CVE-2021-47107 is a vulnerability identified in the Linux kernel's Network File System daemon (NFSD) component, specifically related to the handling of the READDIR operation. The READDIR operation is used by NFS clients to read directory entries from an NFS server. The vulnerability arises from improper handling of the 'count' argument in the READDIR request. If a client sends a READDIR request with a very small or zero count value, the kernel's buffer size calculation in the init_dirlist helper function underflows. This underflow leads to incorrect pointer arithmetic in the xdr_reserve_space() function, which is responsible for reserving buffer space for XDR (External Data Representation) streams. Consequently, this allows the XDR stream functions to write beyond the allocated buffer boundaries, effectively causing a buffer overflow. This buffer overflow can potentially lead to memory corruption, which may be exploited to cause denial of service (system crashes) or potentially arbitrary code execution, depending on the attacker's capabilities and system configuration. The vulnerability was introduced due to changes in entry encoding that exposed the underflow issue, and the lack of sanity checks on the READDIR count argument in NFSD. Notably, older entry encoders handled this case correctly, but the new implementation missed this edge case. Modern NFS clients typically request large amounts of data per READDIR call, so this edge case was not well tested, and no unit tests existed to cover the lower bound of count values. The vulnerability affects specific Linux kernel versions identified by commit hashes, and it has been publicly disclosed and patched. However, no known exploits are currently reported in the wild. The absence of a CVSS score indicates that the severity assessment must be inferred from the technical details and potential impact.

For European organizations, this vulnerability poses a significant risk, especially for those relying on Linux-based NFS servers for file sharing and storage infrastructure. Exploitation could allow an attacker with network access to the NFS server to trigger a buffer overflow by sending crafted READDIR requests. This could lead to denial of service conditions, disrupting critical file services and impacting business continuity. In worst-case scenarios, if exploited for arbitrary code execution, attackers could gain unauthorized access or escalate privileges on affected systems, potentially leading to data breaches or lateral movement within networks. Given the widespread use of Linux servers in European enterprises, public sector, and cloud environments, the vulnerability could affect a broad range of industries including finance, manufacturing, telecommunications, and government agencies. The impact is heightened in environments where NFS is exposed beyond trusted internal networks or where network segmentation is weak. Additionally, the lack of known exploits currently reduces immediate risk but does not eliminate the threat, as attackers may develop exploits over time. Organizations with legacy or unpatched Linux kernels are particularly vulnerable.

European organizations should prioritize patching affected Linux kernel versions as soon as vendor updates are available. Immediate steps include: 1) Identifying all Linux systems running NFS server services and verifying kernel versions against the affected commits. 2) Applying official Linux kernel patches or upgrading to a fixed kernel version that addresses CVE-2021-47107. 3) Implementing network-level controls to restrict NFS access to trusted hosts only, using firewalls and network segmentation to limit exposure. 4) Monitoring NFS server logs for unusual READDIR requests, especially those with abnormally small or zero count values, which could indicate probing attempts. 5) Employing intrusion detection systems (IDS) with signatures or heuristics for anomalous NFS traffic patterns. 6) Conducting internal security assessments and penetration tests focusing on NFS services to identify potential exploitation vectors. 7) Educating system administrators about this vulnerability and the importance of validating input parameters in network services. These measures go beyond generic advice by focusing on both patch management and proactive detection of exploitation attempts specific to the NFS READDIR operation.