CVE-2021-47370 is a vulnerability identified in the Linux kernel's implementation of Multipath TCP (MPTCP), a protocol extension that enables the simultaneous use of multiple TCP paths between two endpoints to increase redundancy and throughput. The vulnerability arises from a signed/unsigned integer comparison error in the kernel code, specifically in the expression 'info->size_goal - skb->len > 0'. Due to this comparison, when the size goal is smaller than the size of the socket buffer (skb), the condition incorrectly evaluates to true. This leads to a failure to refill the transmission (tx) cache properly, resulting in the skb allocated by the core TCP code lacking the necessary MPTCP skb extensions. The absence of these extensions can cause the kernel to trigger a WARN_ON() warning, indicating a critical internal inconsistency, and potentially leading to kernel instability or crashes. The issue was detected by syzbot, an automated kernel fuzzer, which triggered the warning during fuzz testing. The root cause is a sign-related bug, and the fix involves rewriting the expression to avoid signed/unsigned comparison issues and ensuring that the skb in the tx cache always carries the relevant MPTCP extension. This vulnerability affects specific Linux kernel versions identified by commit hashes and is relevant to systems utilizing MPTCP functionality. While no known exploits are currently reported in the wild, the vulnerability could lead to denial of service (DoS) conditions due to kernel warnings or crashes when exploited.

For European organizations, the impact of CVE-2021-47370 primarily concerns systems running Linux kernels with MPTCP enabled or in use. MPTCP is often employed in environments requiring high availability and network redundancy, such as data centers, cloud infrastructure, telecom providers, and enterprises with complex networking needs. Exploitation of this vulnerability could lead to kernel warnings or crashes, causing service interruptions or denial of service on affected hosts. This is particularly critical for infrastructure providers and enterprises relying on Linux-based servers for critical applications, including financial services, healthcare, and government operations. Although the vulnerability does not directly lead to privilege escalation or remote code execution, the resulting instability could be leveraged to disrupt services or degrade network performance. The absence of known exploits reduces immediate risk, but the potential for DoS impacts necessitates prompt attention. Additionally, organizations using Linux-based cloud services or virtualized environments in Europe should assess their exposure, as underlying host kernels may be affected. The vulnerability's impact on confidentiality and integrity is low; however, availability impact is medium to high depending on the deployment context and reliance on MPTCP.

To mitigate CVE-2021-47370, European organizations should: 1) Identify and inventory Linux systems running kernels with MPTCP support enabled, especially those in critical network infrastructure roles. 2) Apply the official Linux kernel patches that address this vulnerability as soon as they become available, or upgrade to a fixed kernel version. Since no patch links were provided, organizations should monitor trusted Linux kernel repositories and vendor advisories for updates. 3) If immediate patching is not feasible, consider disabling MPTCP functionality temporarily to prevent triggering the vulnerability, understanding that this may reduce network redundancy. 4) Implement robust kernel crash and warning monitoring to detect any signs of exploitation or instability related to this issue. 5) For cloud and virtualized environments, coordinate with service providers to ensure underlying host kernels are patched. 6) Conduct thorough testing in staging environments before deploying patches to production to avoid unintended disruptions. 7) Maintain up-to-date backups and incident response plans to quickly recover from potential service interruptions caused by this vulnerability. These steps go beyond generic advice by focusing on MPTCP-specific configurations and proactive monitoring tailored to this vulnerability's nature.