CVE-2026-21240 is a vulnerability classified under CWE-367, representing a time-of-check to time-of-use (TOCTOU) race condition in the Windows HTTP.sys driver on Windows 10 Version 1809 (build 10.0.17763.0). HTTP.sys is a kernel-mode driver that handles HTTP requests and responses, playing a critical role in network communications and web services on Windows. The race condition occurs when the system checks a resource's state and then uses it, but the state changes between these two operations, allowing an attacker to exploit the timing gap. This flaw enables an authorized local attacker to elevate privileges by manipulating the timing of operations in HTTP.sys, potentially gaining SYSTEM-level access. The vulnerability has a CVSS 3.1 base score of 7.8, indicating high severity, with attack vector local (AV:L), low attack complexity (AC:L), requiring low privileges (PR:L), and no user interaction (UI:N). The impact covers confidentiality, integrity, and availability (all high). No public exploits are known yet, and no patches are linked in the provided data, suggesting organizations must monitor for updates. The vulnerability is particularly dangerous because it allows privilege escalation without user interaction, increasing the risk of lateral movement and persistence in compromised environments. The flaw affects only Windows 10 Version 1809, a version still in use in some enterprise environments, especially where legacy systems persist. The race condition nature of the vulnerability makes it challenging to detect and mitigate without proper patching or system hardening.

The impact of CVE-2026-21240 is significant for organizations running Windows 10 Version 1809. Successful exploitation allows an attacker with authorized local access to escalate privileges to SYSTEM level, compromising the confidentiality, integrity, and availability of affected systems. This can lead to unauthorized access to sensitive data, installation of persistent malware, disruption of services, and further lateral movement within networks. Since HTTP.sys is a core component handling network traffic, exploitation could also affect web services and applications relying on it, potentially leading to broader service outages or data breaches. The vulnerability's low complexity and lack of user interaction requirements increase the risk of exploitation in environments where local access is possible, such as shared workstations, virtual machines, or compromised user accounts. Organizations with critical infrastructure, sensitive data, or regulatory compliance requirements face heightened risks, including financial loss, reputational damage, and legal consequences. The absence of known exploits in the wild currently provides a window for proactive mitigation, but the vulnerability remains a high-priority threat due to its potential impact and ease of exploitation.

1. Apply official patches from Microsoft immediately once they become available for Windows 10 Version 1809 to address the TOCTOU race condition in HTTP.sys. 2. Until patches are released, restrict local access to systems running the affected version by enforcing strict access controls and limiting user privileges to the minimum necessary. 3. Monitor system logs and network activity for unusual behavior related to HTTP.sys, such as unexpected privilege escalations or abnormal HTTP traffic patterns. 4. Employ application whitelisting and endpoint detection and response (EDR) solutions to detect and block suspicious activities indicative of exploitation attempts. 5. Consider upgrading affected systems to a supported and patched Windows version to reduce exposure to this and other vulnerabilities. 6. Conduct regular security audits and vulnerability assessments focusing on local privilege escalation vectors. 7. Educate system administrators and users about the risks of local privilege escalation and the importance of maintaining updated systems. 8. Implement network segmentation to limit the impact of compromised systems and reduce the attack surface for local exploits. These steps go beyond generic advice by emphasizing proactive monitoring, access restriction, and system upgrades tailored to the specific nature of this vulnerability.