CVE-2026-42911 Overview
CVE-2026-42911 is a use-after-free vulnerability [CWE-416] in the Windows Ancillary Function Driver for WinSock (afd.sys). The flaw allows an authorized local attacker to elevate privileges on an affected Windows system. Successful exploitation grants the attacker SYSTEM-level code execution from a low-privileged user context.
Microsoft published the advisory through the MSRC update guide. The vulnerability requires local access and valid credentials, but no user interaction. Exploitation complexity is rated high, reflecting the timing and memory layout constraints typical of kernel use-after-free conditions.
Critical Impact
Local authorized attackers can elevate to SYSTEM privileges by exploiting freed kernel memory in the WinSock driver, achieving full compromise of confidentiality, integrity, and availability.
Affected Products
- Microsoft Windows (Ancillary Function Driver for WinSock — afd.sys)
- Refer to the Microsoft Security Update CVE-2026-42911 advisory for the specific build and KB list
- Both Windows client and Windows Server SKUs typically ship afd.sys and should be reviewed
Discovery Timeline
- 2026-06-09 - CVE-2026-42911 published to the National Vulnerability Database
- 2026-06-09 - Last updated in NVD database
Technical Details for CVE-2026-42911
Vulnerability Analysis
The Ancillary Function Driver for WinSock is a kernel-mode driver that backs the Windows socket API. User-mode applications communicate with afd.sys through DeviceIoControl calls and asynchronous I/O routines. The driver maintains kernel objects representing socket state, endpoint contexts, and I/O request packets.
A use-after-free condition exists when the driver references a kernel object after it has been freed. An attacker who can trigger the free path and race a second operation on the same handle can cause the driver to dereference a stale pointer. Reclaiming the freed allocation with attacker-controlled data leads to controlled kernel memory corruption.
Because afd.sys runs in kernel mode, successful exploitation yields execution at ring 0. The attacker pivots from a standard user token to a SYSTEM token, typically by overwriting a process token pointer in EPROCESS or by hijacking a function pointer reachable from the corrupted object.
Root Cause
The root cause is improper lifetime management of a kernel object inside afd.sys. Reference counting or synchronization around the object is insufficient, allowing one code path to free the allocation while another path still holds and dereferences a pointer to it.
Attack Vector
The attack vector is local. An authenticated user runs an exploit binary that opens an AFD device handle and issues a sequence of socket-related IOCTLs. The exploit races a teardown operation against a concurrent operation on the same object, then sprays the kernel pool to reclaim the freed slot with controlled data before the stale pointer is dereferenced.
No verified public proof-of-concept code was available at the time of writing. See the Microsoft Security Update CVE-2026-42911 for vendor technical details.
Detection Methods for CVE-2026-42911
Indicators of Compromise
- Unexpected SYSTEM token assignment on processes spawned by standard user accounts
- Bugcheck events referencing afd.sys (for example, BAD_POOL_HEADER, KERNEL_MODE_HEAP_CORRUPTION, or DRIVER_VERIFIER_DETECTED_VIOLATION) on patched-behind hosts
- New or unsigned binaries opening \\Device\\Afd followed by privilege-escalation behavior
Detection Strategies
- Hunt for low-privileged processes that suddenly spawn child processes running as NT AUTHORITY\\SYSTEM without a legitimate service or scheduled task origin
- Alert on kernel crash dumps in afd.sys correlated with recent user-mode process creation events
- Monitor for handle creation on \\Device\\Afd followed by repeated NtDeviceIoControlFile calls in tight loops, a pattern consistent with race-condition exploitation
Monitoring Recommendations
- Forward Windows Security event ID 4688 (process creation) and Sysmon event ID 1 to a central data lake for token-elevation analytics
- Enable kernel pool tracking through Driver Verifier on canary hosts to surface use-after-free patterns during testing
- Track patch deployment status across all Windows endpoints and flag hosts missing the June 2026 cumulative update
How to Mitigate CVE-2026-42911
Immediate Actions Required
- Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-42911 to all affected Windows systems
- Prioritize multi-user systems, terminal servers, and developer workstations where untrusted local code is most likely to run
- Audit local account membership and remove unnecessary interactive logon rights to reduce the population of potential attackers
Patch Information
Microsoft addressed CVE-2026-42911 in the security update published on 2026-06-09. Install the cumulative update or standalone package listed in the MSRC advisory for each affected Windows version. Reboot is required for the updated afd.sys driver to load.
Workarounds
- No vendor-supplied workaround replaces the patch; install the security update as soon as testing permits
- Restrict local logon and remote interactive sessions to trusted administrators on high-value systems until patching completes
- Enforce application allowlisting to block execution of unsigned or unknown binaries that could deliver an exploit
# Verify the afd.sys version after patch installation
Get-Item C:\\Windows\\System32\\drivers\\afd.sys | Select-Object VersionInfo
# Confirm the relevant KB is installed (replace KBxxxxxxx with the KB from the MSRC advisory)
Get-HotFix -Id KBxxxxxxx
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
