CVE-2021-46952 is a vulnerability identified in the Linux kernel's implementation of the Network File System (NFS), specifically related to the handling of UDP retransmission timeout values during mount operations. The issue arises in the function xprt_calc_majortimeo(), which calculates the major timeout value for NFS mounts using the UDP transport protocol (XPRT_TRANSPORT_UDP). The vulnerability is triggered when an invalid 'retrans' mount option is passed, representing the number of retransmissions. This 'retrans' value is used as a shift count on a 64-bit integer, and if it is equal to or greater than 64, it causes a shift out-of-bounds error. Such an out-of-bounds shift can lead to undefined behavior, potentially causing kernel crashes or memory corruption. The root cause is the lack of proper validation on the 'retrans' parameter before it is used in bit-shifting operations. The fix implemented ensures that if the 'retrans' value is 64 or higher, the mount operation fails gracefully by returning an error, thus preventing the out-of-bounds shift. This vulnerability was discovered through fuzz testing with syzkaller, a kernel fuzzing tool, and affects Linux kernel versions identified by the commit hash 9954bf92c0cddd50a2a470be302e1c1ffdf21d42. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, the impact of CVE-2021-46952 primarily concerns systems running Linux kernels with vulnerable NFS implementations, especially those using UDP transport for NFS mounts. NFS is widely used in enterprise environments for file sharing and storage, including in cloud infrastructure, data centers, and network-attached storage solutions. Exploitation of this vulnerability could lead to denial of service (DoS) conditions by crashing the kernel or causing instability, potentially disrupting critical file-sharing services and impacting business operations. While there is no evidence of remote code execution or privilege escalation directly from this vulnerability, the resulting system instability could be leveraged as part of a broader attack chain. European organizations with large-scale Linux deployments, particularly those using NFS over UDP in production environments, could face operational disruptions. Additionally, sectors with high reliance on Linux-based infrastructure such as finance, telecommunications, government, and manufacturing could be more sensitive to such disruptions. The absence of known exploits reduces immediate risk, but the vulnerability's presence in widely deployed Linux kernels necessitates prompt mitigation to avoid future exploitation attempts.

To mitigate CVE-2021-46952, European organizations should: 1) Apply the latest Linux kernel patches that include the fix for this vulnerability, ensuring that the validation of the 'retrans' mount option is enforced. 2) Audit NFS mount configurations to verify that no invalid or suspicious 'retrans' values are used, particularly values equal to or exceeding 64. 3) Implement strict input validation and monitoring on systems that accept NFS mount parameters, especially if these parameters can be influenced by untrusted users or automated tools. 4) Employ kernel hardening and security modules (e.g., SELinux, AppArmor) to limit the impact of potential kernel crashes and to contain compromised processes. 5) Monitor system logs and kernel messages for unusual NFS mount failures or kernel warnings that might indicate attempted exploitation. 6) Consider using TCP transport for NFS mounts where feasible, as this vulnerability specifically affects UDP transport. 7) Incorporate fuzz testing and continuous security assessments in the development and deployment lifecycle to detect similar vulnerabilities proactively.