CVE-2026-20877 is a use-after-free vulnerability classified under CWE-416 found in Windows Management Services on Microsoft Windows 10 Version 1809 (build 10.0.17763.0). Use-after-free vulnerabilities occur when a program continues to use memory after it has been freed, potentially leading to arbitrary code execution or privilege escalation. In this case, the flaw allows an authorized attacker with low privileges to locally elevate their privileges to higher levels, potentially SYSTEM level, without requiring user interaction. The vulnerability has a CVSS 3.1 base score of 7.8, 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 scope changed (S:C). The impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H), meaning exploitation could lead to full system compromise. Although no known exploits are currently in the wild, the vulnerability poses a significant risk to systems that remain unpatched. The vulnerability was reserved in December 2025 and published in January 2026, with no patch links currently available, suggesting that mitigations or updates may be forthcoming. The vulnerability affects a legacy Windows 10 version, which is still in use in some enterprise environments, especially where upgrade cycles are slow or constrained by compatibility requirements.

For European organizations, this vulnerability presents a serious risk, especially for those still operating Windows 10 Version 1809 in production environments. Successful exploitation could allow attackers to escalate privileges locally, bypassing security controls and gaining administrative access. This could lead to unauthorized access to sensitive data, disruption of critical services, and deployment of further malware or ransomware. Sectors such as government, finance, healthcare, and critical infrastructure are particularly vulnerable due to the potential impact on confidentiality and availability. The requirement for local access limits remote exploitation but insider threats or attackers who gain initial footholds via other means could leverage this vulnerability to deepen their control. The lack of current known exploits provides a window for mitigation, but organizations must act proactively to prevent exploitation. The high attack complexity and privilege requirements reduce the likelihood of widespread exploitation but do not eliminate the risk to targeted attacks.

Organizations should prioritize upgrading or patching Windows 10 Version 1809 systems as soon as security updates become available from Microsoft. Until patches are released, restrict local access to critical systems by enforcing strict access controls and monitoring user activities for suspicious behavior. Employ application whitelisting and endpoint detection and response (EDR) solutions to detect abnormal privilege escalation attempts. Regularly audit and minimize the number of users with local access or administrative privileges. Consider isolating legacy systems from the broader network to reduce the attack surface. Additionally, implement robust logging and alerting mechanisms to identify potential exploitation attempts early. Where possible, accelerate migration to supported Windows versions with active security support to reduce exposure to legacy vulnerabilities.