CVE-2021-47166 is a vulnerability identified in the Linux kernel affecting the Network File System (NFS) implementation, specifically within the function nfs_do_recoalesce(). The issue arises from improper handling of the variable mirror->pg_bytes_written, which tracks the number of bytes written during page operations. The vulnerability occurs because this value is updated regardless of whether the flush operation on the request list succeeds or fails. This can lead to corruption of the pg_bytes_written value, potentially causing inconsistencies in the NFS client's view of data write states. Such corruption could result in data integrity issues, including stale or incorrect data being reported as successfully written, which undermines the reliability of file operations over NFS. The root cause is a logic flaw where the update to pg_bytes_written should only happen after a successful flush, but the current implementation updates it prematurely. The vulnerability was resolved by ensuring that mirror->pg_bytes_written is updated only after a successful flush attempt, preventing the corruption of this critical tracking variable. Although no known exploits are currently reported in the wild, the flaw affects the Linux kernel's NFS client code, which is widely used in enterprise and cloud environments for networked file sharing. The affected versions are identified by a specific commit hash, indicating the vulnerability is present in certain kernel builds prior to the fix. This vulnerability does not have an assigned CVSS score yet, but its impact revolves around data integrity and reliability in NFS operations.

For European organizations, the impact of CVE-2021-47166 could be significant, particularly for those relying heavily on Linux-based infrastructure and NFS for shared storage solutions. Data integrity issues in NFS can lead to corrupted files, inconsistent backups, and potential application errors, especially in environments where data consistency is critical such as financial services, healthcare, research institutions, and government agencies. The disruption caused by corrupted pg_bytes_written values may also affect high-availability systems and cloud service providers operating in Europe that use NFS for distributed storage. While this vulnerability does not directly enable remote code execution or privilege escalation, the potential for data corruption can lead to operational disruptions, loss of trust in data accuracy, and increased recovery costs. Additionally, organizations with compliance requirements around data integrity and auditability may face regulatory risks if corrupted data leads to non-compliance. Since no active exploits are known, the immediate risk is moderate, but the widespread use of Linux and NFS in European IT environments means that unpatched systems could be vulnerable to future exploitation or accidental data corruption.

European organizations should prioritize updating their Linux kernels to versions that include the patch for CVE-2021-47166. Specifically, kernel maintainers and system administrators should apply the fix that ensures mirror->pg_bytes_written is only updated after successful flush operations in nfs_do_recoalesce(). Beyond patching, organizations should audit their NFS client configurations and monitor file system integrity closely, especially in environments with heavy NFS usage. Implementing file integrity monitoring tools can help detect anomalies caused by this vulnerability. Backup strategies should be reviewed to ensure they can recover from potential data corruption scenarios. For critical systems, consider isolating NFS traffic or using alternative, more resilient storage protocols temporarily until patches are applied. Additionally, organizations should maintain strict change management and testing procedures when updating kernels to avoid service disruptions. Network segmentation and limiting NFS access to trusted hosts can reduce the attack surface. Finally, staying informed through Linux kernel security advisories and subscribing to vulnerability feeds will help ensure timely response to any emerging exploits related to this vulnerability.