Microsoft has publicly recorded CVE‑2025‑62221 — a
use‑after‑free (UAF) defect in the Windows
Cloud Files Mini Filter Driver (cldflt.sys) — that can be triggered by a local, authorized user to escalate privileges to SYSTEM, and administrators should treat this as a high‑priority local escalation risk while they map the CVE to exact KBs and deploy vendor updates.
Background / Overview
The Cloud Files mini filter (commonly seen as cldflt.sys and used by OneDrive, Projected File System providers, and other sync clients) implements kernel‑side plumbing that exposes cloud‑backed files as ordinary local objects. Because the component executes in kernel mode and accepts complex inputs from userland (IOCTLs, placeholder metadata, reparse points, provider callbacks), memory‑safety and access‑control errors in this driver are high‑impact: they can convert a low‑privileged local foothold into full SYSTEM control. This class of bugs has surfaced repeatedly in the Cloud Files / ProjFS family over recent years, making new advisories in the same family operationally urgent.
What Microsoft recorded for CVE‑2025‑62221 is concise: a
use‑after‑free condition in the Cloud Files mini filter driver that can be abused by an authorized local attacker to escalate privileges. Public trackers and vendor advisories assign a High severity rating (CVSS 3.1 base ≈ 7.8) and indicate the vector is
local only (no unauthenticated remote exploitation reported). Administrators must rely on the Microsoft Security Update Guide to map the CVE to the correct KB(s) for each Windows build before rolling updates enterprise‑wide.
Why this vulnerability matters
Kernel mode + userland inputs = high leverage
Mini‑filter drivers operate at the kernel / file system boundary. They are attractive escalation targets because:
- They run with kernel/System privileges, so any control flow or memory corruption in the driver can be escalated immediately.
- They accept structured inputs from user mode: IOCTL buffers, reparse point data, provider callbacks and placeholder metadata, increasing opportunities for malformed or raced inputs to reach privileged code.
- A small memory safety bug (UAF, OOB read, integer overflow) often yields an info‑leak or write primitive that attackers chain into reliable elevation primitives.
Operational risk model
- Attack surface: any system where cloud placeholder functionality is present (OneDrive, some SharePoint/third‑party sync clients, ProjFS providers) is potentially exposed.
- Threat model: the attacker must be local and authorized (able to run code or interact with the driver) — but that requirement is increasingly easy for adversaries to meet via phishing, malicious installers, compromised developer machines, or guest VM code that can reach host interfaces.
- Impact: successful exploitation can yield SYSTEM privileges → persistence, credential theft, tampering with defenses, lateral movement, and host or domain compromise.
Because of that domino effect, even local, non‑networked vulnerabilities in kernel drivers are treated as high operational priority in enterprise environments.
Technical analysis — what a use‑after‑free means here
A
use‑after‑free occurs when code retains or dereferences a pointer to an object after that object has been freed. In kernel-mode drivers this leads to several practical exploitation primitives:
- Arbitrary read / information disclosure: the freed memory can be reallocated with attacker‑controlled data; reading it can disclose kernel pointers or sensitive contents that defeat kernel address randomization.
- Arbitrary write / control flow hijack: by reusing freed slots or combining UAF with heap grooming, an attacker can place controlled structures where the kernel will later dereference them, enabling write‑what‑where behavior or function pointer overwrites.
- Reliability challenges: turning a UAF into a stable exploit often requires timing, heap shaping, or chaining to other primitives (TOCTOU windows or additional driver flaws), but once achieved the result is a robust escalation primitive.
Public vendor messaging typically omits low‑level exploit details for kernel bugs. That reduces immediate exploit publication risk but does not eliminate it — historically, Cloud Files mini‑filter vulnerabilities have been weaponized quickly in post‑compromise scenarios. Administrators should therefore act as if an exploitation path is realistic even when PoCs are not public.
Realistic exploitation scenarios
Below are pragmatic, high‑probability paths an attacker would consider when exploiting a cldflt.sys UAF:
1) Device IOCTL abuse (most common)
- Attacker runs code as a standard user.
- The process opens the Cloud Files device object (or triggers a file operation that reaches cldflt.sys) and issues a specially crafted DeviceIoControl/IOCTL or malformed placeholder metadata.
- The driver frees an object but keeps a stale pointer; subsequent operations dereference that pointer and interact with attacker‑controlled memory, enabling a read or write primitive to escalate to SYSTEM.
2) TOCTOU / race conditions combined with path manipulation
Mini filters frequently validate metadata in user mode or perform short‑lived checks in kernel mode. A carefully orchestrated race — where an attacker flips the backing memory or the target path between check and use — can create conditions for UAF dereferences to reach attacker buffers. This is especially plausible in cloud‑sync scenarios where user‑mode providers and kernel drivers communicate rapidly.
3) Chaining info leak → write primitive
Even if the UAF initially yields only an information leak, leaked kernel addresses defeat KASLR and make subsequent heap grooming or exploitation of smaller bugs far more reliable. Attackers commonly use multi‑stage chains: information disclosure → gadget discovery → arbitrary write → SYSTEM token theft.
Practical takeaway: defenders must assume an attacker with a local foothold can convert this vulnerability into a complete compromise if it is not patched.
What is known and what remains uncertain
- Known, verifiable facts:
- Microsoft has recorded CVE‑2025‑62221 in its Security Update Guide and classified it as a Cloud Files mini‑filter driver issue (cldflt.sys). Administrators should use the Update Guide to map CVE → KB → affected OS builds.
- Public trackers report a High severity rating and a CVSS 3.1 base score of 7.8 (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H).
- Check Point and other security vendors have published detection/protection signatures for the CVE and listed affected SKUs in their advisories, indicating industry awareness and protective controls available from third‑party products.
- Less certain / unverifiable at time of writing:
- There is no authoritative public proof‑of‑concept (PoC) or public exploit code for CVE‑2025‑62221 published by Microsoft in the advisory itself; vendor pages deliberately omit low‑level exploit mechanics. If a PoC appears later in public forums, the weaponization risk will rise sharply — treat any PoC release as an immediate trigger to accelerate patching.
Whenever a vendor page leaves technical mechanics out, defenders should assume the worst‑case exploitation path and prioritize patching accordingly.
Detection and hunting guidance
Because kernel exploitation attempts leave both kernel‑level and host‑level artifacts, defenders can tune telemetry to detect likely exploitation attempts even before a public PoC is released.
Key signals to monitor:
- Kernel crashes, BSODs or KMODE_EXCEPTION traces referencing cldflt.sys or other Cloud Files/ProjFS symbols in memory dumps. Preserve full memory dumps for triage.
- Unusual or frequent DeviceIoControl calls from unprivileged processes to cloud‑filter device objects. Audit processes that open handles to filter device names and IOCTL patterns.
- Rapid creation/manipulation of placeholder files, reparse points, or projected file provider operations by uncommon user processes.
- Sudden token impersonation API use, unexpected creation of services/scheduled tasks by low‑privileged accounts, or new SYSTEM‑owned binaries that appear after suspicious DeviceIoControl sequences.
Suggested hunt queries / telemetry combinations:
- Kernel crash dumps referencing cldflt.sys + host process that recently opened cloud filter device handle.
- EDR alerts for DeviceIoControl from non‑standard processes + subsequent creation of SYSTEM services or modified security agent files.
If you suspect exploitation, isolate the host, collect memory and crash dumps, and treat the machine as a high‑priority incident (UAF exploitation in kernel drivers commonly leads to complete host compromise).
Mitigations and recommended remediation path
The single most effective mitigation is to apply the vendor fix. The Microsoft Security Update Guide maps CVE → KB → affected SKUs; use that canonical mapping to build your deployment plan. If you cannot patch every affected machine immediately, apply compensating controls as described below. Immediate 0–72 hour checklist
- Inventory: Identify hosts with cloud placeholder functionality (OneDrive, ProjFS providers, third‑party sync clients) and map Windows build numbers. Use your patch management tools to determine which images require the CVE KB(s).
- Confirm vendor mapping: Use the Microsoft Update Guide entry for CVE‑2025‑62221 and the Microsoft Update Catalog to obtain the precise KB(s) for each affected build. Do not assume a single KB covers all SKUs.
- Pilot: Apply available KB(s) to a small pilot group (admin workstations, jump boxes, VDI images) and verify driver version and system behavior. Check for regressions in cloud sync or projected file functionality during the pilot.
- Deploy: Roll updates in prioritized waves: hosting/hypervisor hosts and admin jump boxes → servers and domain controllers → broad endpoint estate. Ensure reboots complete and verify driver file versions post‑patch.
Short‑term compensating controls (where patching is delayed)
- Restrict local code execution: enforce Windows Defender Application Control (WDAC), AppLocker policies, or strong application allow‑listing to limit ability of untrusted binaries to run.
- Remove unnecessary local admin rights: minimize accounts that can run arbitrary installers or scripts.
- Limit or disable cloud placeholder features on high‑value servers (where practical): server SKUs that do not require OneDrive/ProjFS should have those features removed or disabled until patched.
- Harden device object ACLs: where possible, restrict access to filter device objects or project provider interfaces so only trusted system services can open handles (note: this can break legitimate clients — test before enforcing).
- Update IDS/IPS/EDR content: apply vendor protections and signatures (many vendors released IPS signatures for Cloud Files mini‑filter CVEs the same day Microsoft recorded the CVE).
Verification after patching
- Confirm the KB(s) are installed (Get‑HotFix or enterprise patch reporting tools).
- Verify the updated driver file version for cldflt.sys on representative hosts.
- Monitor EDR for the hunt signals above to detect attempted exploitation attempts that predate or bypass your patching window.
Prioritization guidance for different environments
- High priority (patch first)
- Domain controllers, jump boxes, admin workstations and management servers.
- Hyper‑V hosts, VDI infrastructure and multi‑tenant hosts where a single host compromise would impact many guests.
- Developer machines and build servers where untrusted code is regularly executed.
- Medium priority
- Regular endpoints with cloud sync enabled but limited exposure; schedule patch waves after high‑priority hosts.
- Lower priority
- Isolated systems that do not have cloud placeholder providers installed and where local code execution is tightly controlled; still validate via MSRC mapping.
Practical hardening checklist (numbered)
- Map: Use Microsoft’s Security Update Guide to extract the KB(s) for CVE‑2025‑62221 per OS build.
- Inventory: Identify all systems with OneDrive/ProjFS or third‑party providers.
- Pilot & Test: Apply updates in a controlled pilot ring and verify cloud functionality.
- Deploy: Stage updates by priority and ensure reboots complete.
- Compensate: Apply WDAC/AppLocker, restrict local admin rights and restrict device object access if necessary.
- Hunt: Monitor for cldflt.sys crashes, DeviceIoControl anomalies, and sudden SYSTEM process creations.
- Respond: If exploitation is suspected, isolate, collect full memory dumps, and escalate to incident response with forensic artifacts preserved.
Longer term recommendations and engineering perspective
- Reduce the attack surface: where cloud placeholder features are not essential (servers, jump hosts), configure images without ProjFS/placeholder components. This prevents kernel filter exposure in high‑value hosts.
- Principle of least privilege and application allow‑listing: the fewer ways untrusted code can run, the less opportunity for local‑to‑kernel exploits to be triggered.
- Kernel hardening and diversity: enterprise fleets should prioritize mitigations that detect or block kernel exploit primitives (kernel ASLR verification, kernel control flow integrity where supported). These techniques raise the bar for turning UAFs into stable RCE.
Conclusion
CVE‑2025‑62221 is a use‑after‑free in the Windows Cloud Files mini filter driver that creates a realistic local elevation‑of‑privilege path. Microsoft has recorded the CVE in the Security Update Guide and industry vendors (including Check Point and mainstream CVE aggregators) reflect a High severity score and have produced protections and advisory guidance; administrators must map the CVE to exact KBs for their Windows builds, patch high‑value systems first, and apply compensating controls until updates are installed and verified. Treat every new cldflt.sys advisory as a critical local EoP risk: these filters sit where user inputs meet the kernel, and a single memory‑safety failure can convert a foothold into system control. Prioritize identification, pilot the vendor updates, harden local execution policies, and tune telemetry to detect kernel‑level anomalies while updates are staged and completed.
If further guidance is required for mapping KBs to your specific Windows builds, consult your enterprise patch management tooling and the Microsoft Update Guide entry for CVE‑2025‑62221 as the canonical mapping reference.
Source: MSRC
Security Update Guide - Microsoft Security Response Center