CVE-2026-21253 is a use-after-free vulnerability classified under CWE-416, found in the Mailslot File System component of Microsoft Windows 10 Version 1607 (build 10.0.14393.0). Use-after-free vulnerabilities occur when a program continues to use a pointer after the memory it points to has been freed, leading to undefined behavior such as memory corruption, crashes, or arbitrary code execution. In this case, the flaw allows an authorized attacker with low privileges to execute a local privilege escalation attack by exploiting the improper handling of memory in the mailslot file system. The attacker does not require user interaction but must have local access and some level of authorization on the system. The CVSS v3.1 base score is 7.0, indicating high severity, with attack vector local (AV:L), attack complexity high (AC:H), privileges required low (PR:L), no user interaction (UI:N), and impacts on confidentiality, integrity, and availability all rated high (C:H/I:H/A:H). The vulnerability was reserved in December 2025 and published in February 2026. No patches or exploits are currently publicly available, but the vulnerability poses a significant risk due to its potential to allow privilege escalation and full system compromise on affected Windows 10 1607 systems.

For European organizations, the impact of CVE-2026-21253 is substantial, particularly for those still operating legacy Windows 10 Version 1607 systems. Successful exploitation could allow attackers to elevate privileges from a low-privileged user to SYSTEM level, enabling full control over affected machines. This could lead to unauthorized access to sensitive data, disruption of critical services, and deployment of further malware or ransomware. Sectors such as government, healthcare, finance, and critical infrastructure that rely on legacy Windows environments are especially vulnerable. The lack of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, as attackers may develop exploits once the vulnerability details are widely known. The high impact on confidentiality, integrity, and availability means that exploitation could result in severe operational and reputational damage for European entities.

Given the absence of an official patch at the time of this report, European organizations should implement several practical mitigations: 1) Restrict local access to systems running Windows 10 Version 1607 by enforcing strict access controls and limiting user privileges to the minimum necessary. 2) Employ application whitelisting and endpoint detection and response (EDR) solutions to monitor and block suspicious activities related to mailslot file system operations. 3) Isolate legacy systems from critical network segments to reduce the attack surface and lateral movement opportunities. 4) Regularly audit and monitor logs for unusual privilege escalation attempts or memory corruption indicators. 5) Plan and prioritize upgrading or migrating from Windows 10 Version 1607 to supported, patched versions of Windows to eliminate exposure to this and other legacy vulnerabilities. 6) Once Microsoft releases a security update, apply it promptly following organizational change management procedures.