CVE-2021-47307 is a vulnerability identified in the Linux kernel specifically within the CIFS (Common Internet File System) client implementation. The issue arises in the function cifs_compose_mount_options(), which is responsible for composing mount options when mounting CIFS shares. The vulnerability is due to a potential NULL pointer dereference caused by the optional @ref parameter possibly containing a NULL node_name. If the code attempts to dereference this NULL node_name, it can lead to a kernel NULL pointer dereference, which typically results in a denial of service (system crash or kernel panic). This vulnerability was detected and resolved by preventing the dereference of the NULL node_name, thereby improving the robustness of the CIFS client code. The vulnerability does not have any known exploits in the wild as of the publication date, and no CVSS score has been assigned. The affected versions correspond to specific Linux kernel commits prior to the fix. The issue was identified through static analysis (Coverity scan) and addressed promptly. Since CIFS is widely used for mounting Windows shares on Linux systems, this vulnerability could affect any Linux system that mounts CIFS shares and uses the vulnerable kernel versions. Exploitation would require the attacker to influence the mount options or the parameters passed to the CIFS mount operation, which typically requires local access or specific conditions to trigger the NULL dereference. This vulnerability primarily impacts system availability by causing kernel crashes but does not directly lead to privilege escalation or data confidentiality breaches.

For European organizations, the primary impact of CVE-2021-47307 is the potential for denial of service on Linux systems that mount CIFS shares. Many enterprises in Europe rely on Linux servers for file sharing and network storage, often integrating with Windows environments via CIFS/SMB protocols. A successful exploitation could cause system instability or crashes, leading to downtime of critical file services. This can disrupt business operations, especially in sectors like finance, manufacturing, and public administration where file sharing is integral. Although the vulnerability does not appear to allow remote code execution or privilege escalation, the resulting denial of service could be leveraged in targeted attacks to disrupt services or as part of a multi-stage attack chain. Given the lack of known exploits, the immediate risk is moderate; however, unpatched systems remain vulnerable to accidental crashes or potential future exploitation. Organizations with extensive Linux infrastructure and CIFS usage should prioritize patching to maintain service availability and prevent operational disruptions.

To mitigate CVE-2021-47307, European organizations should: 1) Apply the latest Linux kernel updates that include the fix for this vulnerability as soon as possible. Since the vulnerability is in the kernel, updating to a patched kernel version is the most effective mitigation. 2) Review and audit CIFS mount configurations to ensure that mount options are correctly specified and not influenced by untrusted inputs, reducing the risk of triggering the NULL dereference. 3) Implement monitoring for kernel crashes or system instability that could indicate attempts to exploit this vulnerability. 4) Limit access to systems that perform CIFS mounts to trusted administrators and users to reduce the attack surface. 5) Where feasible, consider alternative file sharing protocols or hardened configurations that do not rely on vulnerable CIFS client implementations. 6) Incorporate this vulnerability into vulnerability management and patching workflows to ensure timely remediation across all Linux systems using CIFS.