CVE-2021-47359 is a vulnerability identified in the Linux kernel related to the CIFS (Common Internet File System) client implementation. The issue manifests as a soft lockup during filesystem stress testing (fsstress), where the system experiences a CPU hang, specifically noted as a CPU stuck for extended periods (e.g., 26 seconds). This soft lockup indicates that a CPU core is stuck in a non-interruptible state, causing the system to become unresponsive or hung. The root cause is linked to the CIFS filesystem driver, which handles SMB (Server Message Block) protocol interactions for network file sharing. Under certain stress conditions, the CIFS driver can enter a state that leads to a deadlock or livelock, preventing normal CPU scheduling and causing the watchdog timer to trigger a soft lockup warning. Although no known exploits are reported in the wild, the vulnerability can cause denial of service (DoS) by making the system unresponsive, which is critical for servers and systems relying on CIFS mounts. The vulnerability affects Linux kernel versions identified by the commit hash 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, and it has been publicly disclosed and patched by the Linux project. The absence of a CVSS score suggests the need for an independent severity assessment based on the impact and exploitability characteristics.

For European organizations, this vulnerability poses a significant risk primarily in environments where Linux servers are used to mount CIFS shares, such as in enterprise file sharing, network-attached storage (NAS), and mixed Windows-Linux network environments. The impact is mainly a denial of service condition, where critical systems may hang or become unresponsive under filesystem stress, potentially disrupting business operations, data access, and services relying on network file systems. This can affect sectors with high dependency on Linux infrastructure, including financial services, telecommunications, government agencies, and cloud service providers. The unavailability of systems due to soft lockups can lead to operational downtime, loss of productivity, and increased incident response costs. Additionally, if exploited in a targeted manner, attackers could induce repeated system hangs to degrade service availability. However, since no remote code execution or privilege escalation is indicated, the confidentiality and integrity impacts are limited. The vulnerability's exploitation does not require user interaction, increasing the risk in automated or unattended environments.

European organizations should prioritize applying the official Linux kernel patches that address CVE-2021-47359 as soon as possible. System administrators should: 1) Identify all Linux systems using CIFS mounts, especially those exposed to untrusted networks or handling high filesystem load. 2) Schedule kernel updates during maintenance windows to minimize disruption. 3) Monitor system logs for watchdog soft lockup warnings or unusual CPU hangs indicative of this issue. 4) Implement proactive filesystem stress testing in controlled environments to detect potential hangs before production deployment. 5) Where patching is delayed, consider temporary workarounds such as limiting CIFS mount usage or isolating affected systems from critical workloads. 6) Review and harden network access controls to CIFS shares to reduce exposure. 7) Employ system monitoring and alerting tools to detect early signs of system unresponsiveness. These steps go beyond generic advice by focusing on CIFS usage patterns, proactive detection, and operational controls specific to this vulnerability.