The vulnerability impacts confidentiality, integrity, and availability by enabling arbitrary code execution on the client machine, potentially leading to system compromise or data theft.

For European organizations, this vulnerability poses a significant risk especially for those relying on Nextcloud Desktop Client version 3.6.0 for file synchronization. Successful exploitation could lead to remote code execution on end-user machines, resulting in unauthorized access to sensitive data, lateral movement within corporate networks, and potential deployment of malware or ransomware. Given Nextcloud's popularity among privacy-conscious organizations and public sector entities in Europe, the impact could be substantial. The vulnerability could disrupt business operations by compromising endpoint security and potentially leading to data breaches or system downtime. The requirement for user interaction (clicking a malicious link) somewhat limits the attack vector but does not eliminate risk, particularly in environments where users frequently share files and links. The ability to execute code on Windows clients is critical because many European enterprises use Windows as their primary desktop OS. Furthermore, the persistence of already existing malicious shares means that organizations must be vigilant not only about new threats but also about legacy malicious content. This vulnerability could be exploited in targeted attacks against sectors such as government, finance, healthcare, and critical infrastructure, where Nextcloud is used for secure file sharing and collaboration.

1. Immediate upgrade of all Nextcloud Desktop Clients from version 3.6.0 to 3.6.1 or later to eliminate the vulnerability. 2. On the Nextcloud server, set the 'minimum.supported.desktop.version' system configuration to '3.6.1' to block clients running the vulnerable version from syncing new files, preventing new exploit attempts. 3. Enable 'sharing.force_share_accept' on the server to require explicit user acceptance of file shares before syncing, reducing the risk of automatic download of malicious files. 4. Conduct a thorough audit of existing file shares and synchronized files to identify and remove any potentially malicious or suspicious content that could be exploited. 5. Educate users about the risks of clicking on nc://open/ links and encourage caution when interacting with shared files and links, especially from untrusted sources. 6. Implement endpoint protection solutions capable of detecting and blocking script execution (e.g., .vbs files) and monitor for unusual process execution patterns on Windows clients. 7. Employ network-level controls to monitor and restrict suspicious traffic related to Nextcloud synchronization activities. 8. Regularly review and update Nextcloud server and client software to incorporate security patches promptly. These measures combined will reduce the attack surface and mitigate the risk posed by this vulnerability beyond generic patching advice.