CVE-2021-47014 is a vulnerability identified in the Linux kernel's network scheduling subsystem, specifically within the act_ct (connection tracking action) module. The flaw arises during the handling of IP packet fragments when using act_ct for re-assembly and re-fragmentation. The vulnerability manifests as a wild memory access caused by improper clearing of fragment data structures. More precisely, when act_ct temporarily stores an IP fragment, it incorrectly restores the socket buffer's (skb) queueing discipline control block (qdisc cb), leading to random data being placed in the fragment control block (FRAG_CB()). This improper memory handling results in out-of-bounds or wild memory accesses during the purging of the red-black tree (rbtree) that manages IP fragments, causing kernel crashes or potential memory corruption. The issue was observed in Linux kernel version 5.12.0-rc7+ and is triggered when tcf_ct_handle_fragments() returns -EINPROGRESS, indicating that fragment processing is incomplete. The root cause is the overwriting of skb control block data when it should be preserved, leading to use-after-free or invalid pointer dereferences. Although no public exploits are currently known, the vulnerability can cause system instability or denial of service (DoS) due to kernel panics. The flaw affects specific Linux kernel commits identified by their hashes, indicating it impacts certain kernel versions prior to the fix. The vulnerability was addressed by ensuring the skb control block is not overwritten when fragment processing is incomplete, preventing wild memory accesses during fragment purging.

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions with act_ct enabled, especially those handling fragmented IP traffic in network-intensive environments such as data centers, cloud infrastructures, and telecommunications. Exploitation could lead to kernel crashes resulting in denial of service, disrupting critical services and operations. In environments where Linux servers act as routers, firewalls, or perform network packet inspection, this vulnerability could be triggered by crafted fragmented packets, potentially from internal or external sources. While no direct evidence of privilege escalation or remote code execution exists, the instability caused could be leveraged as part of a broader attack chain. Disruptions in critical infrastructure, financial services, or government networks relying on Linux-based systems could have cascading effects. Additionally, the vulnerability could be exploited to degrade service availability or cause unexpected reboots, impacting compliance with European regulations on service continuity and data protection. The lack of known exploits reduces immediate risk, but the potential for denial of service in key network components remains a concern.

European organizations should prioritize updating their Linux kernels to versions where this vulnerability is patched, ensuring that the fix preventing skb control block overwrites during fragment handling is applied. Network administrators should audit systems to identify kernels with the affected commit hashes and schedule timely upgrades. For environments where immediate patching is not feasible, implementing network-level controls to filter or drop suspicious fragmented IP packets can reduce exposure. Monitoring kernel logs for KASAN (Kernel Address Sanitizer) warnings or unusual crashes related to inet_frag or act_ct modules can help detect attempts to trigger the vulnerability. Additionally, disabling or limiting the use of act_ct in network scheduling configurations where it is not essential can reduce the attack surface. Organizations should also review firewall and intrusion detection system rules to detect anomalous fragmented packet patterns. Finally, maintaining robust incident response plans to quickly address potential denial of service incidents caused by kernel crashes is recommended.