CVE-2026-85941 in Windows
Summary
by MITRE • 10/14/2026
Concurrent execution using shared resource with improper synchronization ('race condition') in Windows Installer allows an authorized attacker to elevate privileges locally.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability described constitutes a classic race condition within the Microsoft Windows Installer service, specifically involving concurrent access to shared resources without adequate synchronization mechanisms. This flaw arises when multiple processes or threads attempt to read and modify the same data structure simultaneously, but the system fails to enforce mutual exclusion during critical sections of execution. In the context of Windows Installer, this typically involves operations related to package installation, configuration updates, or registry modifications where temporary files or state variables are created and accessed by both the installer service and potentially other local processes. The absence of proper locking mechanisms allows an attacker with authorized but limited user privileges to exploit the timing gap between resource allocation and validation steps. By rapidly executing competing requests that target these shared resources, the attacker can manipulate the outcome of the installation or configuration process, effectively bypassing security checks designed to prevent unauthorized changes.
From a technical perspective, this race condition falls under CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization. The core issue lies in the temporal dependency between checking a resource's state and acting upon it, known as Time-of-Check to Time-of-Use (TOCTOU). When an attacker triggers multiple concurrent operations that modify or read these shared resources, they can force the installer into an inconsistent state. For instance, if the installer checks for file existence before writing new data, a race condition could allow the attacker to replace the target path with a symbolic link pointing to a sensitive system location during the brief window between the check and the write operation. This manipulation enables the overwriting of critical system files or registry keys that are normally protected from standard user-level modifications, thereby facilitating local privilege escalation.
The operational impact of this vulnerability is significant as it allows an attacker with valid credentials on the affected Windows system to elevate their privileges to those of a higher-privileged account, such as SYSTEM or Administrator. Once elevated, the attacker gains unrestricted access to the operating system, including the ability to install arbitrary software, view, change, or delete sensitive data, and create new accounts with full control rights. This compromises the integrity, confidentiality, and availability of the affected system. In enterprise environments, this can lead to lateral movement if the compromised account is used to attack other systems on the network. The vulnerability aligns with MITRE ATT&CK technique T1068: Exploitation for Privilege Escalation, specifically under sub-techniques involving exploitation of software vulnerabilities or misconfigurations within operating system components.
Mitigation strategies primarily involve applying vendor-supplied patches that address the synchronization flaws in the Windows Installer service. Organizations should ensure that all systems are updated with the latest security updates provided by Microsoft to close these race condition gaps. Additionally, implementing strict access controls and least-privilege principles can limit the potential impact if an attacker gains initial foothold. Monitoring for unusual patterns of concurrent file system or registry operations may also help in detecting exploitation attempts in real-time. Security administrators should prioritize patching systems running vulnerable versions of Windows Installer to prevent unauthorized privilege escalation through this specific vector.