CVE-2026-45603 Overview
CVE-2026-45603 is a use-after-free vulnerability in the Windows Ancillary Function Driver for WinSock (afd.sys). The flaw allows an authenticated local attacker to elevate privileges on an affected Windows system. Microsoft classifies the issue under CWE-362, a concurrent execution flaw involving improper synchronization that leads to a race condition. Successful exploitation grants the attacker the ability to run code with higher privileges than originally permitted, compromising confidentiality, integrity, and availability of the host.
Critical Impact
A local authorized user can win a race condition in afd.sys to trigger a use-after-free and elevate privileges to SYSTEM on the affected Windows host.
Affected Products
- Microsoft Windows (Ancillary Function Driver for WinSock, afd.sys)
- Refer to the Microsoft CVE-2026-45603 Update for the specific Windows build list
- Server and client editions running the vulnerable AFD driver
Discovery Timeline
- 2026-06-09 - CVE-2026-45603 published to the National Vulnerability Database
- 2026-06-09 - Last updated in NVD database
Technical Details for CVE-2026-45603
Vulnerability Analysis
The vulnerability resides in the Windows Ancillary Function Driver for WinSock, the kernel-mode component that exposes socket functionality to user-mode applications through the WinSock API. AFD handles IOCTL requests from user-mode and manages object lifetimes for socket-related structures. A race condition between concurrent operations on a shared kernel object causes the driver to reference memory after it has been freed.
An authorized attacker with a local logon can issue carefully timed socket operations to reach the vulnerable code path. Although the attack complexity is high because the attacker must win a narrow race window, success yields full impact on confidentiality, integrity, and availability. The privileges gained are typically those of the kernel, enabling SYSTEM-level code execution.
Root Cause
The defect is a [CWE-362] concurrent execution flaw leading to a use-after-free. Two threads operate on the same AFD object without sufficient synchronization. One thread releases the object while another retains a dangling reference and dereferences it. The freed allocation can be reclaimed by attacker-controlled data, allowing manipulation of kernel object fields such as function pointers.
Attack Vector
The attack vector is local. The attacker must already possess valid credentials and the ability to execute code on the target. From that foothold, the attacker drives afd.sys through repeated socket creation, I/O, and teardown sequences to provoke the race. User interaction is not required. Remote exploitation is not in scope for this issue, though it pairs well with initial-access techniques such as phishing or commodity loaders. Refer to the Microsoft CVE-2026-45603 Update for vendor technical detail.
No public proof-of-concept code is available at publication time. Verified exploitation code is not included here.
Detection Methods for CVE-2026-45603
Indicators of Compromise
- Unexpected SYSTEM-level child processes spawned from medium-integrity user sessions shortly after socket-heavy workloads
- Kernel bugchecks referencing afd.sys or pool corruption on hosts that previously ran stably
- New scheduled tasks, services, or driver loads created by accounts that lack administrative rights
Detection Strategies
- Hunt for processes that suddenly transition from a standard user token to a SYSTEM token without an authentic privilege-granting parent
- Alert on unsigned or anomalous code execution following intensive WinSock IOCTL activity from a single non-administrative process
- Correlate Windows Error Reporting crashes in afd.sys with subsequent privilege changes on the same host
Monitoring Recommendations
- Forward Sysmon Event ID 1, 10, and 13 plus Security Event ID 4688 to a central analytics platform for token elevation analysis
- Monitor kernel pool and driver crash telemetry through Windows Error Reporting and the System event log
- Track non-administrative accounts performing high volumes of socket creation and teardown over short intervals
How to Mitigate CVE-2026-45603
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft CVE-2026-45603 Update to all Windows endpoints and servers
- Restrict interactive and remote logon rights so that only trusted accounts can execute code locally
- Enforce least privilege on service accounts and remove unnecessary local users from multi-user systems
Patch Information
Microsoft has published an advisory and update for CVE-2026-45603. Administrators should consult the Microsoft CVE-2026-45603 Update page for the exact KB articles, affected Windows builds, and replacement binaries for afd.sys. Deploy updates through Windows Update, Windows Server Update Services, Microsoft Intune, or Configuration Manager according to standard patch management policy.
Workarounds
- No vendor-approved workaround replaces patching; deploy the official update as the primary remediation
- Where patching is delayed, limit local logon to administrators and disable non-essential local accounts
- Apply application control policies such as Windows Defender Application Control to block unsigned binaries that could deliver an exploit
# Configuration example: verify patch presence and AFD driver version on Windows
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 20
Get-Item C:\Windows\System32\drivers\afd.sys | Select-Object Name, VersionInfo, LastWriteTime
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
