CVE-2021-47309 is a vulnerability identified in the Linux kernel networking subsystem, specifically related to the handling of lightweight tunnel (lwtstate) data within the skb_tunnel_info() function. The vulnerability arises because skb_tunnel_info() returns a pointer to lwtstate->data as an ip_tunnel_info type without validating the actual data type. Since lwtstate->data can represent various encapsulation types such as mpls_iptunnel_encap and others, which are not compatible with ip_tunnel_info, this lack of validation can lead to improper memory access. The issue manifests as a slab-out-of-bounds read detected by Kernel Address Sanitizer (KASAN), causing a kernel crash (BUG) in the vxlan_get_route function. This crash occurs when the kernel attempts to read memory beyond the allocated buffer, triggered by network operations involving VXLAN or MPLS tunnels. The stack trace indicates that the problem occurs during packet transmission routines (vxlan_xmit_one, vxlan_xmit) and routing functions (mpls_xmit, lwtunnel_xmit), potentially leading to denial of service (DoS) conditions. The vulnerability affects Linux kernel versions prior to the patch that introduced validation checks in skb_tunnel_info(), ensuring that the returned pointer corresponds to the correct tunnel encapsulation type. No known exploits in the wild have been reported yet, but the vulnerability is significant due to its kernel-level impact and the critical role of networking in Linux systems. The vulnerability was published on May 21, 2024, and no CVSS score has been assigned.

For European organizations, the impact of CVE-2021-47309 can be substantial, especially for those relying heavily on Linux-based infrastructure for networking, cloud services, and virtualization. The vulnerability can cause kernel crashes leading to denial of service, disrupting critical network functions such as VXLAN and MPLS tunneling, which are widely used in data centers and enterprise networks for network segmentation and traffic engineering. This disruption can affect availability of services, potentially impacting business continuity. Additionally, while no direct privilege escalation or remote code execution has been reported, kernel crashes can be leveraged by attackers to cause persistent outages or to facilitate further attacks by destabilizing systems. Organizations in sectors such as finance, telecommunications, and critical infrastructure, which depend on stable and secure Linux networking stacks, may face operational risks. The lack of known exploits reduces immediate risk, but the widespread use of Linux kernels in European IT environments means that unpatched systems remain vulnerable to accidental or malicious triggering of this flaw.

To mitigate CVE-2021-47309, European organizations should prioritize updating their Linux kernel to the latest patched versions that include the validation fix in skb_tunnel_info(). Kernel updates should be tested and deployed promptly, especially on systems handling VXLAN and MPLS tunnels. Network administrators should audit their use of lightweight tunnels and encapsulation protocols to identify potentially affected systems. Employing kernel hardening techniques such as enabling Kernel Address Sanitizer (KASAN) in testing environments can help detect similar issues proactively. Additionally, organizations should implement robust monitoring for kernel crashes and network anomalies that could indicate exploitation attempts or accidental triggering of the vulnerability. Where immediate patching is not feasible, temporarily disabling or limiting the use of affected tunneling protocols (VXLAN, MPLS) may reduce exposure. Finally, maintaining strict access controls and network segmentation can limit the ability of attackers to exploit this vulnerability remotely.