CVE-2026-27922: Windows 10 1607 Privilege Escalation Flaw

CVE-2026-27922 Overview

CVE-2026-27922 is a use-after-free vulnerability (CWE-416) in the Windows Ancillary Function Driver for WinSock (afd.sys). The flaw allows an authenticated local attacker to elevate privileges on affected Windows client and server systems. Microsoft published the advisory on April 14, 2026, covering Windows 10, Windows 11, and Windows Server releases from 2012 through 2025. Successful exploitation gives an attacker SYSTEM-level code execution from a low-privileged user context. The Ancillary Function Driver is a kernel-mode component that backs the Winsock user-mode API, making it a recurring target for local privilege escalation research.

Critical Impact

A local authenticated attacker can elevate to SYSTEM by triggering a use-after-free condition in the kernel-mode WinSock driver, resulting in full host compromise.

Affected Products

• Microsoft Windows 10 (1607, 1809, 21H2, 22H2) across x86, x64, and ARM64
• Microsoft Windows 11 (23H2, 24H2, 25H2, 26H1) across x64 and ARM64
• Microsoft Windows Server 2012, 2012 R2, 2016, 2019, 2022, 2022 23H2, and 2025

Discovery Timeline

• 2026-04-14 - Microsoft publishes the CVE-2026-27922 advisory
• 2026-04-14 - CVE-2026-27922 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-27922

Vulnerability Analysis

The vulnerability is a use-after-free condition inside the Ancillary Function Driver for WinSock (afd.sys). The driver exposes kernel functionality to user-mode socket APIs and processes I/O Request Packets (IRPs) issued through NtDeviceIoControlFile against the \Device\Afd device. A use-after-free occurs when kernel code continues to operate on a heap object after its backing memory has been freed and potentially reallocated for an attacker-controlled structure.

When the freed object is reclaimed with data attacker controls, subsequent dereferences of the dangling pointer can be steered to corrupt kernel memory or hijack control flow. Because afd.sys runs in kernel mode, successful manipulation yields execution at ring 0 with SYSTEM privileges. The advisory notes high attack complexity, indicating the attacker must win a timing or state-dependent condition to reliably trigger the dangling reference.

Root Cause

The root cause is improper object lifetime management within afd.sys. The driver releases a reference to a kernel object while another code path retains and later uses a pointer to that object. The flaw is classified as [CWE-416: Use After Free] and is consistent with prior AFD vulnerabilities tied to socket state transitions, asynchronous I/O completion, and reference counting on internal handle structures.

Attack Vector

Exploitation requires local code execution as an authenticated user. The attacker opens a handle to the AFD device, issues a crafted sequence of socket IOCTLs to drive the driver into the vulnerable state, then races a second operation to reclaim the freed allocation with controlled data. No user interaction is required. The local-only vector means the bug is typically chained behind an initial access primitive such as phishing, a browser exploit, or abuse of an existing low-privileged service account.

No verified public proof-of-concept code is available at the time of writing. Technical details are described in prose per Microsoft's CVE-2026-27922 Advisory.

Detection Methods for CVE-2026-27922

Indicators of Compromise

• Unexpected child processes spawned by low-privileged parents that immediately run as NT AUTHORITY\SYSTEM
• Bugcheck events (BSOD) referencing afd.sys or kernel pool corruption on otherwise stable hosts
• Anomalous handle opens to \Device\Afd from non-network applications, followed by rapid DeviceIoControl bursts
• New service installations, scheduled tasks, or token manipulation occurring shortly after a user logon

Detection Strategies

• Hunt for token impersonation and parent-child process integrity mismatches indicative of local privilege escalation
• Correlate kernel crash telemetry (WER, MEMORY.DMP) implicating afd.sys with subsequent privileged process creation
• Baseline normal Winsock IOCTL patterns per process and alert on outliers from non-networking binaries
• Apply behavioral analytics that flag the post-exploitation sequence (SYSTEM shell, credential dumping) rather than the exploit primitive alone

Monitoring Recommendations

• Enable Windows kernel and process auditing (Event IDs 4688, 4672) with command-line logging
• Centralize EDR and Sysmon telemetry to a SIEM for cross-host correlation of privilege escalation patterns
• Monitor patch compliance for the April 2026 cumulative updates across all Windows endpoints and servers
• Track logons from service accounts and review any unexpected interactive session activity

How to Mitigate CVE-2026-27922

Immediate Actions Required

• Apply the April 2026 Microsoft security updates referenced in the Microsoft CVE-2026-27922 Advisory to all affected Windows and Windows Server systems
• Prioritize patching of multi-user systems, terminal servers, and developer workstations where low-privileged local access is common
• Restrict local logon rights and remove unnecessary interactive users from servers
• Audit for signs of prior exploitation on systems that remained unpatched after the disclosure date

Patch Information

Microsoft addressed CVE-2026-27922 through cumulative updates released alongside the April 14, 2026 advisory. Updates are available for Windows 10 (1607 through 22H2), Windows 11 (23H2 through 26H1), and Windows Server 2012 through 2025. Administrators should deploy through Windows Update, Windows Server Update Services (WSUS), Microsoft Update Catalog, or their enterprise patch management platform. Verify installation by confirming the updated afd.sys build number on representative hosts.

Workarounds

• No vendor-supplied workaround is listed; patching is the supported remediation
• Reduce exposure by enforcing least-privilege and removing local administrator rights for standard users
• Apply application control (Windows Defender Application Control, AppLocker) to block execution of untrusted binaries that could deliver the exploit
• Enable attack surface reduction rules and tamper protection on endpoint security agents to limit post-exploitation activity

# Verify patch deployment status on a Windows host
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 10

# Inspect afd.sys version to confirm the updated driver is loaded
Get-Item C:\Windows\System32\drivers\afd.sys | Select-Object Name, VersionInfo