CVE-2025-62470: Windows CLFS Heap Overflow Elevation of Privilege

Microsoft’s public vulnerability record for CVE-2025-62470 confirms a new high‑impact elevation‑of‑privilege flaw in the Windows Common Log File System (CLFS) driver that, according to vendor metadata and multiple independent trackers, is a heap‑based buffer‑overflow allowing a local, authorized user to gain SYSTEM‑level privileges if successfully exploited.

Background / Overview​

The Windows Common Log File System (CLFS) is a kernel‑mode component used across Windows to store transactional logs and other structured log data. Because CLFS operates inside the kernel, flaws in clfs.sys produce high‑value exploitation primitives: a local exploit can often be converted into a full system compromise by converting memory corruption into token manipulation or arbitrary kernel writes. Past CLFS advisories have repeatedly produced high‑severity alerts for enterprise defenders and patching organizations.
CVE‑2025‑62470 was entered into public vulnerability feeds on December 9, 2025. Public trackers list the root cause as a heap‑based buffer overflow in the CLFS driver, with an industry‑typical CVSS v3.1 base score of 7.8 (High) and an exploitability vector that indicates a local attack requiring low privileges and no user interaction. The canonical vendor record is available in Microsoft’s Security Update Guide entry for the CVE, which serves as the authoritative KB mapping for remediation packages, although the public MSRC page for kernel CVEs can be terse or dynamically rendered.

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What the advisory actually says (verified facts)​

• The vulnerability exists in the Windows Common Log File System driver (clfs.sys) and is classified as an elevation of privilege (EoP) vulnerability.
• The reported technical class is a heap‑based buffer overflow (CWE‑122).
• The public severity mapping shows a CVSS v3.1 base score of 7.8 and vector commonly represented as AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H — i.e., local attack, low complexity, low privileges required, no user interaction, high impacts.
• The primary remediation route is the Microsoft security update(s) tied to the CVE entry in the MSRC Security Update Guide; administrators should map the CVE to the correct KB package for their OS builds and apply the update promptly.

These points are corroborated across vendor metadata and industry vulnerability aggregators — the two most load‑bearing claims (existence of the CVE and its heap‑overflow classification plus the high CVSS) are represented in public CVE feeds and the vendor's update guide.

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Technical analysis — how CLFS issues are typically abused​

Why CLFS driver bugs matter​

CLFS is used by multiple Windows subsystems and third‑party applications for reliable, high‑throughput logging. It handles on‑disk structures and exposes APIs that mediate user→kernel transitions. Kernel drivers that parse or load on‑disk structures are inherently risky: an attacker who can craft or manipulate a file or API input that reaches a kernel parser may cause out‑of‑bounds reads/writes, use‑after‑free conditions, or buffer overflows — all of which can yield the primitive needed to escalate privileges. Past CLFS advisories and real‑world exploitation show this pattern repeatedly.

Typical exploitation primitives for heap overflows in clfs.sys​

• Local code execution or file placement: the attacker must already run code as a standard user or be able to write files the CLFS driver will read.
• Triggering the vulnerable parser: crafted log containers, malformed blocks, or specific API calls lead CLFS to allocate memory and then read controlled input into that buffer.
• Corruption conversion: the overflow corrupts adjacent kernel structures or function pointers; an attacker then converts corruption into a reliable token swap, write‑what‑where, or function pointer hijack to obtain SYSTEM.
• Post‑exploit actions: install services, persist drivers, harvest credentials, or move laterally.

This chain is the bread‑and‑butter of local privilege escalation on Windows where kernel‑mode parsing mistakes occur. Security researchers and threat actors have weaponized similar CLFS bugs in the past, and Microsoft’s own telemetry has linked earlier CLFS exploitation to ransomware activity — demonstrating the real‑world value of such vulnerabilities to malicious operators.

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What we can verify and what remains uncertain​

Verified:

• The CVE exists and is recorded in public tracking feeds with a heap‑overflow classification and high CVSS score.
• Microsoft’s Security Update Guide has an entry for the CVE (the Update Guide is the canonical mapping for KB packages), though the page content is rendered dynamically and may not display fully in some crawlers. Administrators must use the Update Guide or Microsoft Update Catalog to obtain the exact KB numbers for their SKUs.

Unverified or not publicly disclosed at the time of writing:

• Public proof‑of‑concept (PoC) exploit code and low‑level exploit specifics (exact function offsets, call sequences, in‑memory layout, or exploit reliability tricks) are not present in the vendor advisory text and were not visible in high‑confidence public write‑ups at time of disclosure. Where PoCs exist, they are often posted to GitHub or disclosed by independent researchers; these increase the risk window once they appear. Treat any claim that a reliable PoC exists in the wild as requiring explicit proof.

Cautionary note: vendor advisories for kernel issues intentionally omit exploitation specifics to limit short‑term weaponization. Public aggregation sites and security vendors sometimes populate CVSS scores and high‑level root causes before full technical write‑ups are published; always reconcile aggregated metadata with the vendor’s KB/package details for remediation.

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Affected platforms and operational impact​

Industry‑visible patterns for CLFS advisories suggest a wide range of Windows client and server builds may be impacted, but the exact set of affected SKUs and fixed package KBs should be obtained directly from Microsoft’s Update Guide and the Microsoft Update Catalog before deploying updates at scale. Because CLFS is a broadly used kernel component, the operational impact can be material:

• Impact on confidentiality/integrity/availability: A successful exploit yields SYSTEM privileges — adversaries can read all data, alter images and binaries, install persistent components, and disable defenses.
• Attack vector: local (AV:L). This reduces the likelihood of wormable, unauthenticated remote exploits but increases the value of the bug for post‑compromise lateral movement and privilege consolidation.
• Likely targeted hosts: admin workstations, jump hosts, servers with interactive users, and any systems where local code execution for standard users is plausible.

Do not assume only legacy builds are affected; CLFS code paths exist across many Windows versions, and Microsoft often maps a CVE across multiple client and server SKUs. Confirm the KB mapping for each build before you declare a host remediated.

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Detection, hunting, and short‑term mitigations​

Because this vulnerability is local and kernel‑level, detection strategies focus both on exploit attempts (crashes, suspicious file activity) and on the post‑exploit artifacts of privilege escalation.
Key telemetry and hunting priorities:

• Monitor for unexpected process elevations to SYSTEM, especially where a non‑privileged process spawns SYSTEM children or duplicates tokens.
• Watch for clfs.sys crashes, BSODs, or kernel dump events correlated to user processes interacting with CLFS files or APIs.
• Alert on unusual writes to system directories or new service/driver installations originating from non‑privileged user contexts.
• Tune EDR to flag anomalous use of token manipulation APIs (DuplicateTokenEx, SetTokenInformation) and suspicious calls to CreateService / RtlInitUnicodeString / NtCreateFile from unexpected parents.

Compensating controls (if immediate patching is impossible):

• Remove unnecessary local admin rights and enforce least‑privilege policies.
• Restrict which accounts can log on locally or via RDP to high‑value systems.
• Implement application allow‑listing (WDAC/AppLocker) on administrative and shared endpoints.
• Apply network segmentation and use jump hosts for administration to reduce the number of systems reachable by a low‑privileged user account.
• Harden file permissions on directories that may be used by CLFS log containers where feasible.

Prioritize remediation for endpoints that permit local code execution by untrusted or lightly‑trusted users (developer machines, research laptops, shared admin consoles).

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Patching guidance — practical steps for administrators​

• Identify affected hosts
• Query inventory for OS builds and clfs.sys file versions.
• Use your vulnerability management tool to map CVE → KB using Microsoft’s Security Update Guide or the Microsoft Update Catalog. Microsoft’s Update Guide is the canonical source; aggregators can lag or mis‑map packages.
• Test and pilot
• Deploy the Microsoft supplied package to a small canary/pilot group first to validate for functional regressions and ensure your critical apps are unaffected.
• Broad rollout
• Use your standard change control and patch orchestration (WSUS, SCCM/ConfigMgr, Intune, or update catalogs) to roll out patches widely and track reboots/completions.
• Verify remediation
• Confirm KB presence on each host and verify clfs.sys version or file hashes where possible before closing tickets.
• Post‑patch monitoring
• Increase EDR sensitivity for privilege escalation indicators for 7–14 days after patch rollout; adversaries often time exploit attempts to follow patch release to target unpatched systems.

If a host cannot be patched promptly, apply the compensating controls above and isolate high‑risk hosts from sensitive networks until remediation is complete.

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Risk assessment: who should worry most and why​

• Small offices/home users: risk is lower if local accounts are tightly controlled, but any device that allows local code execution and retains administrative accounts is at risk.
• Enterprises with large, heterogeneous estates: high priority. Unpatched CLFS EoP vulnerabilities are frequently useful to ransomware and post‑compromise actors because they allow escalation to SYSTEM and persistence.
• Managed service providers and shared admin hosts: extremely high priority. Those hosts are attractive targets because credential theft there yields broad access.

Historical incidents demonstrate that once patches are public or PoCs appear, exploitation accelerates quickly. Microsoft has previously observed CLFS exploitation in targeted campaigns linked to ransomware and post‑compromise operators; this makes rapid validation and patching the pragmatic priority for defenders.

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Strengths and weaknesses in the public response​

Strengths:

• Microsoft’s centralized Security Update Guide gives an authoritative mapping to remediation packages and is the right first stop for patch orchestration.
• The Windows security and third‑party vendor ecosystem is mature; vulnerability trackers and EDR vendors typically publish detection signatures and remediation playbooks within hours or days of MSRC updates.

Weaknesses / risks:

• Vendor advisories for kernel CVEs often omit low‑level exploit details; this is deliberate but it means defenders must triage risk with limited technical signal while attackers with private knowledge may already be developing exploits.
• Many public feeds publish CVE metadata before full KB mappings reach enterprise patch pipelines — causing confusion about which KB applies to which build. Always confirm KB mappings against the MSRC Update Guide or Microsoft Update Catalog.
• Community PoCs and forum discussions can accelerate both defender detection and attacker exploitation; treat community PoCs as a double‑edged sword and validate any third‑party exploit before using in a controlled defensive test.

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Practical checklist for WindowsForum readers (operational takeaways)​

• Immediately consult Microsoft’s Update Guide for CVE‑2025‑62470 and determine the KB(s) that apply to your Windows build. Confirm the KB prior to mass deployment.
• Patch pilot hosts first, validate, then roll out broadly with verification steps for KB installation and clfs.sys file versions.
• If patching is delayed, remove local admin rights from non‑trusted accounts, enable WDAC/AppLocker where possible, and restrict interactive logons on critical systems.
• Tune EDR to detect unexpected SYSTEM process creation, token duplication, and clfs.sys crashes; collect memory dumps and logs for any suspected exploitation attempts.
• Maintain an incident playbook that treats any post‑patch public PoC as a trigger to escalate patching and hunt for exploitation indicators across your estate.

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Wider context: CLFS as an ongoing attack surface​

Windows CLFS has been the subject of multiple CVEs through 2024–2025 and has produced both proof‑of‑concepts and real‑world exploitation in the wild. Microsoft’s MSTIC/MSRC teams and multiple national CERTs have previously documented targeted exploitation of CLFS flaws in ransomware and espionage campaigns, which underscores why a new CLFS CVE like CVE‑2025‑62470 is classed as high priority for detection and patch management. Treat CLFS defects with urgency comparable to other kernel‑level EoP issues — they are powerful enablers for attackers who already have a foothold.

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Final assessment and recommended next steps​

CVE‑2025‑62470 is a credible, high‑severity heap‑based buffer overflow in the Windows Common Log File System driver that enables local elevation of privilege. The vulnerability is listed in public CVE feeds and the vendor’s update guide; however, low‑level exploit details and proof‑of‑concept artifacts were not broadly published at the time of disclosure. That limited public detail should not be mistaken for low risk: the historical pattern for CLFS vulnerabilities is that attackers quickly convert kernel memory‑safety bugs into reliable escalation primitives once exploit details or patch diffs become available.
Immediate actions:

• Map CVE‑2025‑62470 to Microsoft KB(s) for your OS builds using the MSRC Update Guide and the Microsoft Update Catalog.
• Patch pilot hosts, verify, and then perform a prioritized rollout to all at‑risk systems.
• Harden endpoints that cannot be patched immediately by removing local admin rights, enabling allow‑listing, and restricting interactive logons.
• Tune monitoring for clfs.sys crashes, unusual SYSTEM elevations, and token manipulation behaviors.
• If you detect suspected exploitation, collect memory and disk artifacts and treat the host as potentially compromised.

This advisory reinforces a simple truth for Windows security: kernel drivers that parse on‑disk data are rich targets. Rapid patch mapping, disciplined rollout, and vigilant telemetry are the most effective defenses against the threat CVE‑2025‑62470 represents.

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Conclusion
CVE‑2025‑62470 adds another high‑value, kernel‑level elevation‑of‑privilege item to the CLFS vulnerability series that defenders have been monitoring through 2024–2025. The presence of a heap‑based overflow that allows local privilege escalation is a material risk — particularly in environments where attackers can obtain a low‑privilege foothold. The most reliable protective action is to map the CVE to the specific Microsoft updates that apply to your Windows builds and to deploy those updates promptly, while using compensating controls and increased monitoring to reduce the attack surface during any deployment window.

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Source: MSRC Security Update Guide - Microsoft Security Response Center