CVE-2026-27922 Windows WinSock Use-After-Free Privilege Escalation

CVE-2026-27922 is a use-after-free vulnerability affecting the Windows Ancillary Function Driver for WinSock. This vulnerability, reported on April 14, 2026, allows a locally authenticated attacker to elevate their privileges on a vulnerable system. The vulnerability exists within the kernel-mode driver responsible for handling ancillary functions within the Windows Sockets API. Successful exploitation could allow an attacker to execute arbitrary code with elevated privileges, potentially leading to system compromise. This vulnerability is important for defenders to address as it allows an attacker to move laterally within a network and gain complete control over a compromised host.

Attack Chain

1. An attacker gains initial access to a Windows system with valid user credentials.
2. The attacker crafts a malicious application that interacts with the Windows Ancillary Function Driver for WinSock.
3. The application triggers the use-after-free vulnerability by manipulating Winsock API calls to free a memory object.
4. The attacker reallocates the freed memory with controlled data.
5. The Windows Ancillary Function Driver attempts to access the freed memory, now containing attacker-controlled data.
6. This access leads to code execution under the security context of the kernel.
7. The attacker leverages this code execution to escalate privileges to SYSTEM.
8. With SYSTEM privileges, the attacker can perform actions such as installing malware, modifying system configurations, or accessing sensitive data.

Impact

Successful exploitation of CVE-2026-27922 allows a local attacker to elevate their privileges to SYSTEM. This could lead to complete system compromise, including the installation of malware, data theft, or modification of system settings. The impact is significant for any organization using affected versions of Windows, as it provides a pathway for attackers to bypass security controls and gain complete control over critical systems. While the number of victims is currently unknown, the potential for widespread impact is high given the ubiquitous nature of Windows.

Recommendation

• Apply the security update provided by Microsoft to patch CVE-2026-27922 as soon as possible. This can be found at the URL listed in the references.
• Enable Sysmon process-creation logging to activate the process creation rule below.
• Deploy the Sigma rules in this brief to your SIEM and tune for your environment to detect potential exploitation attempts.
• Monitor for unusual or unexpected activity originating from processes interacting with the Windows Sockets API based on the descriptions in the "Attack Chain" section.