CVE-2026-20940: Patch and Hunt Windows Cloud Files Mini Filter Driver

Microsoft’s Security Update Guide lists CVE-2026-20940 as an elevation-of-privilege (EoP) vulnerability in the Windows Cloud Files Mini Filter Driver, and defenders should treat any new Cloud Files mini-filter entry as a high-priority patch-and-hunt item until proven otherwise.

Background / Overview​

The Windows Cloud Files Mini Filter Driver (commonly cldflt.sys) implements kernel-side plumbing that makes cloud-backed placeholder files (OneDrive, Projected File System providers, other sync clients) appear and behave like local files. Because it runs in kernel context and processes structured inputs originating in user mode (IOCTLs, reparse points, placeholder metadata), memory-safety and access-control bugs in this driver consistently yield local privilege escalation primitives. Public advisories and community analysis across multiple Cloud Files CVEs demonstrate the same high-level outcomes: local attackers with some code-execution capability can often convert modest bugs into SYSTEM-level access.
Microsoft’s Update Guide is the canonical, authoritative record for each CVE the vendor tracks; the MSRC entry for CVE-2026-20940 is therefore the first place defenders must consult to map CVE→KB→affected OS builds and obtain the official remediation packages. Because the Update Guide pages render dynamically, administrators should retrieve KB mappings from the Guide or the Microsoft Update Catalog on a secure admin host.

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What we can verify about CVE-2026-20940 right now​

• The vendor has recorded CVE-2026-20940 and associated it with the Windows Cloud Files Mini Filter Driver (cldflt.sys). This fact appears in Microsoft’s Update Guide listing for the CVE.
• Historically and in recent patch cycles, Cloud Files mini-filter defects have been classified by Microsoft and independent trackers as local EoP / information-disclosure issues (common classes: use-after-free, out‑of‑bounds read, and TOCTOU race conditions). Those same technical failure modes are the most-likely root causes for new Cloud Files CVEs unless vendor detail states otherwise.
• Independent community and industry coverage routinely places Cloud Files mini-filter vulnerabilities among high-priority fixes and frequently reports active exploitation when telemetry supports it (for earlier Cloud Files CVEs such as CVE‑2025‑62221). That operational pattern raises the urgency around any new Cloud Files CVE.

Caveat: Microsoft advisories for kernel-mode bugs frequently omit low-level exploit mechanics and may withhold active exploitation details until vendor telemetry can be safely summarized. Where vendor text is absent or deliberately abstract, public technical specifics (exact IOCTLs, routine names, or exploit primitives) may not be available immediately and must be treated as unverifiable until authoritative write-ups or PoCs appear.

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Technical anatomy — why Cloud Files mini-filter bugs are dangerous​

The Cloud Files mini-filter occupies a privileged, user→kernel boundary and accepts multiple structured input surfaces:

• Device IOCTL interfaces and named device objects (user-mode code can open device handles).
• Projected file metadata and reparse-point data that the driver resolves or validates.
• Brokered user→kernel interactions between sync clients (user-mode) and cldflt.sys.

These interaction surfaces make the driver attractive: a single memory-safety error (use‑after‑free, out‑of‑bounds read/write), improper length or bounds checks, or a TOCTOU race can yield one of several powerful primitives in kernel context: information disclosure (leaked kernel pointers defeating KASLR), arbitrary read/write, or reliable token manipulation to spawn SYSTEM processes. The kernel context magnifies impact—small pointer errors quickly convert into full-system compromise.
Typical exploit chains seen across the Cloud Files / ProjFS family follow this path:

• Attacker obtains a local foothold (phishing, malicious installer, compromised VM, or unprivileged process execution).
• The attacker interacts with cldflt.sys (open device handles, issue DeviceIoControl, trigger placeholder metadata flows).
• The driver mishandles the malformed or raced input (UAF/OOB/TOCTOU).
• The attacker leverages an info-leak or write primitive to defeat KASLR and perform token swap or function-pointer hijack.
• SYSTEM privileges are obtained; attacker persists, disables defenses, and moves laterally.

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Verification and cross-checking: what independent sources tell us​

To meet high journalistic standards, the following cross-references underpin the public claims and the operational recommendations:

• Microsoft Security Update Guide (vendor canonical listing for CVE-2026-20940). Administrators must use this to map CVE→KB→OS SKU for remediation.
• Community and incident reporting channels (security sites and patch roundups) show consistent patterns: the Cloud Files driver has produced multiple high‑impact EoP CVEs across late 2024–2025 and into early 2026. Independent outlets have repeatedly flagged active exploitation for related Cloud Files CVEs, reinforcing the operational risk of similar future entries. Use these reports to prioritize patch windows, but validate exploit/active‑use claims against MSRC telemetry or national incident listings (e.g., CISA KEV) before assuming active weaponization of CVE‑2026‑20940.
• Community technical writeups, defensive playbooks, and forums (Windows Forum threads and incident summaries) provide usable detection heuristics and mitigation playbooks derived from past Cloud Files advisories; these are practical for defenders while vendor pages remain intentionally sparse on exploit detail.

If the question is “is this CVE actively exploited?”: at the time of publication the vendor’s Update Guide lists the CVE; independent public telemetry for CVE‑2026‑20940 was not widely and authoritatively documented in accessible threat reports. Given recent trends where Cloud Files CVEs were weaponized quickly (notably CVE‑2025‑62221), defenders should assume high risk and treat the CVE as urgent pending vendor/external telemetry confirming exploitation.

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Practical detection and hunting guidance​

Because Microsoft often omits exploit mechanics for kernel bugs, defenders should hunt with behavioral and kernel-level signals rather than waiting for a PoC. Key telemetry signals to monitor:

• Kernel crash dumps, BSODs, or KMODE_EXCEPTION traces that reference cldflt.sys or Projected File System/mini-filter symbols. Preserve full memory dumps for forensic triage.
• Unusual DeviceIoControl calls: non-privileged user-mode processes opening handles to cloud-filter device objects or issuing repeated/abnormal IOCTL calls. EDR sensors that log DeviceIoControl and device-object access are high value here.
• Rapid creation/manipulation of placeholder files, reparse points, or changed metadata by unexpected processes—especially when followed by token duplication or process elevation events.
• Sudden creation of services, scheduled tasks, or SYSTEM-owned processes spawned shortly after a low-privileged process interacted with cldflt.sys—classic indicator of local EoP usage.

Suggested EDR/SIEM queries and hunt primitives:

• Alert on CreateFile/DeviceIoControl to device paths associated with cloud filter drivers where the caller is a non-elevated account.
• Correlate Process Creation events where parent is a non-admin process that recently accessed cldflt device names.
• Kernel dump triage: Automatic extraction of stack traces that include cldflt.sys for triage and cross-correlation with process timelines.

Document and preserve all evidence (full memory dumps, WER minidumps, Event Logs, driver lists) before rebooting an affected host for forensic value.

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Immediate mitigations and deployment guidance (0–72 hours)​

• Consult Microsoft’s Security Update Guide entry for CVE‑2026‑20940 and map the exact KB(s) to your estate. Apply the KB(s) in accordance with your standard test → pilot → broad rollout model. Verify driver file revisions post-install and reboot as required.
• Prioritize patching for:
• Administrative jump boxes and domain-joined management hosts.
• VDI/VDI host pools and shared workstations.
• Developer machines and build servers where local code execution is allowed.
• If you cannot patch immediately, apply compensating controls:
• Enforce least privilege: remove unnecessary local administrator rights.
• Restrict or disable cloud sync/placeholders on high-risk endpoints (e.g., uninstall OneDrive or disable Projected File System features on jump boxes).
• Deploy WDAC/AppLocker or EDR-based execution restrictions to limit arbitrary binary execution by non-privileged users.
• Network and host isolation for unpatched, high-value hosts until remediation is complete. Segment management networks and enforce stricter authentication for any hosts that cannot be patched promptly.
• Update detection rules and apply vendor-provided signatures or mitigations (several security vendors publish temporary signatures for known IOCTL abuse patterns and kernel exploit attempts). Always test vendor signatures to minimize disruptions.

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Operational risk assessment — strengths, weaknesses, and practical trade-offs​

Strengths of immediate vendor patching:

• Fix is definitive: vendor KB replaces/updates the vulnerable driver; this is the only fully reliable remediation.
• Rapid deployment to prioritized hosts materially reduces risk of post-compromise escalation.

Operational weaknesses and risks:

• Patch fatigue and legacy systems: many organizations run unmanaged endpoints or custom images that complicate rapid rollouts.
• Regressions: kernel-driver updates can affect placeholder semantics; test patches in pilot rings (50–200 hosts representative of different roles) before broad rollout.

Trade-offs:

• Rushed mass rollouts reduce risk window but increase the chance of service disruptions. Maintain tested rollback and snapshot plans for critical systems.

Policy implications:

• Use KEV/KEV-like prioritization where available (if CISA adds a CVE to KEV the remediation timeline shortens for FCEB agencies).
• Strengthen inventory accuracy—if you do not know which hosts have cldflt.sys loaded, you cannot declare remediation complete. Use PowerShell checks or EDR queries to enumerate driver versions across the estate.

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Why defenders must assume urgency even without public PoCs​

Vendor advisories for kernel-mode driver bugs intentionally omit low-level exploit mechanics to limit fallout. However, historical patterns with the Cloud Files mini-filter driver show rapid weaponization: multiple distinct Cloud Files CVEs between 2022–2025 were followed by public PoCs or observed in-the-wild exploitation. Because the attack vector is local but low friction (phishing, malicious installers, compromised VMs, or rogue tenant code), a local EoP is a powerful second-stage tool that converts footholds into full compromise. Treat CVE‑2026‑20940 within that operational context until vendor telemetry says otherwise.

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Quick checklist for Windows administrators​

• 1. Query MSRC Update Guide for CVE‑2026‑20940 and map KB(s) → build(s). Verify targeted KBs in your patch management tool.
• 2. Test the patch in a pilot ring (include jump boxes, VDI hosts, admin workstations). Validate placeholder/OneDrive functionality post-patch.
• 3. Prioritize deployment to high-value hosts. Apply compensating controls for remaining endpoints (WDAC/AppLocker, least privilege, disable cloud sync where feasible).
• 4. Tune detection: DeviceIoControl anomalies, cldflt.sys references in kernel dumps, unusual placeholder/reparse-point activity followed by SYSTEM-level process creation. Preserve memory dumps.
• 5. If compromise suspected: isolate host(s), collect forensic artifacts (memory, event logs, EDR traces), engage IR, and follow reporting obligations (sector-specific or national).

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Final analysis and conclusion​

CVE-2026-20940 sits in a well-understood, repeat-target class: the Cloud Files mini-filter driver. The fundamental reasons this class of bug matters have not changed—kernel-level execution, broad deployment on endpoints with cloud sync enabled, and multiple historical precedents of weaponization mean that any new Cloud Files CVE is a high-priority remediation item. Microsoft’s Update Guide lists the CVE and is your authoritative starting point for KB mapping and remediating affected SKUs; community trackers, vendor analyses, and defensive playbooks provide practical detection and mitigation patterns that have proven effective against analogous Cloud Files CVEs. Actionable priorities for operators remain unchanged: confirm exposure quickly, apply the vendor KBs using a tested rollout plan, harden the local execution surface (remove unnecessary admin rights, block untrusted binaries), and hunt proactively using kernel and IOCTL-level telemetry. Treat the CVE as urgent even where public exploit detail is withheld; the worst-case impact—full SYSTEM compromise—justifies aggressive, evidence-driven remediation and detection. Note on uncertainty: public PoCs, exploit samples, or vendor statements that explicitly declare active exploitation for CVE‑2026‑20940 were not available in authoritative, publicly accessible telemetry at the time this article was produced; where such evidence later appears, defenders should accelerate remediation and broaden incident response activities immediately.

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Security operations teams and Windows administrators should treat CVE-2026-20940 as a priority now: map the KB(s), stage the update, and harden the endpoints that cannot be immediately patched. The combination of kernel context + cloud-file exposure is exactly the kind of failure mode attackers prize; the correct response is decisive patching plus targeted hunting.

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Source: MSRC Security Update Guide - Microsoft Security Response Center