CVE-2025-21972 is a vulnerability identified in the Linux kernel's networking stack, specifically related to the MCTP (Management Component Transport Protocol) implementation over USB. The issue arises from improper handling of packet reassembly when using the frag_list structure, which is responsible for managing fragmented packets. The vulnerability is due to the frag_list being shared among multiple packets during reassembly, which can lead to incorrect packet reassembly and a memory leak caused by circular references between packet fragments and their skb_shared_info structures. This flaw is particularly triggered by the use of skb_clone in the upcoming MCTP-over-USB driver, which clones socket buffers (SKBs) and inadvertently shares the frag_list. Other MCTP drivers do not share SKBs and are not affected by this issue. The Linux kernel developers have addressed this vulnerability by ensuring that the frag_list used during packet reassembly is not shared across packets, preventing incorrect reassembly and eliminating the memory leak. A kernel unit test (kunit) has been added to reproduce and verify the fix. Although no known exploits are currently reported in the wild, the vulnerability affects Linux kernel versions identified by the commit hash 4a992bbd365094730a31bae1e12a6ca695336d57 and potentially others in the same development branch. The vulnerability does not have an assigned CVSS score yet, but it involves kernel-level memory management and packet processing, which are critical components of system stability and security.

For European organizations, the impact of CVE-2025-21972 could be significant depending on their reliance on Linux-based systems that utilize the MCTP-over-USB driver or similar networking components. The vulnerability could lead to memory leaks and incorrect packet reassembly, potentially causing system instability, denial of service (DoS), or unpredictable behavior in network communications. This is particularly critical for organizations operating industrial control systems, telecommunications infrastructure, or data centers where Linux kernels are prevalent. Memory leaks in kernel space can degrade system performance over time and may be exploited in combination with other vulnerabilities to escalate privileges or disrupt services. Although no active exploits are known, the presence of this vulnerability in widely used Linux kernels means that attackers could develop exploits targeting affected systems. European entities in sectors such as finance, healthcare, manufacturing, and government, which often use Linux servers and embedded devices, could face operational disruptions or security breaches if this vulnerability is exploited.

To mitigate CVE-2025-21972, European organizations should prioritize updating their Linux kernel to the latest patched versions that include the fix for this vulnerability. Since the issue is related to the MCTP-over-USB driver and skb_clone usage, organizations should audit their systems to identify if this driver or related networking components are in use. If the MCTP-over-USB driver is not required, consider disabling or blacklisting it to reduce the attack surface. Network administrators should monitor kernel updates and apply them promptly, especially in production environments. Additionally, implementing kernel hardening techniques such as memory protection features (e.g., Kernel Address Space Layout Randomization - KASLR) and using security modules like SELinux or AppArmor can help limit the impact of potential exploitation. Regular system integrity checks and monitoring for unusual memory usage or network packet anomalies can aid in early detection of exploitation attempts. Finally, organizations should maintain a robust patch management process and test kernel updates in staging environments before deployment to avoid service disruptions.