CVE-2025-50170: Local EoP in Windows Cloud Files Driver (cldflt.sys) Patch Now

Microsoft has published an advisory for CVE-2025-50170, a local elevation-of-privilege (EoP) vulnerability in the Windows Cloud Files Mini Filter Driver (cldflt.sys) that—when reached by a local, authorized attacker—can be abused to obtain higher privileges on affected machines. The flaw stems from improper handling of insufficient permissions or privileges in the Cloud Files Mini Filter Driver and Microsoft recommends applying the available security updates immediately. (msrc.microsoft.com)

Background / Overview​

The Cloud Files Mini Filter Driver, commonly referenced by its binary name cldflt.sys, sits in the Windows kernel and is responsible for integrating cloud-based file features (OneDrive Files On‑Demand, Cloud Windows Files APIs, and related placeholder/rehydration behaviors) with the local file system. Because the driver exposes interfaces that mediate file operations and reparse point handling, it must enforce strict permission and boundary checks. Historically, cldflt.sys has been the focus of several high‑severity privilege escalation advisories and proof‑of‑concepts; past vulnerabilities in this component have allowed local attackers to move from limited (standard) user contexts to NT AUTHORITY\SYSTEM by abusing device IOCTLs and malformed reparse data. Independent technical writeups and exploit research have shown that carefully crafted reparse point requests and malformed IOCTL payloads can lead to heap overflows, kernel pointer leaks, and token manipulation—classic building blocks for reliable privilege escalation. (starlabs.sg, avleonov.com)
This advisory—CVE-2025-50170—falls into that family of errors: an attacker who already has some level of local access (for example, via a compromised standard user account, a dropped malicious binary, or an initial foothold from another bug) could call into the Cloud Files driver in a way that the driver fails to validate privileges correctly, opening an escalation path.

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What Microsoft says (summary of the advisory)​

• The vulnerability is an Elevation of Privilege problem in the Windows Cloud Files Mini Filter Driver.
• The root cause is the driver’s improper handling of insufficient permissions or privilege checks for certain operations.
• An attacker who is already authorized locally (i.e., can run code or issue requests on the target machine) could exploit this vulnerability to escalate privileges locally.
• Microsoft’s guidance is to install the security update(s) that address the vulnerability immediately. (msrc.microsoft.com)

This is the concise, actionable core: if a machine is unpatched and an attacker can run code on it, that attacker may be able to gain SYSTEM-level access via cldflt.sys.

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Why this matters: technical and operational context​

Kernel drivers are high‑value targets​

Kernel-mode drivers run with full system privileges. Any logic bug (missing permission check, incorrect buffer handling, or unvalidated caller context) can let an attacker elevate privileges and persist at the highest privilege level on the system. Cloud-integrations are particularly tempting targets because they must mediate between local IO and remote services—an interface that often requires complex parsing and many edge-case code paths. Previous cldflt.sys advisories show that attackers can chain a device handle open + malformed IOCTL to trigger memory corruption or logic bypasses, then leverage kernel objects (WNF state, ALPC tables, tokens) to convert that corruption into a stable privilege escalation. (starlabs.sg, cvedetails.com)

Attack prerequisites reduce but do not eliminate risk​

This class of EoP is typically local in nature: the attacker must already be able to run code on the target machine. That prerequisite does not eliminate practical risk for two reasons:

• In many modern breaches, attackers first gain a foothold with low privileges (phishing, malicious installers, supply‑chain payloads) and then use local privilege escalation to achieve persistence and lateral movement. EoP bugs are frequently the bridge between initial access and total compromise.
• In multi‑user environments, a standard user on a shared host (remote desktop, terminal servers, developer machines) may be able to exploit local vulnerabilities against higher-privileged system services. The localized requirement merely raises the bar—it does not make the bug irrelevant.

Security researchers and incident responders consistently observe privilege escalation bugs being chained with initial-access payloads in real campaigns. The Windows Forum community has tracked multiple related vulnerabilities and warned about the chaining risk for file-system and filter driver bugs.

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A technical snapshot: how similar cldflt.sys exploits have worked​

The technical exploitation approaches used against prior cldflt.sys flaws offer a useful template for defenders and mitigators:

• Attackers open a handle to the device exposed by the driver (commonly available via a symbolic link like .\CldFlt).
• A specially crafted IOCTL or reparse point is sent to the driver; flawed parsing or omitted permission checks allow the driver to write beyond a buffer or corrupt adjacent kernel objects (heap overflow, integer overflow).
• The attacker uses the memory corruption to:
• Leak a kernel pointer (breaking kernel ASLR).
• Forge or corrupt kernel objects such as _WNF_STATE_DATA or PipeAttribute structures to obtain arbitrary read/write primitives.
• Locate and overwrite a token or escalate privileges in the process token to NT AUTHORITY\SYSTEM.

Technical writeups and proof‑of‑concepts for prior cldflt.sys bugs detail multi‑step exploitation (leak → arbitrary read → arbitrary write → token overwrite). These are nontrivial but now well‑understood techniques; once a reliable primitive is obtained, exploitation becomes deterministic. (starlabs.sg, avleonov.com)

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Which systems are likely affected​

Public databases and historical advisories for the Cloud Files driver indicate that many modern Windows 10, Windows 11, and recent Windows Server builds have carried cldflt.sys variants. For earlier cldflt advisories, vendors identified affected OS builds spanning multiple Windows 10/11 feature updates and Server versions. While the exact list for CVE‑2025‑50170 must be obtained from Microsoft’s Security Update Guide and your organization’s patch metadata, the Cloud Files driver is present on most consumer and enterprise installations that include OneDrive or cloud file features enabled by default. Administrators should assume wide exposure until their environment reports otherwise. (cvedetails.com, msrc.microsoft.com)

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Immediate mitigation and remediation steps (what administrators should do now)​

• Patch first
• Apply Microsoft’s security update(s) that address CVE‑2025‑50170 immediately. Prioritize machines that are exposed to high-risk user populations (remote desktops, dev workstations, shared admin consoles). Microsoft’s advisory contains update IDs and KBs — use centralized patch management (WSUS, SCCM, Intune) to deploy at scale. (msrc.microsoft.com)
• Reduce the attack surface
• Where feasible, disable Cloud Files features on critical endpoints that do not require them.
• On servers or locked-down images, consider removing or restricting OneDrive/Cloud File components.
• Harden privilege separation
• Enforce the principle of least privilege: remove unnecessary local administrator rights, use dedicated admin workstations for privileged tasks, and enable User Account Control (UAC) best practices.
• Review and reduce the number of users who can install and execute arbitrary software.
• Detection and monitoring
• Monitor for suspicious accesses to the Cloud Files driver (handles opened against .\CldFlt or unexpected DeviceIoControl calls touching that device).
• Deploy EDR rules to flag unusual process behavior indicative of token manipulation or kernel exploit attempts. Correlate with alerts that indicate unusual memory scanning or kernel module corruption patterns.
• Collect Sysmon logs and monitor for process token changes and rapid privilege shifts. Use kernel integrity alerts where available.
• Incident response readiness
• If a host is suspected, assume full compromise: collect forensic memory images, check for persistence (signed drivers, scheduled tasks, WMI persistence), and isolate the affected host from networks to prevent lateral movement.
• Prioritize public-facing or high-value endpoints
• Remote desktop hosts, build servers, and shared developer systems should be patched first—these are high-value targets where an exploit could deliver maximum benefit to attackers.

These steps are practical, short-term actions that reduce the window of exposure and limit attackers’ ability to chain this EoP into broader intrusions.

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Detection guidance (concrete signals to hunt for)​

• Unusual calls to DeviceIoControl against cldflt.sys or handles to the Cloud Files device object.
• Unexpected process elevation sequences: standard‑user processes spawning SYSTEM processes, or rapid privilege changes.
• Kernel pointer leaks / suspicious kernel read patterns identified by EDR (some EDR vendors will flag typical exploit sequences such as repeated attempts to leak kernel addresses or craft kernel objects).
• Creation of new signed drivers or unexpected modifications in Services/Drivers registry keys after suspected exploitation.

Administrators should bake these detections into routine threat-hunting playbooks and integrate them with SIEM rules to surface correlated anomalies quickly.

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Exploitability and threat assessment​

• Complexity: Medium. The attacker must already have local code execution—or at least be able to interact with local APIs to send specially crafted requests to the driver. However, once local access is present, exploitation is often achievable using established kernel exploitation techniques.
• Impact: High. Successful exploitation results in SYSTEM‑level privileges and effectively full control of the host, persistence, and capability to disable protections.
• Likelihood: Elevated for unpatched systems. Given the historical attention cldflt.sys has received and the existence of detailed writeups and PoCs for prior Cloud Files vulnerabilities, the timeframe between disclosure and exploitable PoC publication is often short. Organizations that delay patching will face sharply increased risk. Evidence from past incidents shows researchers and exploit authors rapidly produce weaponized code after technical details are published. (starlabs.sg, cvedetails.com)

Caveat: For CVE‑2025‑50170 specifically, public proof‑of‑concept exploit code or confirmed in‑the‑wild exploitation was not widely reported in major public sources at the time of writing. That absence is not a guarantee; attackers frequently develop exploits privately or sell code to criminal groups. Treat absence of public PoC as temporary and focus on patching. (This claim of “no public PoC found” is based on searches of public advisories and technical blogs up to August 12, 2025; attackers may have private exploits.) (msrc.microsoft.com, cvedetails.com)

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Why organizations should care even if the issue is “local”​

It’s a common misconception that local-only vulnerabilities are low-severity. In reality:

• Many compromise chains are hybrid: remote access or phishing yields local code execution; local escalation finalizes the compromise.
• Shared environments (VDI, RDP hosts, terminals) permit lower-privileged users to target higher-privileged services on the same machine.
• Attackers who gain limited code execution can weaponize EoP to achieve persistence and evasion, making incident response and clean-up far more difficult.

This vulnerability type is routinely exploited during ransomware campaigns and targeted intrusions. Historically, EoP bugs in foundational components (file systems, drivers) have been paired with initial access exploits to achieve broad compromise.

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Longer-term perspective: code quality, driver attack surface, and defense in depth​

The recurrence of high-severity bugs in the Cloud Files driver underscores broader challenges:

• Drivers and kernel components remain complex and historically shaped by backward-compatibility requirements, increasing the chance of edge-case logic errors.
• Memory-safety issues (heap overflows, use-after-free) and missing permission checks in drivers are recurring root causes. Rewriting critical drivers in memory‑safe languages is impractical in the short term, so platform defenses, micro‑segmentation, and proactive hardening are necessary.
• Defender tooling should prioritize driver monitoring and behavioral detection tuned to kernel exploitation patterns rather than rely solely on signature-based detection.

Security teams should take this advisory as a reminder to treat driver-level vulnerabilities as first-tier risks and to invest in both rapid patch management and robust EDR telemetry.

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Practical checklists for Windows administrators​

• Inventory
• Identify all endpoints with Cloud Files enabled (OneDrive integration, Files On‑Demand, sync clients).
• Map which OS builds and feature updates are in use.
• Patching
• Deploy Microsoft’s update addressing CVE‑2025‑50170 via centralized systems.
• Validate patch installation and reboot where required.
• Temporary mitigations (where patching is delayed)
• Restrict who can execute programs on sensitive hosts (AppLocker / WDAC).
• Remove or disable cloud sync services on high-risk systems.
• Limit local admin membership.
• Detection and hunting
• Add EDR rules to watch for cldflt.sys device interactions and suspicious DeviceIoControl patterns.
• hunt for memory corruption signs and token changes in endpoints.
• Post‑incident playbook
• If exploitation is suspected: isolate host, perform memory and disk collection, restore from known-clean images, rotate credentials, and run a full lateral movement scan.

These steps prioritize speed and certainty: inventory, patch, verify, and monitor.

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Strengths and weaknesses of Microsoft’s response (analysis)​

Strengths:

• Microsoft’s advisory explicitly identifies the vulnerable component and recommends updates—this clarity helps administrators prioritize remediation.
• Vendor-supplied updates for driver‑level problems generally include comprehensive fixes that eliminate the vulnerable IOCTL / permission path, reducing uncertainty for patch testing.

Risks and limitations:

• Advisories for kernel drivers necessarily omit low-level exploit details; defenders must infer exploitability by correlating with past technical reports. This can delay detailed detection rule creation.
• Large enterprises may take days to weeks to roll out kernel driver updates widely, leaving a window for attackers to develop or co‑opt proof‑of‑concept code.
• Detection is nontrivial: kernel exploits often manifest with stealthy primitives (pointer leaks, forged kernel objects) that may evade standard monitoring unless EDR is adapted.

Overall, the combination of a high-impact vulnerability and potentially slow patch deployment makes rapid remediation essential.

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Community and researcher context​

Security researchers who analyzed prior cldflt.sys vulnerabilities published detailed exploitation writeups and proof‑of‑concepts describing the exact sequences used to convert heap corruption into token overwrites and SYSTEM escalation. Those writeups are valuable blueprints for defenders: they demonstrate typical exploit stages and the telemetry patterns EDR should flag. The Windows technical community and enterprise forums have been consistent in recommending rapid patching, elevated monitoring for driver interactions, and restricting cloud file features on sensitive hosts. (starlabs.sg)

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Final assessment and recommendations​

CVE‑2025‑50170 is a serious local elevation‑of‑privilege vulnerability in a sensitive kernel component. While exploitation requires initial local access, the real-world risk is high because local access is frequently obtained through phishing, malicious installers, or secondary exploits. The combination of broad driver presence across Windows releases and historically effective exploitation techniques for cldflt.sys means organizations should treat this advisory as a high-priority patch and remediation item.
Top immediate actions:

• Deploy Microsoft’s security update addressing CVE‑2025‑50170 to all affected systems without delay. (msrc.microsoft.com)
• Harden endpoints by removing unnecessary cloud sync features, minimizing local admin exposure, and enforcing application control policies.
• Update EDR/SIEM rules to detect accesses to the Cloud Files driver and unusual token or privilege changes.
• Prepare incident response playbooks that assume a successful EoP implies full host compromise.

Cautionary note: while public technical writeups and PoCs exist for prior cldflt.sys vulnerabilities, the public proof‑of‑concept landscape for CVE‑2025‑50170 may change quickly. Organizations should treat the advisory as urgent regardless of whether PoC code is publicly visible at any given moment. (starlabs.sg, cvedetails.com)

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The Windows kernel remains a strategic target; driver‑level fixes must be installed quickly, monitored continuously, and complemented with layered defenses to reduce the impact of inevitable future vulnerabilities.

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Source: MSRC Security Update Guide - Microsoft Security Response Center