CVE-2025-21765 is a vulnerability identified in the Linux kernel's IPv6 networking stack, specifically within the function ip6_default_advmss(). This function is responsible for determining the default advertised maximum segment size (MSS) for IPv6 TCP connections. The vulnerability arises because ip6_default_advmss() was not using Read-Copy-Update (RCU) protection when accessing the 'net' structure, which is a critical kernel data structure representing network namespaces. Without RCU protection, there is a risk that the 'net' structure could be freed or modified concurrently while ip6_default_advmss() is reading it, leading to use-after-free conditions or data races. Such conditions can cause kernel memory corruption, potentially resulting in system crashes (denial of service) or enabling privilege escalation attacks if exploited by a malicious actor. The fix involves adding RCU protection to ensure that the 'net' structure remains valid during the function's execution, preventing concurrent modifications or deallocation. Although no known exploits are currently reported in the wild, the vulnerability affects all Linux kernel versions prior to the patch and impacts any system utilizing IPv6 networking. Given the widespread use of Linux in servers, cloud infrastructure, and embedded devices, this vulnerability poses a significant risk if left unpatched.

For European organizations, the impact of CVE-2025-21765 can be substantial due to the extensive deployment of Linux-based systems in critical infrastructure, enterprise servers, cloud environments, and telecommunications. Exploitation could lead to denial of service through kernel crashes, disrupting business operations and service availability. More critically, successful exploitation might allow attackers to escalate privileges on affected systems, potentially gaining root access and compromising sensitive data or control over networked resources. This is particularly concerning for sectors such as finance, healthcare, government, and energy, where Linux servers often handle sensitive workloads. Additionally, IPv6 adoption is increasing in Europe, making this vulnerability more relevant. The absence of known exploits currently reduces immediate risk, but the vulnerability's nature means it could be targeted in future attacks, especially by sophisticated threat actors. Organizations relying on Linux kernel versions prior to the fix are at risk of operational disruption and data breaches if the vulnerability is exploited.

European organizations should prioritize updating their Linux kernel to the latest patched versions that include the RCU protection fix in ip6_default_advmss(). Kernel updates should be tested and deployed promptly across all affected systems, especially those exposed to untrusted networks or running IPv6. Network administrators should audit IPv6 usage and consider temporarily disabling IPv6 where feasible until patches are applied, to reduce exposure. Employing kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR), Control Flow Integrity (CFI), and enabling security modules like SELinux or AppArmor can provide additional defense layers. Monitoring kernel logs for unusual crashes or anomalies related to networking functions can help detect exploitation attempts. Organizations should also review and restrict access to systems with IPv6 enabled, ensuring only authorized users have administrative privileges. Finally, maintaining an up-to-date inventory of Linux systems and their kernel versions will facilitate rapid identification and remediation of vulnerable hosts.