A use‑after‑free vulnerability in the Windows Ancillary Function Driver for WinSock (AFD.sys), tracked as CVE-2025-53137, can be abused by an authorized local user to escalate privileges to SYSTEM on affected Windows hosts — a high‑impact kernel vulnerability that follows a string of similar AFD issues in 2025 and should be treated as an immediate patching and detection priority for enterprises and administrators. (bleepingcomputer.com)
AFD.sys (the Ancillary Function Driver for WinSock) is a kernel‑mode driver that implements core socket and Winsock functionality for Windows. Because it runs in kernel context and proxies many user‑mode networking operations into the kernel, flaws in AFD often yield powerful local privilege escalation primitives when triggered from user space. Multiple distinct AFD vulnerabilities were disclosed and actively patched in 2025, and CVE-2025-53137 fits the same family of memory‑corruption issues that have repeatedly been weaponized or analyzed in public writeups this year. (ibm.com, zeropath.com)
AFD vulnerabilities fall into a handful of technical categories — use‑after‑free, untrusted pointer dereference, heap buffer overflow, and race conditions / concurrent execution — each of which can be converted into a kernel write or control‑flow hijack with the right exploit technique. Public analysis of recent AFD fixes shows that small logic errors and missing mode checks (user vs kernel mode) are common root causes; attackers who can trigger the vulnerable code path and then spray or reallocate kernel pools can escalate to SYSTEM. (ibm.com, cve.news)
This article synthesizes the vendor advisory summary for CVE‑2025‑53137 with independent technical analyses and industry reporting on the family of AFD.sys vulnerabilities to provide a practical, prioritized plan for defenders. Where specific KB identifiers or exploit‑in‑the‑wild confirmations could not be independently recovered in public sources at the time of writing, those items are flagged and administrators are urged to validate KB numbers and exploitation status against their vendor channels and telemetry feeds before finalizing remediation schedules. (ibm.com, zeropath.com)
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background
AFD.sys (the Ancillary Function Driver for WinSock) is a kernel‑mode driver that implements core socket and Winsock functionality for Windows. Because it runs in kernel context and proxies many user‑mode networking operations into the kernel, flaws in AFD often yield powerful local privilege escalation primitives when triggered from user space. Multiple distinct AFD vulnerabilities were disclosed and actively patched in 2025, and CVE-2025-53137 fits the same family of memory‑corruption issues that have repeatedly been weaponized or analyzed in public writeups this year. (ibm.com, zeropath.com)AFD vulnerabilities fall into a handful of technical categories — use‑after‑free, untrusted pointer dereference, heap buffer overflow, and race conditions / concurrent execution — each of which can be converted into a kernel write or control‑flow hijack with the right exploit technique. Public analysis of recent AFD fixes shows that small logic errors and missing mode checks (user vs kernel mode) are common root causes; attackers who can trigger the vulnerable code path and then spray or reallocate kernel pools can escalate to SYSTEM. (ibm.com, cve.news)
What the advisory says (summary)
- The vulnerability is described as a use‑after‑free in the Ancillary Function Driver for WinSock (AFD.sys) that “allows an authorized attacker to elevate privileges locally.”
- The attack vector is local: exploitation requires the ability to run code or processes on the target machine (low‑privileged user). The underlying exploitation is timing or memory‑corruption dependent.
- The impact is local elevation of privilege (EoP) to SYSTEM, enabling attackers to install persistent backdoors, extract credentials, or disable defenses. Public reporting for the AFD family indicates rapid PoC development and active exploitation in earlier 2025 AFD advisories; the broader pattern increases urgency. (bleepingcomputer.com, ampcuscyber.com)
Technical analysis — how this class of bug is abused
Use‑after‑free explained (short)
A use‑after‑free (UAF) occurs when kernel code continues to reference a pointer after the memory it points to has been freed. In kernel drivers like AFD, this can let an attacker shape what is allocated into that freed memory region (via heap spraying or allocation tricks), then cause the driver to read or execute attacker‑controlled data in kernel context. That can convert into a write‑what‑where or function‑pointer overwrite and therefore arbitrary kernel code execution. (cve.news, ibm.com)AFD exploitation pattern
- Trigger a path that frees an AFD internal object while references remain (the UAF).
- Immediately reallocate the freed pool with attacker‑controlled data (heap spraying or using user‑mode handles that allocate kernel objects).
- Force the driver to dereference the dangling pointer, using the attacker‑controlled contents to overwrite kernel pointers or objects.
- Use the resulting primitive to overwrite a process token, vtable, or function pointer to obtain SYSTEM privileges. (cve.news, ibm.com)
Exploit complexity and reliability
The advisory and industry analyses consistently emphasize that exploitability is timing dependent and requires local code execution to attempt exploitation. Skilled attackers can automate the timing and allocation steps, so “timing‑dependent” should not be conflated with “not exploitable” — historically, kernel races and UAFs have been weaponized quickly. Public reporting from earlier 2025 AFD issues shows proof‑of‑concepts and limited in‑the‑wild exploitation appeared within days to weeks of disclosure. (ampcuscyber.com)Affected systems and patching status
- AFD is present in all modern Windows desktop and server SKUs; patch applicability is determined by OS build and cumulative update rollouts. Vendors typically publish fixes for supported Windows 10, Windows 11, and Windows Server versions. Administrators must map the advisory to the specific KB for each OS build via Microsoft Update services. (tenablecloud.cn)
- Because some AFD advisories in 2025 required out‑of‑band rollouts for legacy server SKUs, it is important to verify whether the monthly rollup includes the fix for a particular SKU or whether a separate OOB package is published. Confirm the KB numbers in the MSRC Update Guide or the Microsoft Update Catalog before deploying. (sebastiangogola773121735.wordpress.com, malware.news)
- Where vendors or researchers reported active exploitation for earlier AFD CVEs (for example CVE‑2025‑32709 and CVE‑2025‑21418), organizations prioritized updates for publicly attacked infrastructure — RDS hosts, terminal servers, and developer workstations with broad local access. Treat CVE-2025-53137 with the same prioritization until exploit status is confirmed. (ampcuscyber.com, bleepingcomputer.com)
Detection, telemetry and hunting guidance
Because a kernel UAF EoP may leave subtle artifacts, a layered detection approach is needed:- AFD.sys file‑version inventory: Build a baseline of afd.sys file versions and timestamps across the estate. Patching changes the driver file version; any host that retains an older afd.sys after a fix is published should be high priority. Create queries in inventory and EDR tools to flag mismatches. (ibm.com)
- Monitor suspicious IOCTL and Winsock activity: Look for repeated, tight‑loop IOCTL calls or high‑rate socket control operations originating from non‑privileged processes. Exploit code frequently loops to win races or spray kernel pools, producing characteristic bursts of DeviceIoControl/IOCTL activity. Write EDR hunting rules to surface processes that repeatedly call low‑level Winsock or driver interfaces. (cve.news)
- Process creation and behavior anomalies: Post‑exploit behavior often includes rapid spawning of command shells, attempts to dump credentials, or attempts to load unsigned drivers. Correlate unexpected SYSTEM‑level process actions with prior suspicious IOCTL patterns. (bleepingcomputer.com)
- Kernel integrity checks: Use kernel protection telemetry (HVCI, kernel runtime checks, EDR kernel detectors) to alert on unexpected kernel writes, modifications to DRIVER_OBJECTs, or abnormal allocation/free patterns tied to afd.sys. Where available, collect kernel memory allocation traces around afd.sys operations for deeper forensics. (ibm.com)
- Log and network artefacts: Although the exploit is local, attackers often use the elevated access to perform network reconnaissance or lateral movement. Correlate privilege elevation events with subsequent suspicious outbound connections or credential use.
Mitigations and hardening (short‑term and long‑term)
- Patch first: Prioritize deploying Microsoft’s advisory updates to all affected endpoints and servers; use phased testing but treat multi‑user hosts and RDP/terminal servers as highest priority. If a KB is published as an out‑of‑band update for particular SKUs, apply it according to vendor guidance. (tenablecloud.cn)
- Reduce local attack surface:
- Enforce least privilege for local accounts; remove unnecessary local admin rights.
- Use application control (Windows Defender Application Control / WDAC) and restrict which binaries non‑admin users can execute.
- Harden workstation configurations to limit the ability of unprivileged processes to perform low‑level IOCTLs.
- Enable kernel defenses:
- Where supported, enable HVCI (Hypervisor‑protected Code Integrity) and memory integrity features to make kernel exploitation harder.
- Ensure Windows Defender / EDR kernel sensors and tamper protections are enabled and up to date. (ibm.com)
- Network and operational controls:
- Segment administrative hosts and terminal servers away from general user workstations.
- For high‑value systems, restrict interactive logon and software installation to a small group of hardened management hosts.
- For suspected compromise:
- Assume kernel‑level compromise may require rebuilding (reimaging) rather than in‑place remediation. Collect forensic evidence first, and treat hosts that show kernel modifications with higher caution.
Practical detection rules (examples)
- EDR query: “Flag processes that call DeviceIoControl against afd.* device names more than N times per minute, initiated by a non‑admin user.”
- Inventory script: “Report hosts where afd.sys file version < patched_version_string and last Windows update date < advisory_publish_date.”
- Kernel alert: “Generate high‑priority alert for unexpected writes to kernel token structures, modification of PsInitialSystemProcess token pointers, or new driver loads by non‑system installers.” (ibm.com)
Exploitation status and risk — what’s known and what’s not
- The broader AFD family of vulnerabilities saw active exploitation and fast Proof‑of‑Concept availability in 2025; earlier CVEs in the same family were exploited in the wild. That track record raises the probability that CVE‑2025‑53137 could be weaponized quickly by researchers or adversaries. (ampcuscyber.com, bleepingcomputer.com)
- At the time of publication of this article, independent public PoC code specific to CVE‑2025‑53137 could not be located in open sources. That does not mean an exploit does not exist in private hands; it does mean public PoCs were either not yet published or were behind closed channels. This uncertainty should be treated as high risk, not as reassurance. (zeropath.com)
- Any claim that a vulnerability “is being actively exploited in the wild” should be validated against multiple telemetry feeds (MSRC, vendor telemetry, CISA KEV catalog, and major EDR vendor advisories). Organizations with critical assets should assume high‑risk until proven otherwise and act accordingly. (malware.news)
Why AFD keeps resurfacing (analysis)
AFD sits at a complex intersection of legacy kernel networking APIs, multiple codepaths invoked by many user‑mode libraries, and a need to support backwards compatibility. That makes it both a high‑value target for attackers and a challenging surface to audit. Recent patches show that seemingly small gaps — missing mode checks, inadequate input validation, or subtle reference counting errors — can translate directly into exploitable kernel primitives. The repeated discoveries in 2025 suggest that targeted kernel fuzzing and symbolic analysis of AFD are delivering new findings faster than they can be comprehensively patched across all SKUs. (ibm.com, thesecmaster.com)Recommended response checklist for IT teams
- Identify affected hosts by matching afd.sys file versions and KB rollups in inventory tools.
- Prioritize patch deployment to multi‑user hosts, terminal servers, RDP gateways, and helpdesk workstations.
- Deploy EDR hunting queries for repeated IOCTL/Winsock control calls from non‑admin processes.
- Enable HVCI / memory integrity where compatible and enforce WDAC policies. (ibm.com)
- If exploitation is suspected, isolate the host, capture full memory and kernel dumps, and plan for reimaging after forensic analysis.
Final assessment — strengths, risks and takeaways
- Strengths: Microsoft’s security‑update cadence and collaborative industry analysis have repeatedly reduced the window of vulnerability for AFD issues; published post‑patch analyses (binary diffs and decompilations) help defenders understand exploit mechanics and tune detection. (ibm.com, tenablecloud.cn)
- Risks: The kernel‑level nature of AFD vulnerabilities means successful exploitation yields SYSTEM privileges and potentially persistent, hard‑to‑detect kernel artifacts. Timing‑dependent UAFs and race conditions are exploitable by automation, and the public record in 2025 shows adversaries and researchers can weaponize such bugs quickly. Prioritizing patch deployment and detection is critical. (ampcuscyber.com, bleepingcomputer.com)
- Takeaway: Treat CVE‑2025‑53137 as an urgent EoP advisory — inventory, patch, and hunt. Even if immediate active exploitation is not confirmed in public feeds, the history of AFD vulnerabilities makes preventive action the most effective defense. Confirm KB numbers and rollouts via the MSRC Update Guide or internal patch management channels and apply updates urgently to high‑value hosts.
This article synthesizes the vendor advisory summary for CVE‑2025‑53137 with independent technical analyses and industry reporting on the family of AFD.sys vulnerabilities to provide a practical, prioritized plan for defenders. Where specific KB identifiers or exploit‑in‑the‑wild confirmations could not be independently recovered in public sources at the time of writing, those items are flagged and administrators are urged to validate KB numbers and exploitation status against their vendor channels and telemetry feeds before finalizing remediation schedules. (ibm.com, zeropath.com)
Source: MSRC Security Update Guide - Microsoft Security Response Center
