CVE-2025-54099 Overview
CVE-2025-54099 is a stack-based buffer overflow [CWE-121] in the Windows Ancillary Function Driver for WinSock (afd.sys). The flaw allows an authorized local attacker to elevate privileges on affected Windows client and server systems. Microsoft published the advisory on September 9, 2025, covering supported releases from Windows 10 1507 through Windows 11 24H2 and Windows Server 2008 through Windows Server 2025.
Critical Impact
Successful exploitation grants SYSTEM-level code execution on the local host, providing full control over the operating system kernel context from an authenticated user session.
Affected Products
- Microsoft Windows 10 (1507, 1607, 1809, 21H2, 22H2)
- Microsoft Windows 11 (22H2, 23H2, 24H2)
- Microsoft Windows Server 2008, 2012, 2016, 2019, 2022, 2022 23H2, and 2025
Discovery Timeline
- 2025-09-09 - CVE-2025-54099 published to NVD and Microsoft security advisory released
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-54099
Vulnerability Analysis
The Ancillary Function Driver for WinSock (afd.sys) is a kernel-mode driver that bridges the user-mode Winsock API and the Windows TCP/IP stack. Applications interact with afd.sys through IOCTL calls issued against the \Device\Afd device object. Because the driver executes in ring 0, any memory corruption within its request-handling code translates into a kernel-level primitive.
CVE-2025-54099 is a stack-based buffer overflow triggered when the driver processes attacker-controlled input from user mode. Writing beyond the fixed-size kernel stack buffer overwrites adjacent stack data, including saved return addresses and function pointers used during I/O request completion. Exploitation results in privilege escalation from a standard user context to NT AUTHORITY\SYSTEM.
Root Cause
The vulnerability stems from missing or insufficient bounds validation on a length or offset field supplied through a Winsock IOCTL before the driver copies data into a stack-allocated structure. Because the check occurs after the copy operation or is calculated using untrusted values, oversized inputs corrupt the kernel stack frame [CWE-121].
Attack Vector
Exploitation requires local authenticated access. An attacker running arbitrary code as a low-privileged user opens a handle to \Device\Afd and issues a crafted DeviceIoControl request with a malformed buffer. The high attack complexity in the CVSS vector reflects the need to bypass kernel mitigations such as kASLR, SMEP, kCFG, and stack cookies to achieve reliable code execution. This class of afd.sys flaw is a recurring target in post-exploitation toolchains and commodity local privilege escalation exploits.
No public proof-of-concept or in-the-wild exploitation has been reported for CVE-2025-54099 at the time of writing. See the Microsoft CVE-2025-54099 Advisory for vendor technical details.
Detection Methods for CVE-2025-54099
Indicators of Compromise
- Unexpected System or csrss.exe-parented processes spawned from low-privileged user sessions after Winsock activity.
- Bugcheck or Kernel-Power events referencing afd.sys on hosts where the patch has not been applied.
- Newly created privileged local accounts or scheduled tasks following execution of an unsigned or unusual binary by a standard user.
Detection Strategies
- Hunt for user-mode processes opening handles to \Device\Afd followed by anomalous DeviceIoControl patterns and immediate token manipulation.
- Correlate token elevation events (Windows Event ID 4672 assigned to non-admin users) with the parent process lineage of standard user sessions.
- Baseline legitimate Winsock consumers and alert on unsigned or newly written binaries issuing sockets IOCTLs.
Monitoring Recommendations
- Enable kernel-mode crash dump collection and forward MiniDump artifacts referencing afd.sys to a central data lake for analysis.
- Ingest Sysmon Event IDs 1, 10, and 11 to track process creation, handle access, and file writes tied to suspected LPE tooling.
- Track deployment status of the September 2025 Patch Tuesday cumulative updates across the fleet through configuration management.
How to Mitigate CVE-2025-54099
Immediate Actions Required
- Apply the September 2025 Microsoft security update that addresses CVE-2025-54099 to all supported Windows client and server SKUs.
- Prioritize patching multi-user systems, terminal servers, VDI hosts, and developer workstations where local code execution by non-admin users is routine.
- Audit local group membership and remove unnecessary interactive logon rights for standard users on high-value hosts.
Patch Information
Microsoft released fixes as part of the September 9, 2025 update cycle. Refer to the Microsoft CVE-2025-54099 Advisory for the KB article and build numbers that correspond to each affected Windows version. Legacy platforms such as Windows Server 2008 and 2012 require the appropriate Extended Security Update (ESU) channel.
Workarounds
- No official workaround exists; Microsoft directs administrators to install the security update.
- Restrict local logon and remote interactive sessions to trusted administrative users until patching completes.
- Enforce application allow-listing (for example, Windows Defender Application Control or AppLocker) to block execution of unsigned binaries that could deliver an LPE payload.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
