CVE-2024-26826 is a vulnerability identified in the Linux kernel's implementation of Multipath TCP (MPTCP), specifically related to the handling of stale subflows and data re-injection mechanisms. MPTCP is an extension of TCP that allows a single connection to use multiple paths simultaneously, improving redundancy and throughput. The vulnerability arises because when the MPTCP Path Manager (PM) detects a subflow as stale, the kernel is supposed to re-inject all unacknowledged MPTCP-level data across other active subflows. To optimize performance and avoid unnecessary locking, the code attempts an early check to determine if any unacknowledged data exists in the retransmission (RTX) queue. However, this check is flawed because it uses a TCP-specific helper function on an MPTCP socket, which is inappropriate due to structural differences between TCP and MPTCP socket representations. This results in the check always failing, causing the re-injection process to be skipped erroneously. The issue was exposed by a recent unrelated TCP kernel change that reorganized TCP socket variables, which altered memory layouts and caused the MPTCP code to consistently skip re-injection. Although fuzzers and static analysis tools did not flag this as a memory corruption issue (since the accessed memory still belongs to the MPTCP socket structure), the functional impact is significant: unacknowledged data may not be retransmitted properly, potentially leading to data loss or connection degradation. The fix involved removing the incorrect early optimization check and ensuring the re-injection logic executes correctly on the slow path. This vulnerability affects Linux kernel versions containing the specified commit hashes and was published on April 17, 2024. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, this vulnerability could impact any systems running vulnerable Linux kernel versions with MPTCP enabled or in use. MPTCP is increasingly used in environments requiring high availability and redundancy, such as data centers, cloud infrastructure, mobile networks, and enterprise networks that leverage multipath routing for resilience and performance. Failure to properly re-inject unacknowledged data on stale subflows can lead to degraded network performance, increased latency, or even data loss in multipath TCP sessions. This could affect critical services relying on stable and reliable network connections, including financial services, telecommunications, healthcare systems, and government infrastructure. While the vulnerability does not appear to allow direct code execution or privilege escalation, the disruption of network traffic integrity and availability could have cascading effects on business operations and service continuity. Given the widespread use of Linux in European IT infrastructure, especially in cloud and telecom sectors, the impact could be significant if left unpatched. The lack of known exploits suggests that attackers have not yet weaponized this flaw, but the potential for denial-of-service-like conditions or data transmission failures warrants prompt attention.

European organizations should prioritize updating their Linux kernels to versions containing the patch that fixes CVE-2024-26826 as soon as it becomes available. Since no official patch links are provided yet, monitoring Linux kernel mailing lists and trusted security advisories for the patch release is critical. In the interim, organizations should audit their use of MPTCP, disabling it if not required, to reduce exposure. Network administrators should also monitor network performance and error rates on multipath TCP connections for anomalies that could indicate issues related to this vulnerability. For environments where MPTCP is essential, consider implementing additional network-level redundancy and failover mechanisms to mitigate potential data loss or connection degradation. Furthermore, organizations should review their kernel update policies to ensure rapid deployment of critical security fixes, especially for infrastructure components handling sensitive or critical data. Employing kernel live patching solutions where feasible can reduce downtime and accelerate remediation. Finally, maintaining comprehensive network monitoring and logging will aid in early detection of any exploitation attempts once the vulnerability becomes more widely known.