CVE-2025-54099: Windows AFD.sys Stack Overflow Privilege Escalation Explained

Microsoft’s advisory identifies a vulnerability in the Windows Ancillary Function Driver for WinSock (afd.sys) that can be triggered locally to escalate privileges — described on the vendor page as a buffer overflow in the WinSock ancillary driver — and administrators must treat this as a high-priority patching and detection task. (msrc.microsoft.com)

Background / Overview​

The Ancillary Function Driver for WinSock (AFD, delivered as afd.sys) is a kernel‑mode driver that implements core socket primitives consumed by Winsock and many higher‑level networking APIs. Because AFD runs in kernel context and is reachable from user‑mode networking calls and IOCTL paths, faults in its memory handling often carry outsized risk: crashes, denial‑of‑service, and full SYSTEM compromise are realistic outcomes when local attackers convert kernel bugs into privilege‑escalation primitives. (bleepingcomputer.com)
This article examines the specific advisory identified as CVE‑2025‑54099 — reported as a stack‑based buffer overflow in the Windows Ancillary Function Driver for WinSock that allows an authorized local attacker to elevate privileges — verifies what can be independently confirmed about the issue, explains exploitation mechanics and operational impact, and provides pragmatic detection and remediation guidance for Windows administrators and defenders. The analysis synthesizes the vendor advisory with independent reporting and broader AFD vulnerability patterns observed through 2024–2025. (msrc.microsoft.com) (zerodayinitiative.com)

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Why AFD.sys bugs are especially critical​

AFD is central to Windows networking and is invoked by countless user processes. Its privileged position in the kernel means that a seemingly small pointer or bounds‑checking error can be converted into a kernel memory corruption primitive that allows modification of process tokens, function pointers, or dispatch tables — all common routes to SYSTEM privileges. Past AFD flaws in 2024–2025 included use‑after‑free, heap overflows, null pointer dereferences, and untrusted pointer dereferences; these show a recurring class of risk in the same code paths. (bleepingcomputer.com)
From an operational standpoint, the AFD attack surface is attractive to attackers because:

• It is exposed to normal user processes via Winsock APIs and device IOCTLs.
• It is high value: kernel compromise equals SYSTEM privileges with broad options for persistence and lateral movement.
• Small coding mistakes (missing validation, race conditions) in kernel drivers can be weaponized rapidly by skilled exploit developers.

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What Microsoft says about CVE‑2025‑54099 (summary)​

Microsoft’s Security Update Guide lists CVE‑2025‑54099 as a vulnerability in the Windows Ancillary Function Driver for WinSock that can be used by an authorized local attacker to elevate privileges. The vendor advisory identifies the root cause as a memory safety issue in afd.sys. Administrators are directed to apply the security update distributed by Microsoft for the affected OS builds. (msrc.microsoft.com)
Important operational note: the Microsoft advisory page is the authoritative source for the precise list of affected builds and the KB updates that remediate the issue. Because Microsoft’s Update Guide is rendered dynamically, some third‑party trackers may lag; defenders should confirm KB numbers and package details directly through their standard patch‑management pipeline or the Microsoft Update Catalog. (msrc.microsoft.com)

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Verification and cross‑checks: what is and isn’t independently confirmed​

Independent trackers and security vendors have documented multiple AFD/WinSock CVEs in 2025 (examples: CVE‑2025‑21418, CVE‑2025‑32709, CVE‑2025‑49661, CVE‑2025‑53147 and others), and several of those were explicitly described as heap overflows, use‑after‑free, null pointer dereferences or untrusted pointer dereferences; multiple security advisories and patch analyses confirm that Microsoft patched a string of AFD issues across several Patch Tuesday cycles in 2025. This establishes context that AFD remains a recurring, high‑value target for both researchers and attackers. (zerodayinitiative.com) (bleepingcomputer.com)
However, at the time of writing, third‑party aggregated databases and some public trackers did not consistently show a fully detailed, separately indexed record for the CVE identifier CVE‑2025‑54099. The Microsoft vendor page exists and lists the vulnerability; this makes the MSRC update guide the authoritative record for CVE‑2025‑54099. Because external aggregation can lag vendor postings for newly issued CVEs (or because CVE identifiers and editorial descriptions are sometimes transposed between advisories), defenders should rely on Microsoft’s advisory and KBs for remediation mapping and treat any discrepancy in aggregator listings with caution. Treat the “stack‑based buffer overflow” classification for CVE‑2025‑54099 as reported by the vendor; third‑party trackers may describe similar AFD CVEs using different memory‑corruption labels. (msrc.microsoft.com)
(Flagged claim: the exact technical label "stack‑based buffer overflow" for CVE‑2025‑54099 is taken from the MSRC advisory text provided by the user; independent, third‑party trackers did not uniformly corroborate that specific descriptor for this CVE at the time of verification. Administrators should therefore treat that specific characterization as vendor‑reported and confirm the KB/patch content in test rings before mass deployment.) (msrc.microsoft.com)

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Technical analysis — how a stack‑based buffer overflow in afd.sys could be abused​

A stack‑based buffer overflow occurs when the driver copies more data into a fixed‑size stack buffer than it can hold, overwriting adjacent stack memory such as saved registers, return addresses, or local variables. In a kernel driver, a successful overflow can often be escalated into arbitrary kernel memory overwrite or execution inside the kernel context — a very direct path to SYSTEM compromise. The general exploitation pattern for AFD vulnerabilities in 2025 shows repeated use of IOCTLs, malformed Winsock structures, or crafted socket operations to reach vulnerable code paths. (bleepingcomputer.com)
Typical exploitation chain (high level):

• Attacker obtains the ability to run code or an unprivileged process on the host (local access required).
• Use crafted socket calls or DeviceIoControl invocations to feed a malformed structure to afd.sys that triggers the stack overflow.
• Through precise layout and timing, overwrite a sensitive stack field (for example a saved return pointer or a function pointer frame) and redirect execution flow inside the kernel.
• Convert kernel‑mode execution into a token or process structure overwrite to obtain SYSTEM privileges.

Exploit complexity is non‑trivial: kernel exploitation requires careful control of memory layout and often timing or spraying techniques. But “non‑trivial” does not mean “impractical”. Attackers and exploit developers routinely automate these steps; prior AFD vulnerabilities in 2025 saw rapid weaponization after disclosure. Because the vulnerability is local, the initial access vector is constrained — however, attackers with initial footholds (e.g., via phishing payloads, C2 implants, or malicious local installers) can reliably chain a local EoP into full system compromise. (crowdstrike.com)

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Affected versions, patches and distribution channels​

Microsoft’s update guide lists the OS builds and the cumulative updates that remediate CVE‑2025‑54099; the authoritative advice is to map the MSRC advisory to the KB update matching your exact OS build in the Microsoft Update Catalog or your enterprise WSUS/Windows Update for Business channel. Because vendor pages can be dynamically rendered and third‑party tracking can lag, vendors and administrators should: (a) identify the exact KB/CU for their Windows version, (b) stage in test rings, and (c) deploy broadly after verification. (msrc.microsoft.com)
Patch distribution notes from previous AFD advisories:

• Microsoft has historically delivered AFD fixes in regular Patch Tuesday updates as well as out‑of‑band (OOB) patches when zero‑day exploitation was observed. Organizations should check both the monthly cumulative updates and any OOB advisories. (zerodayinitiative.com)
• For some older server SKUs, Microsoft has released separate OOB updates; confirm whether your server versions require a security‑only package or a monthly rollup plus a supplemental update. (sebastiangogola773121735.wordpress.com)

Immediate remediation steps:

• Apply the vendor KB matching your Windows build as soon as practical.
• If you cannot patch immediately, implement compensating controls described in the vendor guidance (see next section). (msrc.microsoft.com)

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Short‑term mitigations and compensating controls​

When immediate patching is not possible, defenders can reduce risk with the following compensating measures. These are mitigations, not long‑term replacements for patching:

• Restrict capability to execute arbitrary local code: enforce application control (AppLocker/Windows LSA/WDAC) to reduce the chance an unprivileged user can run an exploit binary.
• Limit interactive and non‑essential local accounts: remove unnecessary local administrator privileges and lock down accounts that can log on locally.
• Harden device access: restrict user access to the afd.sys device interface where possible, and monitor for unusual DeviceIoControl or CreateFile calls against the AFD interface. (avertium.com)
• Enable platform mitigations: HVCI/Hypervisor‑enforced Code Integrity (if available and compatible) increases the cost of kernel‑mode payloads. (ampcuscyber.com)
• Network segmentation and principle of least privilege: minimize the hosts that have users with the ability to run code or accept files from risky sources. (cybersecuritynews.com)

These mitigations reduce risk surface but do not eliminate the vulnerability; applying the official patch remains the only complete remediation. (msrc.microsoft.com)

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Detection and hunting guidance for EDR and SIEM teams​

Kernel exploits often leave subtle footprints, but defenders can instrument multiple telemetry sources to increase the likelihood of detection. Practical hunting rules include:

• EDR: alert on user processes issuing repeated or unusual DeviceIoControl requests referencing WinSock/AFD device names or on rapid sequences of IOCTL calls to afd device paths. (windowsforum.com)
• Sysmon/ETW: capture CreateFile and DeviceIoControl calls on device objects that map to afd (or capture the symbolic link names used by WinSock). Flag persistent tight loops or high‑frequency IOCTL sequences from low‑privilege processes.
• Kernel telemetry (where available): watch for unexpected kernel write attempts, changes to process token structures, or sudden SYSTEM‑level process creations following suspicious IOCTL activity.
• File and version inventory: create a baseline of afd.sys version info across the estate and alert on hosts where afd.sys does not match the patched version reported by Microsoft. A quick PowerShell check (Get‑ItemProperty "C:\Windows\System32\drivers\afd.sys" | select VersionInfo) can be rolled into endpoint inventory jobs. (windowsforum.com)

Document and preserve memory and EDR traces for any suspected exploit attempt: kernel exploitation often requires forensic analysis to confirm and determine scope. If a privileged compromise is suspected, consider isolating and reimaging affected hosts because kernel compromises are notoriously difficult to remediate reliably without rebuild.

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Risk analysis: who should be most concerned?​

Prioritize remediation for hosts that, if compromised, present the greatest operational impact:

• Domain controllers, Active Directory servers, and credential‑roaming hosts.
• Remote Desktop Services (terminal servers), VDI hosts and multi‑user server farms.
• Developer workstations and build servers that run untrusted code.
• Systems exposed to contractors, kiosks, or shared machines where an untrusted process could run. (windowsforum.com)

Attackers who escalate to SYSTEM can deploy credential‑dumping tools, install kernel‑level persistence, disable security agents, and move laterally. Because many ransomware and targeted intrusion campaigns rely on local privilege‑escalation chains to gain full control, a local exploit like CVE‑2025‑54099 is operationally significant and often part of multi‑stage attacks. (crowdstrike.com)

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Exploit availability and public proof‑of‑concepts​

Historically, AFD issues in 2025 attracted rapid expert attention and proof‑of‑concept research; some AFD CVEs were reported as exploited in the wild or added to government Known‑Exploited Vulnerabilities lists. For CVE‑2025‑54099 specifically, public exploit PoCs and third‑party writeups were not consistently available at the time of verification; authoritative confirmation should be sought from the Microsoft advisory (which may include mitigation and exploitation guidance). Treat any public PoC as high‑risk and avoid running untrusted exploit code on production hosts. (bleepingcomputer.com)

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Recommended short checklist for sysadmins (24–72 hour window)​

• Identify affected hosts:
• Query afd.sys version across endpoints and compare against patched version metadata from the Microsoft advisory. (windowsforum.com)
• Obtain the exact KB/patch:
• Map the MSRC advisory to the cumulative update or security‑only KB for your OS build and test in a pre‑production ring. (msrc.microsoft.com)
• Patch priority:
• Prioritize domain controllers, RDS/VDI hosts, servers with many local users, and developer/build machines.
• If you cannot patch immediately:
• Apply compensating controls: application control policies, restrict local code execution, enable HVCI when feasible, and increase telemetry on DeviceIoControl/AFD IO patterns. (avertium.com)
• Hunting and SIEM:
• Deploy EDR rules to detect repeated DeviceIoControl activity, rapid IOCTL loops, and rapid process creations following device interactions. Preserve traces of any suspicious behavior for forensic analysis. (windowsforum.com)

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Broader context and lessons learned​

AFD has been a recurring source of kernel vulnerabilities for several years. The pattern through 2024–2025 shows multiple vulnerability classes (heap overflow, use‑after‑free, null dereference, untrusted pointer deref, race conditions) within the same driver module — emphasizing that legacy or critical kernel components exposed to user inputs must be continuously hardened. Defenders should take a posture that combines rapid patching, proactive telemetry, and architectural controls (least privilege, segmentation, application control) to reduce the blast radius when local privilege‑escalation vulnerabilities appear.
Enterprises should also review patch management processes to ensure that when kernel‑level zero‑days or actively exploited issues appear, KBs can be staged and deployed in hours not weeks — particularly for mission‑critical hosts. Historical incidents show that attackers rapidly incorporate local EoP exploits into post‑compromise toolkits. (zerodayinitiative.com)

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Conclusion​

CVE‑2025‑54099 — reported by Microsoft as a stack‑based buffer overflow in the Windows Ancillary Function Driver for WinSock — represents a classic and severe local elevation‑of‑privilege risk: a kernel memory‑safety bug reachable from user mode. The vendor advisory is the authoritative reference for affected builds and the specific KBs that remediate the issue; administrators should identify the matching security update for their Windows versions, stage it in test rings, and deploy broadly with high priority. (msrc.microsoft.com)
While compensating controls and detection rules can reduce immediate exposure, applying Microsoft’s patch remains the only complete remediation. Given AFD’s historical attack surface and the repeated emergence of exploitable kernel bugs in 2024–2025, organizations must treat WinSock/AFD advisories as critical, apply vendor fixes promptly, and strengthen telemetry and least‑privilege controls to limit the operational impact of future kernel vulnerabilities.

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(Verification note: vendor advisory content for CVE‑2025‑54099 was confirmed on Microsoft’s Security Update Guide; independent aggregators listed multiple related AFD CVEs in 2025 and reported active exploitation in several instances, but third‑party databases may lag vendor postings. The specific descriptor “stack‑based buffer overflow” for CVE‑2025‑54099 is taken from the MSRC advisory; defenders should verify the KB patch notes in the Microsoft Update Catalog for the most precise technical details before making compliance or remediation decisions.) (msrc.microsoft.com) (bleepingcomputer.com)

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Source: MSRC Security Update Guide - Microsoft Security Response Center