CVE-2025-49762: AFD.sys Race Condition Enables Local Privilege Escalation

A recently published Microsoft advisory warns that CVE-2025-49762 — a race-condition flaw in the Windows Ancillary Function Driver for WinSock (AFD.sys) — can allow a locally authorized attacker to elevate privileges by exploiting concurrent execution using a shared resource with improper synchronization. (msrc.microsoft.com)

Background​

The Ancillary Function Driver for WinSock (commonly referenced as AFD.sys) is a kernel-mode component that underpins Winsock socket operations on Windows. It mediates many fundamental networking functions and is invoked by a wide range of user-mode networking APIs and libraries. Because AFD operates in kernel mode and touches core networking primitives, bugs in this driver can carry disproportionately high risk — including local privilege escalation to SYSTEM. This class of vulnerabilities has reappeared multiple times across 2024–2025 in different flavors (heap overflows, use-after-free, untrusted pointer dereferences and now a race condition), underscoring AFD’s attack surface and the persistent challenge of securing kernel networking code. (securityintelligence.com)
AFD vulnerabilities that surfaced earlier in 2025 included several high-impact CVEs that required immediate patching in Patch Tuesday cycles; those incidents set a precedent for rapid exploitability and serious real-world impact, making this new race-condition finding especially noteworthy for defenders and system administrators. (cvedetails.com) (nvd.nist.gov)

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What CVE-2025-49762 is (straight facts)​

• Vulnerability type: Concurrent execution using shared resource with improper synchronization — a race condition in the AFD WinSock driver. (msrc.microsoft.com)
• Impact: Local elevation of privilege — an authorized (local) attacker can exploit the race to obtain elevated rights on the host. (msrc.microsoft.com)
• Attack vector: Local — exploitation requires code execution or an ability to run processes on the target machine; it is not described as a remote unauthenticated remote code execution vector in the Microsoft advisory. (msrc.microsoft.com)
• Exploit complexity: Timing-dependent — race conditions typically require precise timing or repeated attempts to win the concurrency window; attackers can automate such attempts, but exploitation is generally more complex than trivial memory-corruption primitives. Security research on similar AFD bugs has shown that once researchers locate the vulnerable code path, reliable exploitation can follow quickly. (securityintelligence.com)

Note: Microsoft’s Security Update Guide lists the advisory and indicates the vulnerability details, but the public item is delivered via the MSRC advisory page; defenders should consult their internal patch-management dashboards for the exact KBs and update rollouts relevant to their OS builds. (msrc.microsoft.com)

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Why this matters: technical and operational impact​

Kernel-mode networking code is high-value​

AFD.sys runs in kernel mode and is reachable from user-mode networking APIs and some device control paths. Any kernel vulnerability that can be escalated into a write-what-where or similar primitive, or that allows arbitrary control flow in kernel context, can lead to SYSTEM-level compromise, kernel rootkits, credential theft, or persistence that is extremely difficult to remediate without full rebuilds. Past AFD vulnerabilities in 2025 confirmed that an attacker can chain local access to full system compromise. (thewindowsupdate.com)

Race conditions are slippery and sometimes more powerful than they first appear​

A race-condition bug in kernel drivers often allows an attacker to exploit a narrow timing window to corrupt state, bypass checks, or cause a synchronized structure to be used unsafely. In some cases, a race can be abused to change a pointer under concurrent use or to flip a flag at just the right time to skip validation — yielding powerful primitives such as kernel writes, overwrites of function pointers, or other conditions that convert into arbitrary code execution at elevated privilege. Public analyses of recent AFD bugs demonstrate how small logic gaps in the driver’s concurrency model can be escalated into potent exploitation primitives. (securityintelligence.com)

Operational risk vectors​

• Multi-user systems, terminal servers, and developer workstations are high-risk targets because local non-admin users may be present and could leverage a local elevation bug for lateral movement.
• Compromised endpoints (phished or malware-infected) can combine local initial access with a local EoP to obtain persistence and deploy ransomware or credential harvesters. Practical campaigns in 2025 reveal adversaries rapidly weaponize kernel bugs that grant SYSTEM privileges. (ampcuscyber.com)

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Exploitation complexity and likelihood​

• Exploitation requires local code execution or the ability to run a malicious process on the system. Because of the timing factor inherent to race conditions, an attacker often needs to perform repeated attempts and precise synchronization to win the race. Skilled attackers and automated exploit frameworks can largely overcome this barrier. (msrc.microsoft.com)
• Historically, once a reliable trigger and exploitation strategy are discovered and disclosed, proof-of-concept (PoC) exploits appear quickly. The Windows AFD family has seen multiple high-severity bugs in 2025; that track record increases the probability that threat actors could produce working exploits for CVE-2025-49762 given time and motivation. (thewindowsupdate.com)

Caveat: at the time of the advisory publication, public proof-of-concept code and evidence of in-the-wild exploitation for CVE-2025-49762 were not clearly documented in public telemetry available to the author; defenders should treat that as a snapshot and verify current exploitation status against threat intel feeds and vendor advisories. (msrc.microsoft.com)

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How Microsoft and others are responding​

Microsoft has published an advisory entry for CVE-2025-49762 in the Security Update Guide. The standard vendor remediation path for Windows kernel driver issues is to deliver fixes via monthly Patch Tuesday updates or out-of-band security updates where required; administrators should expect KB packages and updated AFD.sys versions to be published for affected Windows builds. System owners should track their Windows Update, WSUS, and Microsoft Endpoint Manager channels for the specific patch KB numbers that map to their OS versions and test then deploy according to organizational change processes. (msrc.microsoft.com)
Independent security research groups and incident responders have also been dissecting recent AFD vulnerabilities and publishing technical analyses that reveal how minor driver changes can harden or exacerbate risk; those write-ups are an important complement to vendor advisories for defenders who need to know how to detect exploitation attempts in telemetry. (securityintelligence.com)

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Detection and hunting guidance​

Because the vulnerability is kernel-level and timing-dependent, detection is nuanced. The following practical signals and detection techniques are recommended:

• Monitor for unusual or repeated DeviceIoControl/IOCTL activity targeting the AFD device interface (look for control codes associated with AFD dispatch entries). Repeated failed calls or tight loops in user processes that interact with sockets at a low level can be suspicious. (securityintelligence.com)
• Watch for rapid process creation loops or repeated calls from a non-privileged process that attempt to trigger AFD calls — this often correlates with automated exploitation attempts.
• Endpoint telemetry: compare the loaded AFD.sys file version and timestamp across hosts. Patching will change the driver version; hosts running older AFD.sys versions after the patch publication are high-priority remediations. Security teams should create queries that flag devices where afd.sys file metadata does not match expected patched versions. (securityintelligence.com)
• Kernel integrity alerts: solutions that detect abnormal kernel writes, I/O ring manipulation, or unexpected allocation patterns tied to afd.sys may catch exploit primitives used to convert a race into a write-where or similar primitive. Defender for Endpoint and other EDR products often provide relevant telemetry for this type of activity. (ampcuscyber.com)

Note: public writeups on prior AFD vulnerabilities showed attackers creating primitives that allowed arbitrary kernel writes by manipulating I/O rings and internal driver tables; such techniques are a useful checklist for hunting. (securityintelligence.com)

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Mitigation and remediation: recommended steps​

• Immediately inventory: identify all affected endpoints and servers by querying the AFD.sys version and installed Windows build. Flag any endpoints that have not received the relevant updates.
• Patch promptly: apply the Microsoft security updates that address CVE-2025-49762 as soon as they are available and tested in your environment. Use Windows Update, WSUS, or your enterprise patch management solution to deploy patches. Where MSRC specifies an out-of-band patch for specific SKUs, prioritize those systems. (msrc.microsoft.com)
• Harden local access: reduce the number of standard users who can run arbitrary code on high-value hosts. Apply least privilege to local accounts and restrict software installation via application control policies (e.g., Windows Defender Application Control).
• Telemetry and EDR rules: implement hunting rules described in Detection and hunting guidance above; escalate suspicious findings for immediate containment. (securityintelligence.com)
• Network segmentation: isolate critical servers (e.g., Domain Controllers, RDS hosts) from user workstations to reduce the consequences of a workstation-local compromise. This lowers the risk that a local escalation on one host will enable domain-wide compromise.
• After-action: if exploitation is suspected or confirmed, perform a full forensic capture and consider rebuilding the host from known-good images. Kernel-level compromise is difficult to remediate without reimaging in many cases.

Practical note: some Microsoft advisories for AFD vulnerabilities in 2025 included language that “no known mitigations” were practical beyond patching; organizations should therefore assume patching is the primary defense and prioritize accordingly. (bleepingcomputer.com)

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Recommended prioritization and patching timeline​

• High priority: systems that host multiple users, are used for remote access (RDP), or manage critical identity services should be patched first. These include domain-joined desktops used by helpdesk staff, terminal servers, and developer workstations that run code from untrusted sources.
• Medium priority: typical user endpoints where the risk of immediate lateral movement is lower but where local exploitation could still enable enterprise-wide persistence.
• Low priority: isolated lab systems, air-gapped test machines — still patch but after production-critical systems have been addressed.
• For federal agencies and organizations subject to CISA KEV or equivalent mandates: verify whether the specific CVE has been added to the KEV list and follow required deadlines; some AFD CVEs earlier in 2025 were added to the KEV catalog with mandated timelines for patching. If CVE-2025-49762 is added to that catalog, treat remediation as time-critical. (Check current CISA guidance for confirmation as advisories are updated.) (nvd.nist.gov)

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Technical analysis: how a race could turn into SYSTEM control (exploit outline)​

This section paraphrases common exploitation patterns for kernel race conditions and maps them to AFD-related analysis produced by researchers who dissected recent AFD bugs.

• The attacker triggers two or more concurrent code paths in the driver that assume exclusive access to a shared structure.
• By manipulating scheduling and issuing repeated operations, the attacker forces the driver to act on a stale pointer or to perform a write that raced past validation.
• If the attacker can cause the driver to write a controlled value to a kernel address (or to an I/O ring buffer pointer), this may convert to a write-what-where primitive.
• With a write-what-where primitive, the attacker can overwrite a function pointer, object vtable or critical kernel data, then trigger that overwritten pointer to execute code in kernel context, yielding SYSTEM. Analyses of recent AFD bugs show researchers were able to map IOCTLs and AFD dispatch tables to find the necessary control codes and structure offsets to build these primitives. (securityintelligence.com)

Caveat: the above is a high-level pattern. The exact exploitation steps depend on the vulnerable function, driver dispatch index, and changes Microsoft applied in patches. Relying on vendor patches — and threat-hunting telemetry — remains the appropriate defensive posture.

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Strengths in Microsoft’s approach — and remaining weaknesses​

Notable strengths​

• Rapid advisory publication and KB-based remediation channels enable enterprises to deploy patches quickly (Windows Update, WSUS, MECM). Microsoft’s Security Update Guide centralizes the advisory data for administrators. (msrc.microsoft.com)
• Vendor and independent research communities have been productive in analyzing AFD issues; published technical breakdowns help defenders write detection rules and accelerate mitigation planning. (securityintelligence.com)

Risks and weaknesses​

• Kernel vulnerabilities are inherently risky: a single mistake in synchronization, pointer validation, or boundary checking at the kernel level can enable complete system compromise. Recurrent AFD bugs in 2025 suggest deeper engineering and test gaps in the concurrency controls in the driver.
• Public exploitability often follows disclosure quickly; once PoCs exist, immature patching programs can leave large populations exposed. Historical CVEs from the same AFD family were exploited or rapidly weaponized by attackers. (thewindowsupdate.com)

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Verification and caveats​

• The Microsoft advisory for CVE-2025-49762 is the authoritative description and should be used for KB mapping and official remediation steps. (msrc.microsoft.com)
• Other reputable sources documenting AFD vulnerabilities in 2025 (NVD entries for related CVEs, vendor blogs and technical writeups) corroborate the general risk profile for AFD bugs and the common exploitation patterns for kernel concurrency flaws. Where specific numeric claims (e.g., CVSS scores, exploit-in-the-wild status, KEV inclusion) could not be confirmed for CVE-2025-49762 at the time of writing, those items have been explicitly flagged above and readers should re-check live vendor and government feeds for updates. (nvd.nist.gov, securityintelligence.com)

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Practical checklist for IT teams (actionable)​

• 1.) Inventory afd.sys versions and Windows builds across the estate; produce a prioritized remediation list.
• 2.) Apply Microsoft’s security updates for CVE-2025-49762 promptly after validating on a test cohort. (msrc.microsoft.com)
• 3.) Implement detection queries for abnormal DeviceIoControl/IOCTL patterns targeting AFD and for processes that perform high-frequency Winsock/AFD interactions. (securityintelligence.com)
• 4.) Enforce least privilege on local accounts and tighten software install policies with application control.
• 5.) If suspicious behavior is detected, isolate the host, collect memory and kernel artifacts, and consider reimaging — kernel compromises are hard to cleanse.
• 6.) Monitor vendor and national cyber authorities (MSRC, NVD, CISA) for any updates to exploitation status or required actions; update playbooks accordingly. (msrc.microsoft.com, nvd.nist.gov)

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Conclusion​

CVE-2025-49762 is a timely reminder that even mature components of the Windows networking stack remain fertile ground for high-impact vulnerabilities. Race conditions in kernel drivers are technically challenging to exploit but, once weaponized, can lead to full system compromise. The immediate, practical defense is straightforward: identify affected systems, apply vendor patches without undue delay, and layer detection and least-privilege controls to reduce the blast radius of potential exploitation. Complement vendor fixes with targeted hunting for suspicious AFD interactions and prioritize remediation for multi-user and remote-access hosts. Vigilance, fast patching, and good telemetry remain the best defenses against this class of kernel threats. (msrc.microsoft.com, securityintelligence.com)

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Source: MSRC Security Update Guide - Microsoft Security Response Center