CVE-2025-21764 is a high-severity vulnerability identified in the Linux kernel, specifically within the Neighbor Discovery Protocol (NDP) implementation for IPv6, in the function ndisc_alloc_skb(). The vulnerability arises because ndisc_alloc_skb() can be invoked without holding the RTNL (rtnetlink) lock or Read-Copy-Update (RCU) protection, which are synchronization mechanisms designed to prevent concurrent access issues. The absence of proper RCU protection can lead to a Use-After-Free (UAF) condition, classified under CWE-416. A UAF vulnerability occurs when a program continues to use a pointer after the memory it points to has been freed, potentially allowing attackers to execute arbitrary code, cause kernel crashes, or escalate privileges. The CVSS 3.1 base score of 7.8 reflects a high severity, with attack vector Local (AV:L), low attack complexity (AC:L), requiring low privileges (PR:L), no user interaction (UI:N), unchanged scope (S:U), and high impact on confidentiality, integrity, and availability (C:H/I:H/A:H). This means an attacker with local access and limited privileges could exploit this flaw to gain full control over the affected system without needing user interaction. The vulnerability was resolved by adding RCU protection in the ndisc_alloc_skb() function to ensure safe concurrent access and prevent the UAF condition. No known exploits are currently reported in the wild, but the potential impact warrants prompt attention. The affected versions are identified by specific commit hashes, indicating the vulnerability is present in certain Linux kernel builds prior to the patch. This vulnerability is critical for systems relying on IPv6 networking and running vulnerable Linux kernel versions, especially in environments where local access by untrusted users or processes is possible.

For European organizations, the impact of CVE-2025-21764 can be significant, particularly for enterprises, government agencies, and service providers that rely heavily on Linux-based infrastructure with IPv6 enabled. Exploitation could lead to full system compromise, allowing attackers to access sensitive data, disrupt services, or pivot within networks. This is especially critical for sectors such as finance, healthcare, telecommunications, and critical infrastructure, where confidentiality, integrity, and availability are paramount. Since the vulnerability requires local access, insider threats or compromised user accounts could be leveraged to exploit this flaw. Additionally, many European organizations are accelerating IPv6 adoption, increasing the exposure surface. The potential for privilege escalation and kernel-level code execution could undermine trust in Linux-based systems and lead to costly incident response and remediation efforts. The absence of known exploits in the wild currently provides a window for proactive patching and mitigation before widespread exploitation occurs.

1. Immediate patching: Organizations should prioritize updating their Linux kernels to versions that include the fix for CVE-2025-21764. This involves applying vendor-supplied patches or upgrading to the latest stable kernel releases where the RCU protection has been implemented in ndisc_alloc_skb(). 2. Access control tightening: Restrict local access to Linux systems, especially those exposed to untrusted users or processes. Implement strict user privilege management and monitor for unusual local activity. 3. Network segmentation: Isolate critical Linux systems running IPv6 to limit the ability of attackers to gain local access through lateral movement. 4. Kernel hardening: Employ additional kernel security modules such as SELinux or AppArmor to reduce the impact of potential exploits. 5. Monitoring and detection: Deploy kernel-level monitoring tools and intrusion detection systems capable of identifying suspicious behavior indicative of UAF exploitation attempts. 6. IPv6 usage review: Evaluate the necessity of IPv6 on systems where it is not required and consider disabling it temporarily until patches are applied. 7. Incident response readiness: Prepare for potential exploitation by ensuring backups, forensic capabilities, and response plans are up to date.