Windows Ancillary for WinSock 0-Day Vulnerability Actively Exploited to Gain Admin Access.

Microsoft has confirmed active exploitation of a critical privilege escalation vulnerability in the Windows Ancillary Function Driver for WinSock, tracked as CVE-2025-32709.

This use-after-free flaw enables local attackers with basic user privileges to gain SYSTEM-level access, posing significant risks to unpatched systems.

First publicly documented on 13 May 2025, the vulnerability carries a base score of 7.8 (High) and a temporal score of 6.8, reflecting confirmed in-the-wild attacks prior to patch availability.

The vulnerability resides in the Windows Ancillary Function Driver (AFD), a kernel-mode component managing Winsock API operations for network socket communications.

Attackers exploit a race condition in how AFD handles socket descriptor metadata, where improper memory management allows reallocated buffer regions to maintain stale pointers.

By crafting specific IOCTL (Input/Output Control) requests, threat actors can force the driver to dereference freed memory blocks, enabling arbitrary code execution in kernel space.

Successful exploitation requires local access with standard user privileges but no user interaction, making it particularly effective in multi-user environments or post-breach scenarios.

The flaw’s vector string confirms its localized attack vector with high confidentiality, integrity, and availability impacts.

“Use after free in Windows Ancillary Function Driver for WinSock allows an authorized attacker to elevate privileges locally.”

“An attacker who successfully exploited this vulnerability could gain administrator privileges.” Microsoft stated.

Exploitation Landscape and Mitigation

Microsoft’s security team confirmed limited but targeted attacks leveraging CVE-2025-32709 across healthcare and government sectors since April 2025.

Threat actors combine this vulnerability with phishing campaigns or existing malware installs to elevate privileges, often deploying ransomware payloads or credential harvesters post-exploitation.

Notably, the exploit leaves minimal forensic traces by leveraging legitimate driver functions, complicating detection.

As of 14 May 2025, Microsoft has released security updates through KB5036899 for Windows 10/11 and Server 2022 editions.

Organizations should prioritize patching systems exposed to untrusted users, particularly remote desktop gateways and terminal servers.

For environments requiring delayed updates, Microsoft recommends:

• Enabling Hypervisor-protected Code Integrity (HVCI).
• Restricting administrator privileges via Least User Access policies.
• Monitoring for anomalous AFD.sys memory allocation patterns using Defender for Endpoint.

The active exploitation of CVE-2025-32709 underscores systemic challenges in securing legacy driver components within modern Windows architectures.

AFD’s deep integration with core networking stacks and frequent exposure to user-mode inputs makes it a persistent attack surface.

Microsoft’s decision to classify this as an Important rather than Critical severity has drawn scrutiny from independent researchers, given the vulnerability’s privileged escalation capabilities and confirmed weaponization.

As kernel-level vulnerabilities increasingly feature in ransomware and APT campaigns, organizations must audit driver dependencies and isolate high-risk components through virtualization or sandboxing.

Microsoft continues to enhance driver guardrails in Windows 12 preview builds, including enhanced memory tagging and IOCTL filtering-capabilities expected to mitigate similar flaws in future releases.

Security teams should treat any system showing unexpected privilege escalations or anomalous socket activity as potentially compromised.

Forensic investigations must prioritize AFD.sys memory dumps and Winsock API call logs to identify exploitation attempts.

With nation-state groups historically targeting Windows driver vulnerabilities, CVE-2025-32709 likely represents the first wave of attacks leveraging this vector in 2025.

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