CVE-2021-47129 is a medium severity vulnerability in the Linux kernel's netfilter subsystem, specifically within the nftables connection tracking (conntrack) implementation. The issue arises in the nft_ct_expect_obj_eval() function, which improperly calls nf_ct_ext_add() on a confirmed conntrack entry. According to the kernel's logic, nf_ct_ext_add() should only be invoked for unconfirmed conntrack entries (!nf_ct_is_confirmed()). This flaw can lead to improper handling of connection tracking expectations, potentially causing kernel warnings or crashes due to invalid state transitions. The vulnerability stems from the failure to skip expectations for confirmed conntrack entries, which can result in instability or denial of service conditions. The patch involves adding the connection tracking helper extension only for unconfirmed conntrack entries and skipping rule evaluation if the helper extension does not exist. This limits the creation of expectations to the first packet of a connection, preventing the erroneous state changes. The description also suggests a potential future improvement by introducing a new action to attach a generic connection tracking helper to the first packet, enabling expectations to be created from follow-up packets safely. Additionally, the fix includes skipping template conntrack entries and removing redundant checks for IPCT_UNTRACK flags. The vulnerability has a CVSS 3.1 score of 4.6 (medium severity), with an attack vector of network, low attack complexity, requiring privileges and user interaction, and impacts confidentiality and integrity but not availability. No known exploits are reported in the wild at this time.

For European organizations, this vulnerability could lead to kernel instability or denial of service on Linux systems that utilize nftables for firewalling and connection tracking, which is common in enterprise and cloud environments. The flaw could be exploited by an attacker with some level of local privileges and user interaction to cause unexpected behavior in network packet processing, potentially disrupting network services or enabling further privilege escalation or information leakage. While the direct impact on confidentiality and integrity is limited (partial impact), the disruption of network filtering and connection tracking could affect critical infrastructure, especially in sectors relying heavily on Linux-based firewalls and routers. Given the widespread use of Linux in European data centers, telecommunications, and government networks, unpatched systems could face service interruptions or be leveraged as a stepping stone for more advanced attacks. However, the requirement for privileges and user interaction reduces the risk of remote exploitation without prior access.

European organizations should prioritize applying the Linux kernel patches that address CVE-2021-47129 as soon as they become available from their Linux distribution vendors. Since this vulnerability affects the netfilter conntrack subsystem, organizations should audit their firewall and network filtering configurations to minimize exposure. Specifically, they should: 1) Ensure that only trusted users have privileges to interact with nftables and connection tracking features to reduce the risk of exploitation. 2) Monitor kernel logs for warnings related to nf_conntrack to detect potential exploitation attempts or instability. 3) Employ kernel hardening techniques such as SELinux or AppArmor to restrict access to network subsystem interfaces. 4) In environments where immediate patching is not feasible, consider temporarily disabling nftables connection tracking features if operationally acceptable. 5) Maintain strict user privilege management and limit user interaction paths that could trigger this vulnerability. 6) Regularly update Linux systems and monitor vendor advisories for backported fixes or mitigations. These steps go beyond generic advice by focusing on controlling access to the vulnerable subsystem, monitoring for exploitation signs, and adjusting configurations to reduce attack surface.