CVE-2025-21295 is a high-severity use-after-free vulnerability (CWE-416) affecting Microsoft Windows 10 Version 1809 (build 10.0.17763.0). The flaw resides in the SPNEGO Extended Negotiation (NEGOEX) security mechanism, which is part of the authentication protocols used in Windows environments to negotiate security mechanisms between clients and servers. A use-after-free vulnerability occurs when a program continues to use a pointer after the memory it points to has been freed, potentially leading to arbitrary code execution, memory corruption, or system crashes. In this case, the vulnerability allows a remote unauthenticated attacker to execute arbitrary code on the affected system without requiring user interaction. The CVSS v3.1 base score is 8.1, indicating a high severity, with the attack vector being network-based (AV:N), requiring high attack complexity (AC:H), no privileges (PR:N), and no user interaction (UI:N). The impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H), meaning successful exploitation could lead to full system compromise. The vulnerability is publicly disclosed but no known exploits are currently observed in the wild, and no patches have been linked yet. Given the nature of NEGOEX in authentication, exploitation could allow attackers to bypass security controls and execute code remotely, potentially compromising enterprise environments that rely on Windows 10 Version 1809 for critical operations.

For European organizations, this vulnerability poses a significant risk, especially for those still operating legacy Windows 10 Version 1809 systems. The ability for an unauthenticated remote attacker to execute arbitrary code could lead to widespread compromise of enterprise networks, data breaches, ransomware deployment, or disruption of critical services. Sectors such as finance, healthcare, government, and critical infrastructure, which often have stringent security and compliance requirements, could face severe operational and reputational damage. Additionally, since NEGOEX is involved in authentication, exploitation could undermine trust in identity and access management systems, potentially allowing lateral movement within networks. The lack of user interaction and privileges required for exploitation increases the risk of automated attacks and wormable scenarios. The absence of known exploits currently provides a window for proactive mitigation, but organizations must act swiftly to prevent future attacks.

European organizations should immediately identify and inventory all systems running Windows 10 Version 1809 (build 17763.0) to assess exposure. Given the lack of an official patch link, organizations should monitor Microsoft security advisories closely for updates or emergency patches. In the interim, network-level mitigations should be applied: restrict inbound access to services using SPNEGO/NEGOEX protocols, especially from untrusted networks; implement strict network segmentation and firewall rules to limit exposure; enable enhanced logging and monitoring for unusual authentication attempts or network activity related to NEGOEX. Organizations should also consider upgrading affected systems to supported Windows versions with active security updates. Employing endpoint detection and response (EDR) solutions to detect anomalous behavior indicative of exploitation attempts is recommended. Finally, conducting penetration testing and vulnerability assessments focusing on authentication mechanisms can help identify and remediate related weaknesses.