Windows WinSock Race Condition Privilege Escalation (CVE-2026-26173)

CVE-2026-26173 describes a race condition vulnerability within the Windows Ancillary Function Driver for WinSock. This vulnerability enables an authorized, local attacker to achieve privilege escalation on a vulnerable system. The specifics of exploitation aren’t detailed, but the core issue lies in the improper synchronization when the driver handles shared resources under concurrent execution. This vulnerability, reported on 2026-04-14, could allow an attacker to gain elevated system privileges and potentially take control of the compromised machine. While the exact scope of exploitation is yet unknown, successful exploitation would have a significant impact on the confidentiality, integrity, and availability of the targeted system.

Attack Chain

1. The attacker gains local access to the target Windows system.
2. The attacker triggers concurrent execution of specific operations within the WinSock driver using a crafted application.
3. The race condition occurs when multiple threads attempt to access and modify shared resources within the Ancillary Function Driver simultaneously.
4. Due to improper synchronization, one thread may read or write data in an inconsistent or unexpected state, leading to memory corruption.
5. The attacker exploits the memory corruption to overwrite critical system data structures related to privilege levels.
6. The attacker manipulates their own process token or security context by modifying the overwritten system data.
7. The attacker’s process gains elevated privileges, such as SYSTEM, allowing them to perform privileged operations.
8. The attacker leverages these elevated privileges to install malware, modify system settings, or exfiltrate sensitive data.

Impact

Successful exploitation of CVE-2026-26173 allows a local attacker to elevate their privileges to SYSTEM. This privilege escalation could allow attackers to install programs; view, change, or delete data; or create new accounts with full user rights. The impact is significant as it allows a complete compromise of the affected system. This could lead to data theft, system instability, or the deployment of ransomware.

Recommendation

• Apply the security update provided by Microsoft to patch CVE-2026-26173 as soon as possible (https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-26173).
• Monitor for unusual process creation events originating from system processes related to WinSock using the provided Sigma rule.
• Enable auditing of privilege use, and deploy the provided Sigma rule to identify potential privilege escalation attempts.