CVE-2021-47515 is a vulnerability in the Linux kernel related to the handling of IPv4 packets encapsulated within IPv6 Segment Routing Header (SRH) tunnels. Specifically, the issue arises from the interaction between the IPv4 and IPv6 socket control blocks (IPCB and IP6CB) which share the same memory area (skb->cb). When an IPv4 packet is received, the receiving interface index (iif) is set in the IPv4 socket control block. However, if the packet is encapsulated in an IPv6+SRH header, the function seg6_do_srh_encap clears the IPv6 socket control block memory area using memset, inadvertently erasing the iif information. This loss of the receiving interface index can lead to a NULL pointer dereference when certain forwarding statistics code paths are executed, specifically after commit 0857d6f8c759 was applied. The root cause is the clearing of the shared memory area without preserving the iif field, causing the kernel to lose track of the interface on which the packet was received. The fix involves explicitly restoring the iif field in the IPv6 socket control block after the memset operation to maintain correct interface tracking. This vulnerability affects Linux kernel versions containing the specified commits and is related to the segment routing IPv6 (seg6) implementation and IPv4-in-IPv6 encapsulation. While no known exploits are reported in the wild, the flaw can cause kernel crashes due to NULL pointer dereferences, potentially leading to denial of service (DoS) conditions on affected systems.

For European organizations, this vulnerability poses a risk primarily to systems running affected Linux kernel versions with segment routing IPv6 features enabled, especially in environments utilizing IPv4-in-IPv6 encapsulation. The impact includes potential denial of service through kernel crashes triggered by crafted network packets, which could disrupt critical network infrastructure, servers, or services relying on Linux-based systems. Organizations operating data centers, cloud services, or network equipment that leverage advanced IPv6 routing features may experience service outages or degraded network performance. Although the vulnerability does not appear to allow remote code execution or privilege escalation, the resulting instability could be exploited to cause operational disruptions. Given the increasing adoption of IPv6 and segment routing in European telecom and enterprise networks, the vulnerability could affect ISPs, cloud providers, and enterprises with complex network topologies. The absence of known exploits reduces immediate risk, but the vulnerability should be addressed promptly to prevent potential exploitation or accidental crashes.

European organizations should take the following specific actions: 1) Identify and inventory Linux systems running kernel versions containing the affected commits, focusing on those using IPv6 segment routing and IPv4-in-IPv6 encapsulation features. 2) Apply the latest Linux kernel patches or updates that include the fix for CVE-2021-47515 as soon as they become available from trusted Linux distributions or upstream sources. 3) If immediate patching is not feasible, consider disabling segment routing IPv6 features or IPv4-in-IPv6 encapsulation on critical systems to reduce exposure. 4) Monitor network traffic for unusual IPv4-in-IPv6 encapsulated packets that could trigger the vulnerability, using advanced network monitoring tools capable of inspecting IPv6 SRH headers. 5) Implement robust kernel crash monitoring and automated recovery mechanisms to minimize downtime in case of exploitation or accidental triggering. 6) Engage with Linux distribution vendors and security mailing lists to stay informed about patch releases and related advisories. 7) For environments using custom or embedded Linux kernels, coordinate with vendors or internal development teams to backport the fix promptly.