CVE-2021-47132 is a vulnerability identified in the Linux kernel's implementation of Multipath TCP (MPTCP), specifically related to the handling of the sk_forward_memory field during retransmissions. MPTCP is an extension of TCP that allows the simultaneous use of multiple paths between endpoints to increase redundancy and throughput. The vulnerability arises because the sk_forward_memory field, which is protected by the msk socket spin_lock (a synchronization primitive), is being updated in certain code paths without properly acquiring this lock. The affected code path is __mptcp_retrans() calling __mptcp_clean_una_wakeup(), where several helper functions update sk_forward_alloc without holding the necessary msk spin lock. This improper locking can lead to memory corruption of the sk_forward_memory field, potentially causing kernel instability or crashes. The issue was reported by a security researcher named Matthieu and addressed by introducing a new variant of the implicated function that explicitly acquires the msk spin lock before modifying the field, ensuring proper synchronization and preventing corruption. This vulnerability affects specific Linux kernel versions identified by their commit hashes, indicating it is present in certain recent kernel builds prior to the patch. No known exploits have been reported in the wild as of the publication date (March 15, 2024).

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions with MPTCP enabled or in use. Potential impacts include kernel memory corruption leading to system instability, crashes, or denial of service (DoS). In environments where MPTCP is leveraged for critical network redundancy or performance (such as data centers, telecom infrastructure, or cloud services), exploitation could disrupt network communications or degrade service availability. Although no remote code execution or privilege escalation is indicated, the instability could be exploited by local attackers or malicious processes to cause denial of service or potentially facilitate further attacks by destabilizing kernel memory. Given the widespread use of Linux in European enterprise servers, cloud infrastructure, and embedded systems, unpatched systems could face operational disruptions. The absence of known exploits reduces immediate risk, but the vulnerability's nature warrants prompt remediation to avoid potential future exploitation.

European organizations should prioritize updating their Linux kernel to versions that include the patch for CVE-2021-47132. Specifically, kernel maintainers have introduced a variant of the affected function that properly acquires the msk spin lock, eliminating the race condition and memory corruption. Organizations should: 1) Identify all systems running vulnerable Linux kernel versions, especially those utilizing MPTCP features. 2) Apply official kernel updates or patches from trusted Linux distributions as soon as they become available. 3) If immediate patching is not feasible, consider disabling MPTCP functionality temporarily to mitigate risk, if operationally acceptable. 4) Monitor system logs and kernel messages for signs of memory corruption or instability related to MPTCP retransmissions. 5) Employ kernel hardening techniques and runtime protections to reduce the impact of potential kernel memory corruption. 6) Maintain strict access controls to limit local user capabilities, as exploitation would likely require local access. These steps go beyond generic advice by focusing on MPTCP usage and kernel synchronization issues specific to this vulnerability.