CVE-2026-33100: Windows WinSock Use-After-Free Privilege Escalation

CVE-2026-33100 is a use-after-free vulnerability present within the Windows Ancillary Function Driver for WinSock. This flaw enables an attacker with local access and a degree of authorization to escalate their privileges on the system. The vulnerability stems from improper memory management within the WinSock driver, leading to potential access of freed memory. Exploitation of this vulnerability would allow an attacker to execute arbitrary code with elevated privileges. Microsoft has acknowledged this vulnerability and assigned it a CVSS v3.1 base score of 7.0, highlighting the potential for significant impact if exploited. Defenders should prioritize patching systems to prevent potential exploitation and privilege escalation.

Attack Chain

1. An attacker gains local access to a Windows system with some level of authorization.
2. The attacker crafts a malicious program that triggers the use-after-free vulnerability in the Windows Ancillary Function Driver for WinSock (afunix.sys).
3. The malicious program interacts with the WinSock API to allocate and free memory related to ancillary functions.
4. The attacker exploits the timing of memory allocation and deallocation to cause the WinSock driver to access freed memory.
5. By manipulating the freed memory, the attacker can overwrite critical data structures within the kernel.
6. The attacker overwrites function pointers or other security-sensitive data, allowing them to redirect execution flow.
7. The attacker executes arbitrary code within the kernel context.
8. The attacker achieves elevated privileges, potentially gaining full control over the system.

Impact

Successful exploitation of CVE-2026-33100 allows an attacker to elevate their privileges from a standard user account to SYSTEM level. This could allow them to install programs; view, change, or delete data; or create new accounts with full user rights. The vulnerability could be exploited as part of a post-exploitation phase in a targeted attack to gain complete control of a compromised system. The number of potential victims is very large, as it affects a core component of the Windows operating system.

Recommendation

• Apply the security update released by Microsoft to patch CVE-2026-33100 and prevent exploitation of the use-after-free vulnerability in the Windows Ancillary Function Driver for WinSock. Refer to the Microsoft Security Response Center advisory for specific patch information (https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-33100).
• Enable Sysmon process creation logging to potentially detect malicious processes spawned by an exploited WinSock vulnerability.
• Deploy the Sigma rule provided to detect exploitation attempts of CVE-2026-33100 based on suspicious process execution.