CVE-2025-55336 Info Disclosure in Windows Cloud Files Driver Patch Guide

Microsoft has recorded CVE-2025-55336 — an information-disclosure vulnerability in the Windows Cloud Files Mini Filter Driver (cldflt.sys) that permits an authorized local actor to read sensitive data from affected hosts; the issue is assigned a CVSS v3.1 base score of 5.5 (Medium) and Microsoft has published a remediation for the vulnerability.

Background / Overview​

The Cloud Files Mini Filter Driver (cldflt.sys) is a kernel-mode component used by Windows to implement local representations of cloud-hosted files (the placeholder/placeholder-present model used by services such as OneDrive and other cloud-integrations). Because the driver runs in the operating system’s kernel context, bugs in its logic can expose high-value artifacts — kernel memory, cached tokens, or other secrets — to attackers who already have some level of local presence on the system.
This particular CVE is categorized as an information disclosure (not remote code execution). That means the primary impact is loss of confidentiality rather than integrity or availability, but the security consequences are real: leaked tokens, pointers or credentials often become the fuel for privilege escalation, lateral movement, or persistence in multi-stage attacks. Independent vulnerability trackers and vendor summaries report the attack vector as local, the attack complexity as low, and the privileges required as low — making multi-user systems and shared VDI/RDP hosts especially important to harden and patch quickly.

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What we know: confirmed facts and current vendor posture​

• The vulnerability exists in the Windows Cloud Files Mini Filter Driver and permits a local, authorized actor to disclose information from the host.
• The published severity is CVSS v3.1 5.5 (Medium) with a confidentiality-focused impact.
• Microsoft’s Update Guide lists the CVE and indicates that patches are available; third-party CVE aggregators likewise report a vendor-supplied fix. Administrators should treat the MSRC Update Guide as the canonical source for exact KB/CU mappings and the correct packages to install. Because the MSRC Update Guide is delivered as a JavaScript-driven web application, automated scrapers may not show the full advisory — query it from an interactive, secured admin workstation to capture the definitive KB mapping for your OS SKUs.

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Technical analysis — how an information-disclosure in cldflt.sys typically behaves​

Nature of the flaw​

Public vendor summaries and aggregate analyses characterize CVE-2025-55336 as an information disclosure: code in a privileged component returns or exposes memory that it should have either sanitized or prevented the caller from reading. Typical kernel-mode information disclosure root causes include:

• Uninitialized buffers returned to user-mode without zeroing, leaking residual kernel/process memory.
• Incorrect length or status reporting in IOCTLs or read/write handlers (driver reports more bytes than it actually filled, copying leftover memory to user buffers).
• Insufficient validation of user-supplied parameters, enabling out‑of‑bounds reads that include neighboring memory.

These patterns have been the root cause of similar cldflt.sys and other kernel-mode driver vulnerabilities in the past; the practical result is that an attacker who can trigger the vulnerable path can receive fragments of memory containing secrets, pointers, or state useful for follow-on attacks.

Likely exploitation model​

• The attacker must be able to run code locally or interact with the driver from a local process (the vector is local).
• The simplest exploitation pathway for such an information leak is to call the driver’s IOCTLs, read functions, or file operations with crafted inputs that cause the driver to return more data (or different data) than intended.
• The leaked bytes may include session tokens, cached credentials, internal object pointers (helping defeat KASLR), or configuration strings. Even a small fragment (an authentication token or GUID) can materially reduce the cost of privilege escalation and lateral movement.

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Threat model and who is most at risk​

The vulnerability’s local attack vector means broad internet-exposure is not the primary concern — however, practical risk depends heavily on environment:

• High risk: multi-user systems, shared VDI / cloud desktop hosts, CI/build servers, or developer workstations where non‑privileged users or automated jobs can execute code or interact with system drivers. In such environments, a low-privileged actor can exploit local leaks to escalate privileges or harvest secrets.
• Moderate risk: servers with administrative or service accounts that cache long-lived secrets in memory; even limited memory disclosure can expose those secrets.
• Lower impact: strictly single-user consumer machines with robust local account hygiene and no shared or multi-tenant workloads, though patching is still required.

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Evidence of exploitation and public proof-of-concept status​

At the time of writing, public trackers report no confirmed proof-of-concept (PoC) in the wild and no observed exploitation campaigns tied to CVE-2025-55336. That said, the lack of public PoC does not mean the vulnerability is harmless — information-disclosure bugs are frequently used as reconnaissance or chaining primitives in targeted intrusions. Treat the absence of PoC as a temporary comfort, not a mitigation strategy.

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Patching and remediation: practical, prioritized checklist​

Apply vendor updates as the primary mitigation. Because Microsoft may deliver the fix as part of a cumulative update, driver update, or out‑of‑band patch, administrators must confirm the exact KB/CU for their OS SKU in the Microsoft Security Update Guide. The Update Guide is authoritative — map the CVE to the KB(s) listed there before confirming remediation. If MSRC’s page does not render in an automated tool, fetch it interactively from a secured admin workstation.
Immediate actions (0–72 hours)

• Inventory and identify exposed hosts at scale. Use your endpoint management tool (WSUS/SCCM/Intune, or third‑party EPM) to map OS build numbers and the presence of the Cloud Files Mini Filter Driver.
• Apply the Microsoft-supplied security update for CVE-2025-55336 to pilot rings and test hosts first, then to production. Validate installed KBs on each host (Get-HotFix, Windows Update history) to confirm the update landed.
• Where immediate patching is not possible, implement compensating controls: restrict local logon and execution privileges on high-value hosts, limit who may mount or interact with cloud-file services, and rotate long‑lived secrets that could have been exposed.

Short-term mitigations and hardening

• Enable Memory Integrity (HVCI) and related virtualization-based security features where supported; these raise the cost of exploiting kernel-level issues.
• Enable Microsoft’s Vulnerable Driver Blocklist and enforce driver signing policies to reduce the chance a malicious driver gets loaded.
• Restrict local accounts and privileges on servers, VDI hosts, and multi-user systems. Remove unneeded local accounts and enforce least privilege.

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Detection and hunting guidance​

Because the vulnerability leaks data to a caller, detection focuses on abnormal local interactions and memory‑harvesting behavior rather than network-based exploit signatures.
Suggested detection signals

• Unusual IOCTL or driver interaction from non‑privileged processes contacting cldflt.sys or related service binaries.
• Process creation anomalies where low-trust processes spawn utilities that normally require elevated privileges (telltale of local reconnaissance).
• EDR telemetry showing frequent reads/queries of privileged service endpoints or unexpected memory dump activity immediately following driver calls.
• Hunting for token exfiltration patterns: secondary evidence of credential reuse, lateral authentication attempts, or sudden credential rotation requests.

Hunt queries and triage steps

• Search EDR for processes invoking device I/O (DeviceIoControl) against the Cloud Files driver from low-privilege accounts.
• Correlate those invocations with process creation (Event ID 4688) and network connections that could indicate data exfiltration.
• If you detect suspicious invocation patterns, collect memory and EDR artifacts for forensic analysis to determine whether secrets were retrieved. Rotate any secrets that might have been exposed and apply the vendor patch to that host immediately.

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Critical assessment: strengths and remaining risks​

What Microsoft did well

• Microsoft published a CVE entry and made a patch available in a recognized update channel; prompt vendor remediation reduces the window of opportunity for attackers. Aggregators report the presence of a vendor fix, and administrators can obtain the KB mapping from Microsoft’s Update Guide.

What remains concerning

• Local attack vector is deceptive. Many organizations treat anything that is not remotely exploitable as lower priority. In practice, local information leaks on shared hosts (VDI, RDP, cloud desktops) are extremely valuable to attackers. A small memory disclosure can yield credentials, session tokens, or kernel pointers that meaningfully lower the cost of full compromise.
• Dynamic vendor pages complicate automation. The MSRC Update Guide’s client-side rendering can obscure KB mapping for automated ingestion pipelines; defenders must confirm the KB mapping in an interactive browser before scheduling remediation at scale. This creates process friction that can slow timely patch deployment.
• No public PoC is not the same as no exploitation. Information-disclosure bugs are frequently weaponized as reconnaissance primitives in targeted attacks long before a public PoC shows up. Treat the absence of public exploit code as a narrowing of the immediate attack surface — not elimination of risk.

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Operational playbook: recommended steps for IT and SecOps​

• Confirm the KB for CVE-2025-55336 in MSRC’s Update Guide using a secured admin workstation. Record the KB IDs and the applicable OS/build mappings.
• Stage the update to a pilot group of representative hosts (VDI/built images, domain controllers not in critical paths, and a sample of multi-user hosts). Test for compatibility, performance, and any known update issues.
• Deploy to production following normal change-control procedures; prioritize multi-user hosts, cloud‑desktop pools, and servers with cached secrets.
• While patching rolls out, reduce local attack surface: remove unnecessary local accounts, limit who can spawn interactive sessions on high-value hosts, disable or constrain cloud-file features for machines with broad access requirements (where feasible).
• After patching, hunt for indicators described earlier and rotate credentials that were stored in memory on threatened hosts (service accounts, long-lived tokens).

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Caveats and unverifiable claims — be precise when making security decisions​

• Public CVE aggregators reflect vendor information, but exact affected SKUs and KB mappings must be confirmed on the Microsoft Security Update Guide; third‑party mirrors sometimes lag or mis-map patches to OS builds. Do not rely on a single aggregator for roll‑out decisions.
• Some public summaries speculate on the precise root cause (heap overflow, out‑of‑bounds read, logging error, etc.). While those inferences are credible given typical driver bug classes, they are inferred until Microsoft or an independent researcher publishes a full technical post‑mortem. Flag any such speculative assumptions inside your incident response process and treat them accordingly.

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Wider context — precedents and why this pattern matters​

The Cloud Files Mini Filter Driver has been a repeat focus for security research and patching in recent years. Prior CVEs affecting cldflt.sys exhibited both information‑disclosure and elevation‑of‑privilege behaviors; that historical pattern demonstrates two things: kernel-mode file-system filter code has repeated opportunities for subtle boundary-check and marshalling mistakes, and exploitation of those mistakes yields high-value outcomes for attackers who already possess or can gain local execution. Patching and driver-hardening practices remain essential defensive layers.

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Conclusion​

CVE-2025-55336 is a concrete, vendor‑acknowledged information-disclosure vulnerability in cldflt.sys that should be treated with urgency in environments that permit local code execution by untrusted users or automated jobs. While the flaw does not permit unauthenticated remote code execution, its high confidentiality impact and utility as a chaining primitive make it a priority for security teams overseeing multi-user hosts, VDI/cloud desktops, developer build fleets, and shared services.
Action summary (short):

• Confirm the exact KB/CU mapping in the Microsoft Security Update Guide and schedule the vendor update immediately for high-priority hosts.
• If immediate patching is not possible, harden local access, enable kernel integrity protections where available, and rotate sensitive tokens/credentials.
• Hunt for anomalous driver interactions and collect EDR traces for hosts that showed suspicious cldflt.sys activity prior to remediation.

Treat this patch like a critical hygiene item: apply the vendor update, confirm remediation, and continue to monitor for any follow‑on public disclosures or proof‑of‑concept exploit code that could change the immediate threat calculus.

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Source: MSRC Security Update Guide - Microsoft Security Response Center