Windows LUAFV TOCTOU Vulnerability Allows Local Privilege Escalation (CVE-2026-27929)

A time-of-check time-of-use (TOCTOU) race condition vulnerability, identified as CVE-2026-27929, exists within the Windows LUAFV (likely referring to a component related to Least-Privilege User Account Filtering). This vulnerability enables a locally authenticated attacker to elevate their privileges on the system. The vulnerability stems from the way LUAFV handles file operations, creating a window where an attacker can manipulate a file between the time it is checked for permissions and the time it is actually used. Microsoft has assigned this vulnerability a CVSS v3.1 score of 7.0, indicating a high severity. Successful exploitation leads to unauthorized privilege escalation, potentially granting the attacker administrative control over the compromised system.

Attack Chain

1. The attacker, with limited privileges, identifies a file or resource protected by LUAFV.
2. The attacker crafts a malicious program designed to exploit the TOCTOU vulnerability.
3. The malicious program initiates a file operation (e.g., accessing, modifying, or executing) on the target resource.
4. LUAFV performs a security check to determine if the attacker has the necessary permissions for the requested file operation.
5. The attacker leverages a race condition to modify the target resource between the security check and the actual file operation, potentially bypassing the intended access controls. This might involve rapidly replacing a legitimate file with a symbolic link pointing to a sensitive system file.
6. LUAFV, acting on the outdated or manipulated state of the resource, grants the attacker elevated privileges.
7. The attacker leverages the elevated privileges to execute arbitrary code, install malicious software, or access sensitive data.
8. The attacker achieves persistent access to the system with escalated privileges.

Impact

Successful exploitation of CVE-2026-27929 allows a local attacker with limited privileges to escalate their privileges to SYSTEM level. This would allow the attacker to perform actions such as installing programs, viewing, changing, or deleting data, or creating new accounts with full user rights. Given the local nature of the attack, its impact is primarily confined to individual systems; however, in environments where users share systems or rely on specific permission models, this vulnerability poses a significant threat.

Recommendation

• Apply the security update released by Microsoft to address CVE-2026-27929 as soon as possible. Refer to the Microsoft Security Response Center advisory linked in the references.
• Enable process creation logging to monitor for suspicious processes launched by low-privileged users that might indicate exploitation attempts (e.g., running whoami /priv from different contexts).
• Implement the provided Sigma rule to detect potential exploitation attempts by monitoring for unexpected modifications within protected LUAFV areas.
• Monitor for registry modifications related to LUAFV configurations, as attackers may attempt to weaken or disable security measures after privilege escalation.