CVE-2025-62457: Patch Cloud Files Mini Filter Driver for LPE (OOB Read)

An out‑of‑bounds read in the Windows Cloud Files Mini Filter Driver (cldflt.sys) can be abused to escalate privileges locally, and administrators should treat the resulting CVE — reported under CVE-2025-62457 by Microsoft — as a high‑priority patching item for any systems that expose or use cloud file placeholder functionality.

Background / Overview​

The Windows Cloud Files Mini Filter Driver (cldflt.sys) is a kernel‑mode component that underpins on‑demand cloud file scenarios: OneDrive, SharePoint, and other sync clients rely on the driver to present placeholder files and to resolve data from cloud storage only when requested. Because the component runs in kernel context, bugs in its parsing, validation, or IOCTL handling can lead to severe outcomes — including local privilege escalation (LPE) where a non‑privileged or authorized user is able to gain SYSTEM or equivalent rights.
Microsoft’s advisory for CVE‑2025‑62457 describes an out‑of‑bounds read inside the Cloud Files mini filter driver that could allow an authorized (local) attacker to elevate privileges. The reported weakness is consistent with a pattern seen repeatedly in cldflt.sys advisories over recent years: insufficient validation of user‑supplied inputs or race/validation windows in path/metadata handling that allow kernel code to read outside intended buffers and disclose data or reach an exploitable state.
This feature article explains what the vulnerability class means in practical terms, evaluates attack complexity and likely exploitation scenarios, and lays out mitigation, detection, and deployment guidance Windows administrators need right now.

---

What the vulnerability is — technical summary​

The core defect: out‑of‑bounds read​

An out‑of‑bounds read (CWE‑125) occurs when code reads memory past the end of a buffer or structure. In kernel code, such reads can:

• Leak kernel memory contents (including pointers and sensitive data).
• Reveal layout information that helps defeat Address Space Layout Randomization (ASLR).
• Create the preconditions needed for follow‑on exploitation (e.g., turning information disclosure into a write primitive).

In the Cloud Files driver, the affected code path processes inputs coming from user‑mode services and sync clients. The advisory indicates that a specific input or IOCTL results in the driver reading beyond a validated boundary — an action an attacker with local access can trigger to obtain privileged information or to craft an exploit chain that escalates privileges.

Attack prerequisites and scope​

• Local, authorized attacker: The vulnerability requires local access to the host. It is not reported as remotely exploitable by unauthenticated actors.
• Authenticated user context: An attacker who can run code as a standard user (or an attacker that already controls a user process) can attempt exploitation.
• Affected component: Systems with the Cloud Files mini filter active (for example, machines running OneDrive or other cloud‑sync integration that uses cldflt.sys) are in scope.

Potential impact​

• Elevation of privilege (EoP): If exploitation is successful, an attacker could obtain SYSTEM privileges on the host.
• Information disclosure: Out‑of‑bounds reads often expose kernel memory; attackers can use leaked addresses or tokens to sequence additional kernel exploits.
• Persistence & post‑compromise pivoting: SYSTEM privileges allow installing persistence mechanisms, tampering with security agents, and moving laterally.

---

Why kernel driver bugs in cloud sync code matter​

The Cloud Files mini filter lives at a privileged boundary between user processes and kernel file handling. Several properties make this component a high‑value target for attackers:

• It is commonly present on desktop, laptop, and server SKUs where cloud integration is enabled.
• It exposes device interfaces and IOCTLs that user‑mode sync clients rely upon, increasing the opportunities for malformed inputs to reach privileged code.
• The kernel context magnifies the impact of memory‑safety failures: a single pointer error can be escalated into full SYSTEM compromise.
• Cloud sync features interact with user documents and network resources, creating meaningful opportunities for local or least‑privileged compromise to cascade into enterprise‑level incidents.

Historical advisories for cldflt.sys over the past few years repeatedly show the same high‑level outcome: local attackers can craft inputs that trick kernel code into unsafe reads or writes. That pattern means defenders should treat new Cloud Files CVEs as priority updates.

---

Exploitation scenarios — realistic threat models​

Below are practical exploit pathways defenders should consider. Each assumes a local foothold (malware, malicious insider, or an attacker who lures a user to run code).

1) Malicious local binary invoking a device IOCTL​

• Attacker runs code as a standard user and opens the Cloud Files device or triggers a file‑system operation that reaches cldflt.sys.
• The exploit crafts a specially formed request buffer or manipulates placeholder metadata so the driver performs an out‑of‑bounds read.
• The read leaks kernel pointers or sensitive values to user space.
• The attacker uses leaked addresses to defeat KASLR and construct a reliable kernel exploitation chain (token swap, function pointer overwrite), elevating to SYSTEM.

This path is common when drivers expose DeviceIoControl interfaces that read variable‑length buffers without robust bounds checks.

2) Race/TOCTOU combined with path manipulation​

• A time‑of‑check/time‑of‑use (TOCTOU) window or insufficient locking allows the attacker to change a file or reparse point between a validation step and its use.
• When the driver subsequently dereferences a pointer or reads metadata, it reads past the intended buffer or from an attacker‑controlled region.
• The attacker obtains kernel structure data or crafts a memory layout favorable to exploitation.

TOCTOU scenarios are especially plausible in file‑sync code that performs rapid user‑space/kernel‑space coordination.

3) Chaining an information leak into a write primitive​

• Out‑of‑bounds reads often leak critical mapping information. Once an attacker knows kernel addresses, they can combine that leak with another bug (even a modest one) to obtain write capabilities — and from there, SYSTEM.

In short: an out‑of‑bounds read is rarely the final step; it usually primes the pump for a full privilege escalation chain.

---

Who is at risk​

• Desktop and laptop endpoints with cloud sync enabled (OneDrive is the typical example).
• Managed devices in enterprise fleets where users have write access to sync origins.
• Servers or shared workstations with sync features active, especially in multi‑user or multi‑tenant contexts.
• Systems where users can run arbitrary code (developers, test boxes, student lab machines).

Priority patches should go to:

• Machines with privileged or sensitive data.
• Jump boxes, admin workstations, domain‑joined machines used for management.
• Shared systems or virtualization hosts that accept user images or mounts that could indirectly trigger cldflt.sys code paths.

---

Detection: what to look for​

Out‑of‑bounds read exploitation is often noisy at the kernel level or leaves telltale trails. Hunting guidance:

• Monitor kernel crash logs and oops traces. Unexpected faulting in cldflt.sys or stack traces involving Cloud Files handlers is high value.
• Watch for repeated failures or anomalous DeviceIoControl calls to Cloud Files‑related device objects from user processes.
• Endpoint detection rules:
• User processes opening unusual handles to device objects that correspond to the cloud filter.
• Rapid creation and removal of placeholder file metadata or reparse points.
• Unusual Local service processes making file operations that could exercise the filter.
• EDR telemetry: search for sequences where low‑privileged processes spawn commands shortly after interacting with cloud sync APIs or files — could indicate a chain from local exploit to privilege spike.
• Post‑compromise artifacts: unexpected service creation, new scheduled tasks, or modifications to security agent binaries in the presence of suspicious cldflt.sys activity.

Because vendors sometimes omit low‑level exploit details in official writeups, defenders should rely on behavioral indicators and kernel telemetry for reliable detection.

---

Mitigations and immediate steps​

• Install Microsoft’s security update for CVE‑2025‑62457 immediately. The vendor’s patch is the definitive remediation. Prioritize high‑value endpoints and expand to the rest of the estate in a staged fashion.
• Restrict local code execution until patched. Where possible, use application allow‑listing (WDAC/AppLocker), enforce least privilege, and restrict who may install or run untrusted binaries.
• Limit cloud sync features on high‑risk endpoints. For servers, jump boxes, or systems that do not require OneDrive/sync functionality, disable cloud file integrations or the related service until updates are applied.
• Harden ACLs for shared folders and placeholder locations. Reduce write permissions to directories used by sync clients; many LPE attacks rely on writable paths that allow an attacker to influence driver inputs.
• Use centralized patch management. Map affected KBs to OS builds and push updates via SCCM/Intune/WSUS. Stagger deployment with pilot rings to catch compatibility issues before broad rollout.
• Consider temporary compensating controls when immediate patching is impossible:
• Restrict access to device interfaces if possible.
• Remove non‑admin write access to critical sync paths.
• Increase monitoring and alerting for cldflt.sys‑related errors.

---

Hardening guidance for security teams and administrators​

• Apply the vendor update across all supported Windows client and server SKUs in your inventory. Confirm successful installation by validating installed KBs or build revisions.
• Review group policies or provisioning scripts that enable cloud sync by default; only enable on systems where required.
• In enterprise environments, restrict who can become a local administrator and use privileged access management (PAM) to reduce the blast radius of a local compromise.
• Update detection content in SIEM/EDR to include new kernel fault patterns and to watch for post‑exploit behaviors (token impersonation, service manipulation).
• For organizations using image templates or golden images, ensure those images include the updated OS and drivers before redeployment.
• Conduct focused hunts on devices handling high‑value data or acting as management hosts (jump boxes). Look for indicators of local exploitation attempts prior to patching.

---

Risk assessment — how urgent is this?​

• The vulnerability is local in nature, so it is not a direct remote worm vector. That lowers the immediate global exploitation risk compared to an unauthenticated network RCE.
• However, local LPEs are routinely used as the second stage in multi‑stage attacks. An attacker who gains a foothold via phishing, malicious installer, or compromised app can use a local LPE to achieve SYSTEM and persist.
• In multi‑user or multi‑tenant environments and on privileged endpoints, the impact is high: SYSTEM on a domain‑joined admin workstation or a virtualization host is a severe exposure.
• Therefore, treat CVE‑2025‑62457 as high priority for patching in any environment where:
• Users can run arbitrary code, or
• Cloud sync features are enabled on privileged devices, or
• Devices are shared/multi‑user or host sensitive credentials.

---

How to validate the fix in your environment — a short checklist​

• Identify affected systems:
• Query your inventory for OS builds and for installations of OneDrive or other sync clients that register the cloud filter.
• Obtain the exact KB mapping for your build from Microsoft’s update guidance and download the appropriate packages.
• Patch a pilot group (10–50 endpoints) and validate:
• Reproduce typical workflows: file sync, placeholder access, and user file operations.
• Check for stability regressions or unexpected reboots.
• Monitor telemetry:
• Verify that kernel errors related to cldflt.sys no longer appear.
• Confirm that the earlier detection signals are absent or dramatically reduced.
• Roll out broadly in prioritized waves:
• Domain controllers and admin workstations
• Jump boxes and management endpoints
• Developer and build hosts
• Remaining endpoints and servers
• Post‑deployment audit:
• Ensure all systems report the expected KB/build level.
• Update runbooks and incident playbooks to reflect the remediation.

---

What we could not verify and cautionary notes​

• At the time of writing, details published by the vendor are intentionally concise to limit short‑term weaponization — this is standard vendor practice for kernel vulnerabilities. Some public trackers and writeups reference Cloud Files driver flaws in 2024–2025 with similar impact patterns, which supports the high‑level description above.
• Specific exploit code, proof‑of‑concepts, or in‑the‑wild telemetry tied uniquely to CVE‑2025‑62457 were not broadly available in public feeds at the time of this report. Absence of public PoCs does not guarantee absence of private exploitation.
• Administrators should rely on the Microsoft Security Update Guide and their enterprise telemetry for authoritative KB mappings and post‑patch validation rather than third‑party summaries alone.

---

Practical incident response notes (if compromise is suspected)​

• Isolate the host immediately from the network to prevent lateral movement.
• Preserve volatile data: collect kernel crash dumps, event logs, EDR telemetry, and any suspicious DeviceIoControl calls or open handles.
• Perform an image backup before remediation to keep an evidence trail for forensics.
• Rotate credentials and keys that may have been present on the host or accessible to the impacted processes.
• Rebuild the host from known‑good image if evidence indicates kernel compromise or persistence at SYSTEM level — in many cases, a rebuild is the most reliable remediation after a kernel exploit.
• Hunt for follow‑on activity across the estate (new admin accounts, scheduled tasks, remote access tools, lateral authentication attempts).

---

Long‑term recommendations​

• Treat kernel drivers and platform integration points (filters, filesystem drivers, virtualization VSPs) as high‑risk assets and include them in vulnerability scanning and asset inventory tooling.
• Enforce least privilege and make local admin access an exception, not a default. Use PAM/JIT to grant temporary elevated rights.
• Harden developer/test fleets to reduce the occurrence of locally executable untrusted code; build pipelines should run in isolated, patched environments.
• Maintain a rapid patching cadence for high‑impact CVEs and keep a tested rollback plan for any update that destabilizes critical services.
• Enhance endpoint telemetry capture for kernel faults and device interface activity, as these are frequently the earliest signs of kernel‑level exploitation attempts.

---

Conclusion​

CVE‑2025‑62457 — an out‑of‑bounds read in the Windows Cloud Files Mini Filter Driver — is a textbook example of why kernel‑mode memory‑safety defects remain urgent to fix. While the attack vector requires local access, the ability to turn an OOB read into reliable privilege escalation makes the flaw materially serious for desktops, servers, and management hosts that expose cloud file placeholder functionality.
Actionable priorities for Windows administrators and security teams:

• Apply the Microsoft update that addresses CVE‑2025‑62457 without delay and validate installation across the estate.
• Harden endpoint configurations by restricting cloud sync on high‑value machines and limiting local administrative privileges.
• Update detection rules to flag anomalous cldflt.sys activity and hunt proactively for any signs that the vulnerability was abused before patching.

This class of vulnerability demonstrates a continuing truth for Windows security: keep kernel components up to date, limit local attack surface, and pair patching with detection and response plans so an exploited flaw does not become a full domain compromise.

---

Source: MSRC Security Update Guide - Microsoft Security Response Center