CVE-2025-53141: Null Pointer in AFD.sys Enables Local SYSTEM Elevation (WinSock)

Microsoft’s advisory confirms that a null pointer dereference in the Windows Ancillary Function Driver for WinSock (AFD.sys) can be triggered by a locally authorized attacker to elevate privileges to SYSTEM, creating a high-impact local elevation-of-privilege (EoP) risk for affected Windows hosts.

Background​

The Windows Ancillary Function Driver for WinSock (commonly known as AFD.sys) is a kernel-mode driver that implements core socket and networking primitives used by Winsock and a broad set of user-mode network APIs. Because AFD operates in kernel context, flaws in the driver can produce disproportionately severe outcomes — from system instability and denial-of-service to full SYSTEM compromise when an attacker chains a local EoP into broader persistence and credential theft. This class of issues has been an active focus across multiple Microsoft security updates in 2024–2025, making any new AFD-related advisory especially important for defenders. (bleepingcomputer.com, action1.com)
Microsoft’s published entry for CVE-2025-53141 states the core symptom succinctly: a null pointer dereference in the Ancillary Function Driver for WinSock allows a local, authorized attacker to elevate privileges. The vendor’s guidance emphasizes that the attack vector is local — exploitation requires the ability to run code on the target machine — and that the underlying fault is a kernel dereference of a pointer that can be controlled or made null by user-mode inputs.

---

What the vulnerability is — technical summary​

• Vulnerability type: Null pointer dereference in the WinSock AFD.sys kernel driver.
• Impact: Local elevation of privilege to SYSTEM for an attacker who can execute code on the host.
• Attack vector: Local; requires running a process or code on the target system.
• Exploit complexity: Timing and context dependent. Null pointer dereferences often cause crashes, but attackers can sometimes manipulate execution or memory layout to convert seemingly benign faults into privilege-elevation primitives.
• Typical trigger path: crafted socket or IOCTL interactions that reach the vulnerable AFD code path and cause the kernel to dereference an invalid (NULL) pointer. (msrc.microsoft.com, zeropath.com)

Why a null pointer dereference in kernel code matters​

Null pointer dereferences are commonly associated with crash-only behavior, but in kernel drivers they are not necessarily harmless. Attackers and researchers have long shown that memory-management and concurrency flaws can be abused to:

• Force an unexpected code path that leaves kernel state exploitable.
• Convert a crash-prone bug into a memory-corruption primitive by manipulating timing, allocations, or kernel object lifetimes.
• Combine the bug with other local primitives (e.g., information leaks, uninitialized memory reads, or controlled allocations) to achieve arbitrary kernel memory writes or function-pointer overwrites.

In the case of the AFD family, previous AFD.sys bugs in 2024–2025 included use-after-free, heap overflow, and untrusted pointer dereference variants — a pattern that highlights both the attack surface and the speed with which AFD bugs have been weaponized when PoCs surface. Public reporting and vendor advisories from 2025 demonstrate this recurring pattern and the operational urgency to patch AFD-related holes. (zeropath.com, bleepingcomputer.com)

---

Affected systems and deployment guidance​

Microsoft’s advisory entry lists AFD.sys as the affected component; the common impact matrix across recent AFD advisories has included a wide range of supported Windows client and server SKUs. While the MSRC page for CVE-2025-53141 is the canonical vendor entry, defenders should map the advisory to their environment using Windows Update, WSUS, or their enterprise patch-management tooling to determine the exact KBs and patched file versions for the builds they operate. Microsoft typically delivers kernel driver fixes as part of cumulative updates or out‑of‑band security updates depending on severity and exploit activity. (msrc.microsoft.com, thewindowsupdate.com)
Practical guidance for operations teams:

• Prioritize endpoints with multiple non-admin users, terminal servers, development workstations, and any system where local processes may be launched by lower-privileged users.
• Verify the AFD.sys file version and timestamp across hosts; create detections that flag devices still running the pre-patch driver. Patching AFD changes the driver binary metadata and is a reliable scope indicator.

---

Detection and hunting guidance​

Because the vulnerability is kernel-level and often timing-dependent, detection is nuanced. The following detection strategies are recommended based on public guidance and recent analyst write-ups for similar AFD bugs:

• Monitor for abnormal DeviceIoControl / IOCTL activity targeting the AFD device interface and unusual socket-level interactions from non-privileged processes. Repeated tight loops or rapid, automated attempts to invoke low-level AFD operations can indicate exploitation attempts.
• Look for rapid process creation loops or repeated calls from a user-mode process that correspond with socket operations; attackers attempting to “win” a timing window often drive high-frequency syscalls from a single worker process.
• Compare AFD.sys version metadata across endpoints to identify unpatched hosts; create queries in your EDR/SIEM that flag older afd.sys signatures.
• Watch for kernel-integrity alerts that detect abnormal kernel allocations, writes, or tampering of kernel structures associated with afd.sys. Modern EDRs and Microsoft Defender for Endpoint can emit useful signals for kernel anomalies.

Note: public detections for AFD exploitation can produce noisy alerts; teams should tune thresholds and correlate with process behavior to reduce false positives.

---

Exploitation risk and likelihood​

• Local-only vector reduces blast radius relative to remote RCEs but does not eliminate urgency. An attacker obtaining initial access via phishing, a malicious installer, or a user-mode code execution bug can chain this EoP to gain SYSTEM and achieve full control. Multiple public AFD bugs in 2025 show that once a reliable trigger is found, proof-of-concept code and weaponization can follow quickly. (ampcuscyber.com, bleepingcomputer.com)
• Timing and complexity: Null pointer dereference bugs often require careful timing or repeated attempts, but attackers can automate attempts. Skilled adversaries and exploit frameworks can overcome timing hurdles, particularly when the vulnerable code path is predictable.
• Evidence of in-the-wild exploitation: At the time of Microsoft’s advisory publication, public telemetry and vendor write-ups for some AFD bugs showed active exploitation for other CVEs in the same family (examples include use‑after‑free and heap overflow AFD bugs earlier in 2025). For CVE‑2025‑53141 specifically, defenders should consult active threat-intel feeds and Microsoft telemetry statements to confirm whether an exploit has been observed in the wild; public reporting does not always align immediately with vendor advisories. If there is uncertainty, treat the vulnerability as high priority until proof otherwise. (zeropath.com, msrc.microsoft.com)

Caveat: some third-party write-ups conflate different CVE identifiers in the AFD family; verification directly against Microsoft’s MSRC advisory and your patch-management tools is the authoritative route. Public lists and news coverage are useful context but can occasionally mix CVE numbers for distinct but related bugs. (bleepingcomputer.com, zeropath.com)

---

Recommended mitigation and response steps​

Follow a practical, prioritized remediation workflow:

• Apply vendor patches immediately where applicable.
• Use Windows Update, WSUS, Microsoft Endpoint Manager, SCCM, or whichever enterprise patching system you operate.
• Validate the KB/package that corresponds to your OS build and confirm afd.sys file replacement on remediation candidates.
• Where immediate patching is not possible:
• Restrict local code execution: enforce least privilege, restrict interactive logons where feasible, and use AppLocker / WDAC to prevent arbitrary process execution by non-admin users.
• Monitor for exploitation indicators (see Detection section) and elevate any suspicious IOCTL or socket-based behavior for investigation.
• Harden kernel exploit surface:
• Enable virtualization-based security features supported by your platform (e.g., Hypervisor-Protected Code Integrity, HVCI), and ensure secure boot / driver signing enforcement is active where feasible.
• Keep EDR/AV agent definitions and kernel sensors up-to-date to maximize detection fidelity for kernel anomalies.
• Post‑incident: if you suspect exploitation, treat the affected host as fully compromised. Perform full forensic triage, credential resets, and consider reimaging as the safest remediation for kernel-level compromise. Use EDR telemetry to determine lateral movement, persistence, and data exfiltration.

---

Attack scenarios and operational impact​

• Enterprise lateral movement: A foothold on a single workstation can be escalated to SYSTEM using this EoP, enabling attackers to harvest cached credentials, create persistent services, or move laterally across networks. Terminal servers and multi-user systems are particularly sensitive.
• Cloud and multi‑tenant hosts: On shared infrastructure or multi-tenant hosts, a kernel-level compromise of a host OS can impact tenant isolation and lead to broader compromises if the host is leveraged by a malicious workload. While cloud hypervisors and guest isolation reduce this risk, host-level EoP is a high-severity operational concern.
• Critical infrastructure: Systems handling industrial control, health-care endpoints, or regulated workloads could experience severe operational disruption if patched slowly or if the vulnerability is exploited in place. The cost of downtime, regulatory penalties, and sensitive-data exposure makes prompt patching essential.

---

Why AFD.sys keeps appearing in advisories​

AFD.sys sits at the intersection of user-mode network APIs and kernel-mode network handling, making it both widely invoked and complex. The driver has a repeatedly exercised code path surface, multiple IOCTL interfaces, and long legacy code paths historically extended to preserve compatibility. Those three characteristics combine to produce risk:

• Large codebase with legacy interactions increases the likelihood of logic gaps or insufficient sanitization.
• Kernel-mode execution means coding errors have greater consequence.
• Networking features are frequently invoked by third-party libraries and apps, producing diverse input patterns that can expose corner-case bugs.

Security teams and vendor researchers have repeatedly recommended aggressive kernel fuzzing of AFD and similar networking drivers to discover these classes of bugs earlier; several public analyses in 2024–2025 document how fuzzing and targeted research led to AFD flaws being found and fixed.

---

Cross-check and verification notes​

• The Microsoft Security Response Center (MSRC) advisory entry for CVE‑2025‑53141 remains the primary authoritative source for the vulnerability description and Microsoft’s remediation guidance; however the MSRC web UI uses client-side rendering and may be blocked or truncated in some scraping contexts — consult your internal MSRC feeds or enterprise vulnerability management dashboards for the exact KB mappings applicable to your OS builds.
• Public reporting and vendor blogs covering AFD vulnerabilities in 2024–2025 (examples include mainstream security outlets and independent analysis blogs) corroborate that the AFD driver has been the subject of multiple high-severity advisories this year. Those sources provide technical context and historical exploitation patterns that reinforce the need for rapid action, but readers should not assume identical exploitability or exploit code presence across distinct CVE identifiers — always cross-reference the specific CVE number on MSRC and your patch-management records. (bleepingcomputer.com, zeropath.com)
• If any public write-ups or third-party feeds claim active exploitation of this exact CVE (CVE‑2025‑53141), verify those claims against vendor telemetry or CISA/CNAs before treating them as confirmed. Some early news reports have conflated different AFD CVEs; treat each advisory on its own merits. Unverified or conflated claims are flagged here deliberately; confirm with authoritative telemetry before escalation. (ampcuscyber.com, bleepingcomputer.com)

---

Practical checklist for system administrators (quick action items)​

• 1. Identify hosts running pre-patch AFD.sys binaries and prioritize them for remediation.
• 2. Deploy the Microsoft updates (cumulative update or security-only KB) that correspond to your OS build as soon as they are validated in test.
• 3. Apply application whitelisting and least-privilege controls to reduce the ability of non-admin users to launch arbitrary code.
• 4. Tune EDR/SIEM to flag repeated AFD/IOCTL activity and rapid process spawn loops.
• 5. If a host exhibits signs of kernel compromise, isolate and treat the system as compromised — collect forensic artifacts and consider full reimage.

---

Strengths in Microsoft’s response — and residual risks​

Strengths:

• Microsoft published a vendor advisory and rolled the fix into its security update channels, enabling enterprise patching workflows to operate as intended. The vendor’s advisory model remains the authoritative mechanism for patch distribution and vulnerability details.
• The security community and EDR vendors continue to publish actionable detection and mitigation guidance for AFD-driven vulnerabilities, giving defenders pragmatic hunting signatures and remediation playbooks.

Risks and caveats:

• Patch adoption lag: historical telemetry shows a substantial portion of endpoints remain unpatched for weeks after initial updates; adversaries frequently exploit that window. Prioritize patch deployment for high-risk systems.
• Conflation of CVEs: multiple AFD.sys CVEs have been disclosed across 2024–2025 with different technical root causes. Public reporting sometimes conflates these issues; rogue or delayed PoCs may focus on earlier CVEs that are no longer directly relevant. Verify the exact CVE and KB mapping before triage. (bleepingcomputer.com, zeropath.com)
• Kernel compromise fallout: if exploitation is successful, remediation may require full reimage and credential resets; the operational cost is high. Early detection and swift patching remain the best defenses.

---

Conclusion​

CVE‑2025‑53141 — a null pointer dereference in the Windows Ancillary Function Driver for WinSock — represents a classic but serious kernel-level elevation-of-privilege risk: local exploitation can escalate an attacker to SYSTEM, enabling broad compromise. Microsoft’s advisory establishes the vulnerability’s existence and the local attack vector; defenders must map the advisory to their OS builds, prioritize patching, and deploy detection hunts for suspicious AFD interaction patterns.
Given the historical pattern of AFD-related bugs being quickly weaponized once a reliable trigger is publicly known, treat this advisory as high priority: apply the vendor-provided updates promptly, harden local execution controls where patching is delayed, and hunt for evidence of exploitation using the IOCTL/socket indicators described above. Where public reporting is uncertain or multiple AFD CVEs are discussed in the wild, rely on MSRC advisories and your patch-management tooling to resolve CVE-to-KB mappings and scope. (msrc.microsoft.com, zeropath.com)

---

---

Source: MSRC Security Update Guide - Microsoft Security Response Center