A use‑after‑free vulnerability in the Windows Ancillary Function Driver for WinSock (AFD.sys) — tracked as CVE-2025-53147 — can allow an authorized local attacker to escalate privileges to a higher level on affected Windows systems by forcing the kernel driver to operate on freed memory. Microsoft’s advisory marks this as a kernel‑mode issue affecting the Winsock ancillary driver; defenders should treat the flaw as high‑priority for patching and detection because of the driver’s privileged context and repeated history of serious AFD vulnerabilities.
AFD.sys (the Ancillary Function Driver for WinSock) is the kernel component that implements low‑level socket semantics for Windows networking. Because it runs in kernel mode and services user‑mode networking calls across countless libraries and applications, bugs in AFD present disproportionately high risk: a local exploit that gains kernel control or corrupts kernel memory can be converted into SYSTEM‑level compromise. This class of vulnerabilities in AFD has recurred through 2024–2025 in multiple flavors (heap overflows, pointer validation issues, race conditions and use‑after‑free bugs), and they have been actively addressed in Microsoft security updates.
Defenders have a clear, practical path: inventory affected hosts, apply vendor patches immediately, harden local access and application control, and implement focused detection for abnormal AFD interactions and kernel integrity changes. The speed with which AFD vulnerabilities have been weaponized in 2025 underscores that rapid patching paired with solid EDR hunting is the only defensible strategy.
For authoritative mapping of CVE‑to‑KB and to confirm which builds are affected in your fleet, consult Microsoft’s Security Update Guide entry for CVE‑2025‑53147.
Acknowledgement of verification steps and gaps: Microsoft’s advisory provides the primary technical description that classifies CVE‑2025‑53147 as a use‑after‑free in AFD.sys and confirms local elevation potential; independent CVE databases and vendor analyses document the broader set of AFD issues addressed across 2025 and the operational urgency to patch. Where specific KB or CVSS metadata for CVE‑2025‑53147 was not visible in secondary sources at the time of analysis, administrators should verify those numeric and build mappings directly via the Security Update Guide and their enterprise patch channels. (msrc.microsoft.com, nvd.nist.gov)
Source: MSRC Security Update Guide - Microsoft Security Response Center
AFD.sys (the Ancillary Function Driver for WinSock) is the kernel component that implements low‑level socket semantics for Windows networking. Because it runs in kernel mode and services user‑mode networking calls across countless libraries and applications, bugs in AFD present disproportionately high risk: a local exploit that gains kernel control or corrupts kernel memory can be converted into SYSTEM‑level compromise. This class of vulnerabilities in AFD has recurred through 2024–2025 in multiple flavors (heap overflows, pointer validation issues, race conditions and use‑after‑free bugs), and they have been actively addressed in Microsoft security updates. What the advisory says (short)
- Vulnerability: Use‑after‑free in the Windows Ancillary Function Driver for WinSock (AFD.sys).
- Impact: Local elevation of privilege — an authorized local attacker may be able to escalate privileges.
- Attack vector: Local (requires ability to run code or create processes on the target).
Note: Microsoft’s Security Update Guide entry for this CVE is the authoritative source for affected builds and the specific KB updates that remediate the issue; administrators must map the advisory to their patch‑management channels to obtain the exact KB numbers.
Background: why AFD bugs matter
AFD.sys is invoked by Winsock APIs and many high‑level networking stacks. Because it manipulates kernel memory and interacts with user‑supplied socket state, a logic or memory‑management bug in AFD can yield powerful primitives for attackers:- A use‑after‑free may let an attacker cause the driver to operate on a freed object and then coerce the allocator to reuse that memory with attacker‑controlled data — a classic pathway to hijack kernel control-flow or create a write‑what‑where primitive.
- Kernel‑level primitives can be turned into overwrites of function pointers, I/O ring structures, or vtables to execute code at SYSTEM privilege. Real exploit writeups for similar AFD bugs show how I/O ring and dispatch table manipulation were converted into kernel writes.
- Historically, AFD vulnerabilities have been patched in Patch Tuesday and out‑of‑band cycles; several AFD issues in 2025 were included in emergency and monthly update packages — some of them were also added to government “Known Exploited Vulnerabilities” catalogs because of active exploitation.
Technical analysis
Class of bug: use‑after‑free (CWE‑416)
A use‑after‑free occurs when code frees a memory region but then later dereferences a pointer to that freed region. In kernel drivers, this can allow an attacker to:- Reuse freed memory for attacker data, then cause the kernel to dereference attacker‑controlled pointers.
- Overwrite kernel objects (function pointers, callback tables, vtables) to hijack kernel control flow.
- Produce subtle faults that evade basic crash‑based detection and, in some cases, allow quiet privilege escalation.
Exploitability and complexity
- Attack complexity is timing‑ and state‑dependent in many use‑after‑free cases, but automated attempts and fuzzers reduce the human effort needed to find a reliable exploitation window. Security researchers have rapidly developed reliable exploitation techniques for similar AFD use‑after‑free bugs in 2025.
- Exploitation requires the attacker to run code locally (or otherwise obtain local process execution). However, environments where users can run arbitrary code (helpdesk PCs, developer machines, terminal servers, RDS hosts) present a practical path for adversaries to escalate an initial foothold to SYSTEM.
- Historical precedent: once the public trigger/technique is known, proof‑of‑concept (PoC) exploits and weaponized code often appear quickly; that shortens the defense window and increases urgency to patch.
Verifying technical details and what could not be independently verified
- Microsoft’s Security Update Guide page for CVE‑2025‑53147 is the vendor advisory and the authoritative record for this CVE. The entry confirms the vulnerability class and impact as a use‑after‑free that can be abused for local privilege escalation. Administrators should consult the MSRC entry to map to the exact KB(s) for their Windows builds.
- Independent vulnerability databases and multiple security blogs corroborate that multiple AFD use‑after‑free issues were disclosed and patched in mid‑2025, and that several AFD CVEs were added to important mitigation catalogs (e.g., CISA KEV) because of active exploitation. However, if specific KB numbers or CVSS scoring for CVE‑2025‑53147 are not clearly shown in public copies of the advisory at the time of reading, that should be treated as a data gap — verify directly in the Security Update Guide or through your enterprise patch management feed. (nvd.nist.gov, cisa.gov)
Practical remediation steps (actionable checklist)
- Identify affected devices
- Query endpoint inventories for the afd.sys file version and Windows build. Flag devices where afd.sys does not match the patched version after the vendor rollout.
- Patch immediately
- Apply the Microsoft security update(s) that remediate CVE‑2025‑53147 through Windows Update, WSUS, MECM/Intune, or your enterprise patch tool. Prioritize systems with multi‑user access, remote access roles, or those that store sensitive credentials.
- Prioritize targets
- High: domain controllers, RDS/Terminal Servers, helpdesk machines, build/dev workstations.
- Medium: standard user desktops with sensitive access.
- Low: isolated labs and air‑gapped test machines (still patch, later).
- Temporary hardening where patching is delayed
- Enforce least privilege: remove local admin rights where feasible.
- Apply application control (Windows Defender Application Control / WDAC) and restrict unsigned drivers.
- Enable Kernel‑mode protections where available: HVCI / Memory integrity (if supported by hardware). These are complementary but not replacements for the patch.
- For suspected compromise
- Perform a full forensic capture of memory and disk, and consider reimaging if kernel compromise cannot be ruled out. Kernel‑level persistence is often not reliably removed without a rebuild.
Quick verification commands (examples)
- Check afd.sys file version and path on a host (PowerShell):
- Get‑ItemProperty -Path "C:\Windows\System32\drivers\afd.sys" | Select‑Object VersionInfo
- Compare the reported FileVersion/ProductVersion against your patch‑management’s expected patched version.
- Use centralized queries (EDR / SIEM):
- Flag devices where afd.sys timestamp or file version is older than the KB release timestamp.
- Create alerts for repeated DeviceIoControl/IOCTL loops coming from non‑privileged processes.
Detection and hunting guidance (practical)
- Monitor for unusual or repeated DeviceIoControl or IOCTL activity against the AFD device interface (control codes tied to AFD dispatch entries). Tight loops or repeated failed IOCTLs from a non‑privileged process are suspicious.
- Watch for rapid or repeated local process creation from low‑privilege accounts (automation loops that try to win timing windows).
- Kernel integrity and memory‑corruption telemetry:
- Use EDR telemetry to detect unexpected kernel writes, manipulated I/O ring structures, or abnormal allocations correlated to afd.sys.
- Defender for Endpoint, other EDRs, and kernel integrity monitors often expose such signals and should be tuned to flag anomalous afd.sys behavior. (ibm.com, ampcuscyber.com)
- Create a prioritized hunt:
- Search for processes performing DeviceIoControl calls to afd device path.
- Look for processes exhibiting tight retry loops, high kernel interaction, or multiple simultaneous socket operations from a single non‑privileged user context.
- Correlate these with post‑exploit behaviors: credential dumping, new local admin account creation, or unusual scheduled tasks.
Why this is urgent: operational impact and threat landscape
- AFD vulnerabilities have been repeatedly targeted because local escalation to SYSTEM unlocks the ability to uninstall defenses, dump domain credentials, and deploy ransomware payloads with full privileges. Multiple AFD issues in 2025 were handled as zero‑days and drew rapid exploit development from researchers and (potentially) adversaries. (securityblotter.com, cisa.gov)
- The short window from disclosure to PoC and weaponization — documented in public analyses of prior AFD bugs — increases the consequence of delayed patching. Even “local‑only” flaws are powerful in modern enterprise attack chains where initial access often comes via phishing, malware, or stolen credentials.
Critical analysis: strengths, weaknesses and residual risk
Strengths in the vendor and community response
- Microsoft publishes Security Update Guide entries and KB updates that enable enterprises to identify and remediate affected builds quickly; this centralization helps large fleets map advisories to specific patches.
- The security research community and commercial vendors produce rapid technical analyses and detection guidance after advisories, accelerating defender readiness and enabling targeted EDR hunts. (ibm.com, zeropath.com)
Notable weaknesses and risks
- Kernel‑mode networking code is large, complex and historically prone to subtle concurrency and memory‑management bugs; the repeated AFD CVEs in 2024–2025 suggest recurring engineering challenges in concurrency controls and pointer validation.
- Many practical mitigations beyond patching are limited; Microsoft advisories for previous AFD flaws often stated that no practical workarounds were available other than applying the vendor fix. That means organizations with slow patching processes face a persistent high risk.
- Detection of a successful kernel exploit is difficult; adversaries aim for stealth and often remove or disable detections at the kernel level once SYSTEM access is obtained. Reimaging is frequently the safest remediation after suspected kernel compromise.
Residual risk after patching
- Patch deployment reduces the direct risk of the specific CVE, but attackers can (and historically have) pivot to other kernel or user‑mode flaws. A robust posture therefore requires: timely patching, strict least‑privilege enforcement, application control, EDR coverage with kernel‑level telemetry, and practiced incident response playbooks.
Tactical recommendations for IT teams (concise)
- Map CVE‑2025‑53147 to KB numbers for every Windows build in your environment and deploy patches ASAP. Verify installation by checking afd.sys file versions across devices.
- Treat multi‑user and remote access hosts as highest priority. Reassess local admin population and lock down developer and helpdesk machines.
- Implement and tune EDR hunts for IOCTL/DeviceIoControl activity against afd.sys, and add alerts for tight process loops from non‑privileged users.
- If a system is suspected of kernel compromise, perform full forensic captures and consider reimaging rather than attempting surgical remediation. Kernel‑level persistence is notoriously hard to remove.
Final assessment
CVE‑2025‑53147 is another reminder that kernel networking components are high‑value targets. The combination of privileged execution context (AFD runs in kernel mode), repeated memory‑management classes of bugs (use‑after‑free, race conditions, buffer overflows), and an environment where local code execution is commonly obtained by adversaries (phishing, malware, lateral movement) makes this vulnerability category particularly dangerous.Defenders have a clear, practical path: inventory affected hosts, apply vendor patches immediately, harden local access and application control, and implement focused detection for abnormal AFD interactions and kernel integrity changes. The speed with which AFD vulnerabilities have been weaponized in 2025 underscores that rapid patching paired with solid EDR hunting is the only defensible strategy.
For authoritative mapping of CVE‑to‑KB and to confirm which builds are affected in your fleet, consult Microsoft’s Security Update Guide entry for CVE‑2025‑53147.
Acknowledgement of verification steps and gaps: Microsoft’s advisory provides the primary technical description that classifies CVE‑2025‑53147 as a use‑after‑free in AFD.sys and confirms local elevation potential; independent CVE databases and vendor analyses document the broader set of AFD issues addressed across 2025 and the operational urgency to patch. Where specific KB or CVSS metadata for CVE‑2025‑53147 was not visible in secondary sources at the time of analysis, administrators should verify those numeric and build mappings directly via the Security Update Guide and their enterprise patch channels. (msrc.microsoft.com, nvd.nist.gov)
Source: MSRC Security Update Guide - Microsoft Security Response Center