CVE-2025-59242 Overview
CVE-2025-59242 is a heap-based buffer overflow [CWE-122] in the Windows Ancillary Function Driver for WinSock (afd.sys). The flaw allows an authenticated local attacker to corrupt kernel heap memory and elevate privileges to SYSTEM. Microsoft published the advisory on October 14, 2025, covering client and server editions from Windows 10 1507 through Windows 11 25H2 and Windows Server 2008 through Windows Server 2025. Because afd.sys is a core kernel-mode component reachable from user mode through standard socket handles, exploitation requires only a low-privileged local account and no user interaction.
Critical Impact
Successful exploitation grants attackers SYSTEM-level privileges on the affected Windows host, enabling full compromise of the operating system, credential theft, and disabling of security controls.
Affected Products
- Microsoft Windows 10 (1507, 1607, 1809, 21H2, 22H2)
- Microsoft Windows 11 (22H2, 23H2, 24H2, 25H2)
- Microsoft Windows Server 2008, 2012, 2016, 2019, 2022, 2022 23H2, and 2025
Discovery Timeline
- 2025-10-14 - CVE-2025-59242 published to NVD and Microsoft releases security update
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-59242
Vulnerability Analysis
The vulnerability resides in the Ancillary Function Driver for WinSock, afd.sys, which brokers user-mode socket API calls into the Windows kernel networking stack. A heap-based buffer overflow in the driver allows an attacker holding a valid socket handle to write data beyond the bounds of a kernel pool allocation. Since afd.sys runs in kernel mode, corrupted memory can be leveraged to overwrite adjacent pool objects, kernel structures, or function pointers.
Attackers with local access can trigger the flaw through crafted DeviceIoControl requests against the \Device\Afd device object. The result is arbitrary kernel memory corruption leading to privilege escalation from a standard user to NT AUTHORITY\SYSTEM. This class of flaw in afd.sys has been repeatedly abused in the wild for local privilege escalation chains.
Root Cause
The root cause is insufficient bounds validation on data written to a heap allocation inside afd.sys when processing input received through the WinSock IOCTL interface. The driver allocates a buffer based on caller-controlled parameters but writes more data than the destination region can hold, resulting in a classic heap-based buffer overflow [CWE-122].
Attack Vector
Exploitation requires local access and low privileges. An attacker executes code as any authenticated user, obtains a handle to the AFD device, and issues a crafted IOCTL that triggers the overflow. Successful shaping of the kernel pool typically leads to overwrite of adjacent objects and controlled kernel-mode code execution. No user interaction is required and the attack completes entirely on the target machine.
The vulnerability manifests inside the AFD IOCTL dispatch path. Technical specifics of the vulnerable code path have not been released; refer to the Microsoft CVE-2025-59242 Advisory for authoritative details.
Detection Methods for CVE-2025-59242
Indicators of Compromise
- Unexpected loading or crash dumps referencing afd.sys on Windows endpoints or servers.
- New SYSTEM-level processes spawned from a parent that is a non-privileged interactive user process.
- Kernel bugchecks (BSODs) with codes such as KERNEL_MODE_HEAP_CORRUPTION or DRIVER_OVERRAN_STACK_BUFFER originating in afd.sys.
Detection Strategies
- Monitor for anomalous DeviceIoControl calls targeting \Device\Afd from processes that do not normally perform socket operations.
- Correlate low-integrity processes suddenly performing SYSTEM-level actions such as service creation, LSASS access, or security tool tampering.
- Baseline kernel crash telemetry and alert on new crash signatures involving the Ancillary Function Driver.
Monitoring Recommendations
- Enable Windows Event Log forwarding for kernel and application crashes, and centralize WER reports.
- Track process lineage and token elevation events (Event ID 4672 in combination with 4688) for unexpected escalations.
- Ingest EDR telemetry into a centralized data lake to hunt across fleets for patterns of AFD exploitation attempts.
How to Mitigate CVE-2025-59242
Immediate Actions Required
- Apply the October 2025 Microsoft security updates that address CVE-2025-59242 across all affected Windows client and server SKUs.
- Prioritize patching multi-user systems such as terminal servers, VDI hosts, and developer workstations where local access is broadly available.
- Audit local user and service accounts; remove unnecessary interactive logon rights to reduce the attacker pool.
Patch Information
Microsoft released fixes for all supported Windows versions on October 14, 2025. Refer to the Microsoft CVE-2025-59242 Advisory for the specific KB article and cumulative update applicable to each Windows build. Legacy versions such as Windows Server 2008 and 2012 require Extended Security Updates (ESU) coverage to receive the patch.
Workarounds
- No official workaround exists; the patch is the authoritative mitigation.
- Restrict local logon and remote desktop access to trusted administrative accounts until patches are deployed.
- Enforce application allowlisting to block execution of untrusted binaries that could deliver an exploit payload.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
