CVE-2026-21240 is a time-of-check to time-of-use (TOCTOU) race condition vulnerability identified in the Windows HTTP.sys driver component of Microsoft Windows 10 Version 1809 (build 10.0.17763.0). HTTP.sys is a kernel-mode device driver that handles HTTP requests and is critical for web server functionality on Windows systems. The vulnerability arises because of a race condition between the time a resource or permission is checked and the time it is used, allowing an attacker with local authorized access to exploit the timing gap to elevate their privileges. Specifically, an attacker could manipulate the timing of operations in HTTP.sys to gain higher privileges than originally granted, potentially leading to full system compromise. The vulnerability does not require user interaction but does require the attacker to have some level of local privileges (PR:L). The CVSS v3.1 base score is 7.8, indicating high severity, with impacts rated high on confidentiality, integrity, and availability. No public exploits have been reported yet, but the vulnerability is significant due to the critical role of HTTP.sys and the potential for privilege escalation. The issue is tracked under CWE-367, which covers race conditions related to TOCTOU flaws. As of the published date, no patches have been linked, suggesting organizations should monitor for updates and apply them promptly once available.

For European organizations, this vulnerability poses a serious risk, especially for those running legacy Windows 10 Version 1809 systems in environments where local user access is possible. Successful exploitation could allow attackers to escalate privileges from a limited local user account to SYSTEM-level access, compromising the confidentiality, integrity, and availability of critical systems. This could lead to unauthorized data access, system manipulation, and disruption of services, particularly impacting sectors such as government, finance, healthcare, and critical infrastructure where Windows 10 1809 remains in use. The lack of known exploits in the wild currently reduces immediate risk, but the high severity and ease of exploitation by authorized users mean that threat actors could develop exploits rapidly. European organizations with strict regulatory requirements for data protection (e.g., GDPR) could face compliance and reputational damage if exploited. Additionally, the vulnerability could be leveraged as a stepping stone for lateral movement within networks, increasing the scope of potential damage.

Organizations should prioritize the following mitigations: 1) Apply official patches from Microsoft immediately once they become available for Windows 10 Version 1809 systems to eliminate the race condition. 2) Restrict local user access to systems running vulnerable versions, enforcing the principle of least privilege to minimize the number of users who could exploit the vulnerability. 3) Implement robust endpoint detection and response (EDR) solutions to monitor for unusual privilege escalation attempts or suspicious activity involving HTTP.sys. 4) Harden system configurations by disabling unnecessary services that rely on HTTP.sys if feasible, reducing the attack surface. 5) Conduct regular audits of local user accounts and permissions to ensure no unauthorized accounts exist. 6) Employ network segmentation to limit lateral movement opportunities if an escalation occurs. 7) Educate IT staff about the vulnerability and encourage vigilance for signs of exploitation. 8) Maintain up-to-date backups to enable recovery in case of compromise. These targeted actions go beyond generic advice by focusing on controlling local access, monitoring HTTP.sys activity, and preparing for patch deployment.