CVE-2021-47593 is a vulnerability identified in the Linux kernel related to the Multipath TCP (MPTCP) implementation. MPTCP is an extension of the traditional TCP protocol that allows a single TCP connection to use multiple paths to maximize resource usage and increase redundancy. The vulnerability arises from improper handling of fallback sockets within the MPTCP Upper Layer Protocol (ULP) extension. Specifically, the kernel flag 'kern' is not cleared from fallback sockets, which causes the socket to be incorrectly identified as a kernel socket. This misclassification allows the setsockopt system call with the TCP_ULP option set to 'mptcp' to be applied to plain TCP sockets, which should normally be prevented for user-space sockets. When accept() returns a plain TCP socket as a fallback, the socket remains tagged as 'kernel', enabling the setsockopt call to succeed improperly. This leads to a null pointer dereference in the subflow_data_ready function, causing a kernel crash (BUG: KASAN: null-ptr-deref). The crash occurs because the subflow extension attempts to access a NULL ctx->conn pointer. This vulnerability can cause denial of service (DoS) by crashing the kernel, potentially leading to system instability or reboot. The vulnerability affects Linux kernel versions identified by the commit hash cf7da0d66cc1a2a19fc5930bb746ffbb2d4cd1be and was published on June 19, 2024. There are no known exploits in the wild at this time, and no CVSS score has been assigned yet.

For European organizations, this vulnerability poses a significant risk primarily in environments running Linux kernels with MPTCP enabled or where fallback sockets might be used. The impact is mainly a denial of service due to kernel crashes, which can disrupt critical services, especially in data centers, cloud infrastructure, telecommunications, and enterprise environments relying on Linux servers. Systems that use MPTCP for network redundancy or performance improvements are particularly vulnerable. The kernel crash could lead to downtime, loss of availability, and potential cascading failures in dependent services. Although this vulnerability does not directly lead to privilege escalation or data leakage, the resulting service interruptions could affect business continuity, compliance with service-level agreements, and operational reliability. European sectors such as finance, healthcare, and public administration, which rely heavily on Linux-based infrastructure, could face operational disruptions. Additionally, organizations using Linux in embedded systems or network appliances might experience outages or degraded network performance.

To mitigate this vulnerability, European organizations should: 1) Apply the latest Linux kernel patches that address CVE-2021-47593 as soon as they become available. Monitoring official Linux kernel repositories and vendor advisories is critical. 2) Temporarily disable MPTCP if it is not essential for operations, reducing the attack surface until patches are applied. 3) Implement kernel crash monitoring and automated recovery mechanisms to minimize downtime in case of exploitation. 4) Conduct thorough testing of network applications using MPTCP to identify fallback socket usage and ensure they handle socket options correctly. 5) Employ network segmentation and strict access controls to limit exposure of vulnerable systems to untrusted networks or users. 6) Maintain up-to-date backups and disaster recovery plans to recover quickly from potential service interruptions. 7) Collaborate with Linux distribution vendors to ensure timely patch deployment and validation in production environments.