CVE-2021-46915 is a vulnerability identified in the Linux kernel's netfilter subsystem, specifically within the nft_limit module. The issue arises from an incorrect mathematical operation in the nft_limit_init() function, where a 64-bit unsigned integer (u64) is divided by another 64-bit unsigned integer using a function designed to divide a u64 by a 32-bit unsigned integer (div_u64()), instead of the correct function div64_u64() that handles u64 by u64 division. This improper division can lead to a divide error, causing kernel faults such as crashes or panics. The vulnerability was observed in kernel version 5.12.0-rc4, with the error trace indicating a failure in the division operation during initialization of nft_limit expressions. The netfilter nft_limit module is responsible for limiting the rate of packets processed by nftables, a packet filtering framework used for firewall and network traffic control. A divide error in this context can cause denial of service (DoS) conditions by crashing the kernel or triggering kernel address sanitizer (KASAN) faults. The vulnerability does not appear to have known exploits in the wild at the time of publication, and no CVSS score has been assigned. However, the flaw stems from a fundamental arithmetic error in kernel code that can be triggered during netfilter configuration or operation, potentially by local or remote actors with the ability to manipulate nftables rules or netlink messages. The vulnerability affects Linux kernel versions containing the faulty code, as identified by the commit hashes provided. The fix involves replacing the incorrect division function with the appropriate div64_u64() to safely handle 64-bit division operations.

For European organizations, the impact of CVE-2021-46915 could be significant, particularly for those relying heavily on Linux-based infrastructure for networking, servers, and cloud environments. The vulnerability can lead to kernel crashes or system instability, resulting in denial of service conditions. This can disrupt critical services, including web hosting, internal applications, and network security controls. Organizations using nftables for firewalling and traffic shaping are especially at risk, as exploitation could be triggered by malformed netfilter configurations or crafted netlink messages. While exploitation requires the ability to interact with netfilter configurations, this may be possible for privileged users or through compromised systems. The disruption caused by kernel panics can lead to downtime, loss of productivity, and potential cascading failures in networked systems. Additionally, the instability could be leveraged as part of a broader attack chain to facilitate privilege escalation or lateral movement if combined with other vulnerabilities. Given the widespread use of Linux in European data centers, cloud providers, and enterprise environments, the vulnerability poses a tangible risk to availability and operational continuity.

European organizations should take the following specific mitigation steps: 1) Immediately identify and inventory Linux systems running vulnerable kernel versions, focusing on those using nftables and netfilter subsystems. 2) Apply the official Linux kernel patches or upgrade to a kernel version where the div64_u64() fix has been implemented. Since no patch links are provided, organizations should monitor official Linux kernel repositories and vendor advisories for updates. 3) Restrict access to netfilter configuration interfaces and netlink sockets to trusted administrators only, minimizing the risk of unauthorized rule manipulation. 4) Implement strict access controls and monitoring on systems that allow nftables rule changes, including audit logging of netlink messages and nft commands. 5) Employ kernel hardening techniques such as enabling Kernel Address Sanitizer (KASAN) in testing environments to detect similar faults proactively. 6) For cloud environments, coordinate with providers to ensure underlying host kernels are patched and that tenant VMs are protected. 7) Conduct thorough testing of nftables configurations after patching to ensure stability and correct operation. 8) Maintain up-to-date incident response plans to quickly address potential DoS events caused by kernel crashes.