CVE-2026-50401

What it actually does

CVE-2026-50401 is a local information disclosure vulnerability in the Windows Cloud Files Mini Filter Driver, commonly associated with cloud-backed placeholder file functionality such as Files On-Demand. The flaw is an out-of-bounds read, meaning a locally authenticated user can cause the kernel-mode driver to read memory outside the intended bounds of a buffer. Successful exploitation can expose sensitive information from local system memory, potentially including data belonging to privileged processes or the kernel. The vulnerability does not directly provide code execution, integrity modification, or denial of service, but disclosed memory may help an attacker bypass protections or support follow-on local privilege escalation attempts.

Why it matters to the business

An attacker who already has local access to an affected Windows workstation or server could extract sensitive memory contents from the system. This may expose credentials, tokens, file metadata, or other confidential data and could help deepen compromise after an initial foothold. The risk is most relevant for shared workstations, virtual desktop environments, developer systems, jump hosts, and servers where low-privileged users or compromised accounts may have interactive or local execution capability. The vulnerability is not currently in CISA KEV, is not known to be used in ransomware campaigns, and has a low EPSS score, so broad exploitation pressure appears limited; however, because it affects multiple supported Windows client and server versions, patching should still be prioritized through normal Windows security update processes.

How an attack unfolds

An attacker first obtains the ability to run code locally as an authenticated, low-privileged user on an affected Windows system. They then interact with cloud-file placeholder or Cloud Files Mini Filter Driver functionality in a way that triggers the vulnerable out-of-bounds read in the kernel driver. By repeatedly invoking the vulnerable code path and capturing returned or observable data, the attacker may recover memory contents that should not be accessible to their security context. The disclosed data may then be analyzed for secrets, addresses, tokens, or other material that can support credential theft, defense evasion, or a separate privilege escalation chain. Because the issue is local and requires prior authorization, it is most likely to be used after phishing, malware execution, abuse of exposed remote access, or compromise of a low-privileged account.

How to fix it

Apply the Microsoft security update that addresses this vulnerability for all affected Windows versions, including Windows 10 1809, 21H2, and 22H2; Windows 11 24H2, 25H2, and 26H1; Windows Server 2019; Windows Server 2022; and Windows Server 2025. Where exact fixed build or KB information is not available from the supplied data, use Microsoft Update, Windows Server Update Services, Microsoft Configuration Manager, Intune, or an approved patch management platform to install the cumulative security update released on or after the July 2026 security update cycle for each OS branch. Reboot systems where required so the updated kernel driver is loaded. As interim risk reduction, enforce least privilege, remove unnecessary local interactive access, restrict execution of untrusted binaries, monitor local code execution by non-administrative users, and disable or limit cloud-file placeholder functionality only if it is not operationally required and Microsoft guidance supports doing so.

How to confirm the fix worked

Confirm that affected Windows endpoints and servers have installed the relevant cumulative security update for their OS branch and have been rebooted after installation. Validate through centralized vulnerability scanning, endpoint management compliance reports, Windows Update history, and OS build/patch inventory. On representative systems, confirm that the Cloud Files Mini Filter Driver file, typically cldflt.sys, has a post-remediation file version and timestamp consistent with the installed Microsoft security update. Security teams should also verify that vulnerability scanners no longer report CVE-2026-50401 and that no systems remain on unsupported or unpatched Windows builds. If temporary mitigations were applied, confirm they did not break required file synchronization or cloud storage workflows before removing or relaxing compensating controls.

Typical attack path

1. Gain local authenticated access to an affected Windows 10, Windows 11, or Windows Server system.
2. Identify that the Cloud Files Mini Filter Driver is present and active on the target system.
3. Run a crafted local program or script that exercises vulnerable cloud-file or placeholder-file operations.
4. Trigger the out-of-bounds read condition in the kernel-mode driver.
5. Collect leaked memory contents returned or exposed through the vulnerable interaction.
6. Analyze the disclosed data for credentials, tokens, memory addresses, or other sensitive information.
7. Use any recovered information to support follow-on compromise, lateral movement, or local privilege escalation using a separate technique.

How to detect it

There is no reliable single log entry that proves exploitation of this vulnerability, and information disclosure flaws often leave minimal artifacts. Detection should focus on signs of suspicious local execution and abnormal interaction with the Windows Cloud Files Mini Filter Driver. Review Windows Security event logs for process creation events if enabled, especially Event ID 4688, and correlate with Sysmon Event ID 1 process creation, Event ID 7 image load, Event ID 11 file creation, and Event ID 15 file stream creation where available. Look for newly introduced executables running from user-writable locations such as Downloads, Temp, AppData, or synced cloud folders shortly before suspicious credential access or privilege escalation activity. Review System logs, Windows Error Reporting, endpoint detection telemetry, and crash artifacts for driver faults, bugchecks, or repeated errors involving cldflt.sys or the Cloud Files Mini Filter Driver. Investigate unusual access to OneDrive, cloud-synchronized, or placeholder-file directories by unsigned or uncommon local binaries. If exploitation is suspected, collect volatile data quickly, preserve EDR telemetry, examine prefetch and Amcache/Shimcache artifacts for recently executed local tools, and hunt for follow-on behavior such as credential dumping, token abuse, suspicious LSASS access, new persistence mechanisms, or lateral movement. Threat actor exploitation has not been identified in the supplied data, so detections should be behavior-based rather than actor-specific.

• cldflt.sys
• CldFlt
• Cloud Files Mini Filter Driver
• \SystemRoot\System32\drivers\cldflt.sys
• C:\Windows\System32\drivers\cldflt.sys
• Event ID 4688
• Event ID 1
• Event ID 7
• Event ID 11
• Event ID 15
• OneDrive
• Files On-Demand

What your security team should do

• Prioritize deployment of the applicable Microsoft cumulative security update across affected Windows clients and servers, with special attention to shared systems, VDI, jump hosts, and servers allowing local or interactive user sessions.
• Use endpoint management and vulnerability scanning to identify all affected Windows 10, Windows 11, Windows Server 2019, Windows Server 2022, and Windows Server 2025 systems.
• Require reboot completion after patch installation so the updated kernel-mode driver is actually loaded.
• Harden local access paths by limiting RDP, console access, scheduled task creation, and arbitrary local code execution for low-privileged users.
• Monitor for suspicious local processes interacting with cloud-file locations, OneDrive-synchronized paths, or Cloud Files API behavior, especially when launched from temporary, user-writable, or recently downloaded locations.
• Review endpoint telemetry for unusual crashes, driver errors, or repeated access patterns involving cldflt.sys before and shortly after patch deployment.
• Treat any suspected exploitation as a post-compromise activity and investigate for the initial access vector, credential theft, persistence, and lateral movement.
• Because this vulnerability is not currently KEV-listed and has low EPSS, handle it as important routine patching rather than an emergency internet-facing exposure, unless high-risk local multi-user systems are present.