CVE-2025-55680 Patch Cloud Files Mini Filter Driver Privilege Elevation

Microsoft has recorded an elevation-of-privilege vulnerability in the Windows Cloud Files Mini Filter Driver (cldflt.sys) that permits a local actor to obtain sensitive kernel-level information and, in some attack chains, progress to SYSTEM privileges—an urgent issue for shared desktops, VDI hosts, and any environment where unprivileged processes can run locally.

Background​

The Windows Cloud Files Mini Filter Driver (cldflt.sys) implements the placeholder / cloud-backed file model used by OneDrive and other cloud-integrated file systems. Operating as a kernel-mode mini-filter, it mediates file-system operations and therefore runs with the highest privileges available on the host. Kernel-mode faults in filter drivers historically produce high-impact outcomes because they can leak kernel memory, reveal kernel object addresses (defeating KASLR), or introduce primitives that enable privilege escalation.
Microsoft’s public entry for the CVE indicates that the vulnerability allows local information disclosure through the kernel, and that vendor-supplied fixes have been published. Independent trackers and community analyses generally agree on the classification and remedial guidance, but public technical details remain deliberately sparse in Microsoft’s advisory. Administrators should therefore treat the vendor advisory as authoritative for exact KB mappings while using independent analyses for operational guidance.

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What we know right now​

• The vulnerability exists in the Windows Cloud Files Mini Filter Driver (cldflt.sys) and is categorized by Microsoft as a local information-disclosure/EoP class issue.
• The attack vector is local: an adversary must be able to execute code or interact locally with the driver to trigger the condition. This makes mass remote exploitation (worming) unlikely, but the vulnerability is highly useful as a post‑compromise tool.
• Microsoft has published updates to remediate the issue; the Microsoft Security Update Guide is the authoritative place to map the CVE to the exact KBs for each Windows SKU. Administrators must confirm KB→build mappings before declaring a host remediated.
• Public technical specifics (e.g., exact IOCTLs, routine names, or code paths) remain limited in the vendor text; defensive teams must assume realistic exploitation models for kernel info-leaks until third-party technical writeups appear.

These points are corroborated by multiple independent summaries in the posted materials and by the vendor’s advisory guidance; the convergence of vendor acknowledgement and third‑party reporting raises confidence in both the vulnerability’s existence and the broad technical classification.

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Why Cloud Files driver vulnerabilities matter​

Kernel-mode mini-filters like cldflt.sys occupy a privileged execution context and frequently process externally supplied data (file metadata, reparse points, cloud placeholders). That combination creates a high-value attack surface:

• Kernel memory disclosures can leak authentication tokens, cryptographic keys, or internal pointers that defeat kernel address space layout randomization (KASLR).
• Small information leaks are often the final piece of a multi-stage privilege-escalation chain: once an attacker learns pointer layouts or token fragments, previously unreliable techniques become reliable.
• Multi-user or shared-host environments (VDI, RDP servers, CI runners) magnify the risk because non-privileged tenants often run arbitrary code or mount user-supplied images.

In short, an information-disclosure in cldflt.sys is not merely about confidentiality loss; it frequently becomes the enabling primitive for integrity and persistence compromises when chained with other foothold techniques.

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Technical analysis: likely root causes and exploitation model​

Microsoft’s limited public description points at improper input validation and a local information-disclosure outcome, which matches common kernel leak patterns:

• Returning uninitialized or partially-initialized buffers to user mode (leftover kernel memory copied into user buffers).
• Incorrect reporting of returned lengths/status in IOCTL handlers (driver says it returned N bytes while only M < N were actually filled).
• Out-of-bounds reads caused by insufficient parameter validation or reparse-point parsing errors.

A typical exploit path for such vulnerabilities is straightforward in concept:

• A low-privilege process repeatedly invokes the vulnerable driver interface (IOCTL, read, or query) with carefully crafted parameters.
• The driver returns buffers containing kernel-resident data fragments.
• The attacker scans returned buffers for recognizable artifacts (token structures, pointer patterns, GUIDs).
• Extracted artifacts can be used to bypass KASLR, craft kernel memory corruption primitives, or extract credentials/tokens for lateral movement.

Because the driver operates in kernel context, even a small information leak can materially lower the difficulty of converting a local foothold into SYSTEM privileges. For defenders, that means treating information leaks with the same urgency as higher-severity code-execution flaws when those leaks can be chained with other available primitives.

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Validation and confidence in the public details​

Multiple independent trackers and community analyses replicate the same core facts: CVE recorded by Microsoft, local information disclosure in cldflt.sys, and vendor-supplied fixes. This pattern—vendor acknowledgement plus rapid third-party corroboration—provides a high degree of confidence that the vulnerability exists and that the remediation path provided by Microsoft is required operationally.
However, because Microsoft’s advisory intentionally omits low-level details, several specific technical claims remain unverifiable from public material alone (for example, the exact function names, the IOCTL codes involved, or an authoritative exploit proof-of-concept). Until detailed technical analyses or PoCs appear from trusted researchers, those deeper implementation details must be treated cautiously. Defensive guidance should therefore be prioritised on patching and operational mitigations rather than on chasing speculative exploitation paths.

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Practical remediation and rollout guidance​

Applying Microsoft’s supplied updates is the primary and preferred remediation. Follow this prioritized, pragmatic plan:

• Inventory: Map all Windows hosts to exact OS builds and installed updates using your endpoint management system (WSUS, SCCM, Intune, or third-party EPM). Confirm presence of the Cloud Files Mini Filter Driver on hosts where it matters.
• Identify KBs: Consult the Microsoft Security Update Guide for CVE→KB mappings for each build. Do not assume a single KB covers all SKUs. Validate the exact KB ID before deployment.
• Test: Apply the update to a representative test ring. Exercise storage/backup agents, virtualization host agents, and applications that interact with OneDrive or other cloud file services to confirm there are no regressions.
• Deploy: Use an emergency patch ring for domain controllers, privileged admin workstations, and backup servers. Then roll out to broader populations during a controlled maintenance window.
• Verify: Confirm the KBs are installed (Get-HotFix, Windows Update history) and that reboots are applied where indicated. Log the remediation for audit purposes.

If patching cannot occur immediately, apply short-term hardening:

• Enforce least privilege: remove unnecessary local admin accounts and lock down who can run unsigned binaries.
• Restrict access to cloud-file features on high-value servers: consider disabling OneDrive or cloud-file placeholder mounting on jump hosts and servers where not required.
• Enable Memory Integrity (HVCI) and the Microsoft Vulnerable Driver Blocklist where supported; these raise the cost of kernel exploits and prevent loading of known bad drivers.

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Detection, hunting, and forensic guidance​

Because the vulnerability leaks data to local callers, detection focuses on local driver interaction patterns and post-exploit behaviors rather than network signatures.
Recommended detection signals and hunt logic:

• Alert on non-standard processes invoking DeviceIoControl against cldflt.sys or other cloud-files-related device objects from non-privileged users.
• Correlate suspicious DeviceIoControl/driver calls with process creation events (Event ID 4688) and unusual parent-child relationships.
• Hunt for sudden credential reuse, lateral authentication events, or anomalous token usage—these can indicate that leaked secrets were used for follow-on escalation.
• Collect memory and EDR artifacts for any host where suspicious driver interaction is observed and rotate potentially exposed credentials immediately.

EDR rules should prioritize telemetry for kernel-mode interactions, and SOC teams should treat any low-privilege process interacting unexpectedly with kernel drivers as high-fidelity triage candidates.

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Risk prioritization: who should patch first​

• Privileged admin workstations and jump boxes—compromise here yields broad access.
• Domain controllers and backup servers—targets with cached credentials or backup data magnify attacker value.
• Shared VDI/RDP hosts and CI-build servers—multi-tenant environments where unprivileged code is often executed.
• Developer workstations and imaging servers—these often mount user-supplied images and are high-risk for guest→host attack chains.

Consumer single-user machines with strong local account hygiene are a lower deployment priority only in the immediate term, but broad patching across consumer and enterprise devices remains recommended to reduce the overall attack surface.

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Potential operational risks of the patch​

Kernel and driver fixes can sometimes introduce compatibility or stability regressions, particularly on bespoke or older images and with third-party drivers. Best practice is to:

• Test the patch in a staging environment that mirrors production as closely as possible, including backup software and virtualization host agents.
• Monitor for kernel crashes and driver issues immediately after deployment and have rollback plans if necessary.

If your environment includes legacy systems where the patch cannot be applied, document compensating controls and isolate those hosts to reduce local execution risk.

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Attack scenarios and likely threat actors​

Because the vulnerability requires local interaction, the most likely real-world usage is as part of a multi-stage intrusion:

• Opportunistic criminals and commodity ransomware actors could weaponize this as an escalation path after gaining a low-privilege foothold (phishing, malicious installer).
• Targeted threat actors (state-level or espionage groups) find value in kernel info leaks for stealthy lateral movement and persistence inside well-defended networks.

There were no widely reported public proofs-of-concept at the time of the advisory snapshot, which reduces immediate mass-exploitation risk, but history shows that once kernel primitives are publicly demonstrated, weaponization accelerates quickly. Treat absence of PoC as a temporary comfort—not a mitigation.

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Strengths and limitations of public information​

Strengths:

• Vendor acknowledgement and published fixes create a clear remediation path and high confidence in the vulnerability’s existence.
• Independent trackers and community reporting converge on the same classification and operational guidance, enabling defenders to prioritize properly.

Limitations:

• Microsoft’s advisory intentionally omits low-level implementation details; exact exploitation mechanics are therefore not public and cannot be independently verified from the advisory alone. Defensive teams must avoid speculative conclusions about precise code paths until third-party technical analyses are available.

When technical specifics are absent, defenders should prioritize patching, operational mitigations, and detection over speculative exploit development or "reproduction" efforts that could increase risk.

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Executive summary for decision-makers​

• This is a kernel-level information-disclosure / elevation-of-privilege vulnerability in the Windows Cloud Files Mini Filter Driver (cldflt.sys) that requires local interaction.
• Microsoft has published patches; the Security Update Guide is the authoritative source for mapping CVE to KBs. Apply the vendor-supplied updates after appropriate testing.
• Prioritize remediation for privileged admin systems, domain controllers, shared-host environments (VDI/CI), and any hosts where multiple users can execute code.
• Where patching is delayed, enforce least privilege, isolate high-value hosts, enable HVCI/Memory Integrity, and hunt for suspicious local driver interactions.

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Recommended checklist (0–72 hours)​

• Query inventory for Windows build numbers and identify hosts that load cldflt.sys.
• Look up CVE→KB mappings in Microsoft’s Security Update Guide and pull the exact KBs into your test ring.
• Test updates in a staging ring targeting storage/backup and virtualization agents.
• Deploy to high-priority hosts: admin workstations, domain controllers, backup servers, and shared hosts.
• Enable temporary mitigations if patching delayed: restrict OneDrive/cloud placeholder mounting, enforce least privilege, enable HVCI, and apply driver blocklists.

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Conclusion​

Kernel-mode vulnerabilities remain one of the most consequential threat classes in Windows. CVE-2025-55680 (the Cloud Files Mini Filter Driver issue) exemplifies the classic risk: a local bug that, while not remotely exploitable on its own, dramatically empowers attackers who already have a foothold. The vendor’s patch is the definitive fix; organizations should reconcile the CVE against their Windows builds in the Microsoft Security Update Guide, test the KB(s) in representative environments, and prioritize rapid deployment to admin and shared-host systems. Meanwhile, defenders should harden environments with least-privilege controls, memory integrity protections, and EDR hunts focused on local driver interactions—because in practice, information leaks are often the pivot point that turns a local foothold into a full compromise.

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Source: MSRC Security Update Guide - Microsoft Security Response Center