CVE-2026-62771 Overview
CVE-2026-62771 is a heap-based buffer overflow [CWE-122] in the Windows Cloud Files Mini Filter Driver (cldflt.sys). An authorized local attacker can trigger the overflow to elevate privileges on affected Windows client and server systems. Microsoft rates the flaw HIGH with a CVSS 3.1 score of 7.8. The vulnerability spans a broad set of supported Windows releases, including Windows 10, Windows 11 (23H2 through 26H1), and Windows Server 2019, 2022, and 2025. No public exploit code or in-the-wild exploitation has been confirmed at publication, and the CVE is not listed in the CISA Known Exploited Vulnerabilities catalog.
Critical Impact
Successful exploitation grants SYSTEM-level privileges from a low-privileged local account, enabling full compromise of confidentiality, integrity, and availability on the affected host.
Affected Products
- Microsoft Windows 10 (1809, 21H2, 22H2) on x86, x64, and ARM64
- Microsoft Windows 11 (23H2, 24H2, 25H2, 26H1) on x64 and ARM64
- Microsoft Windows Server 2019, 2022, and 2025
Discovery Timeline
- 2026-08-11 - CVE-2026-62771 published to the National Vulnerability Database
- 2026-08-13 - Last updated in NVD database
Technical Details for CVE-2026-62771
Vulnerability Analysis
The Cloud Files Mini Filter Driver (cldflt.sys) implements the kernel-mode component of the Windows Cloud Files API, which powers on-demand file synchronization for OneDrive and other cloud storage providers. The driver processes reparse point data and placeholder file metadata supplied by user-mode sync engines. CVE-2026-62771 arises when the driver copies attacker-controlled data into a heap allocation without correctly validating the destination size. The resulting out-of-bounds write corrupts adjacent pool memory in the kernel.
Because the flaw resides in a kernel filter driver, corruption of pool metadata or adjacent objects can be steered toward arbitrary kernel read/write primitives. Attackers commonly convert such primitives into token replacement, elevating a low-integrity process to NT AUTHORITY\SYSTEM.
Root Cause
The root cause is classified as a heap-based buffer overflow under CWE-122. The driver allocates a fixed or size-derived buffer on the paged or non-paged pool, then writes user-influenced data past the allocation boundary. Missing or incorrect length validation between the request parser and the destination allocation is the underlying defect.
Attack Vector
Exploitation requires local access and low-level authenticated privileges. An attacker sends crafted I/O control requests or reparse point data through the Cloud Files interfaces exposed by cldflt.sys. No user interaction is required. Because the vulnerable code path runs in kernel mode, a successful overwrite yields full system compromise from a standard user account.
No verified proof-of-concept code is publicly available. Refer to the Microsoft Security Update Guide for CVE-2026-62771 for authoritative technical detail.
Detection Methods for CVE-2026-62771
Indicators of Compromise
- Unexpected cldflt.sys bugchecks or SYSTEM_SERVICE_EXCEPTION, KERNEL_MODE_HEAP_CORRUPTION, or PAGE_FAULT_IN_NONPAGED_AREA crash dumps referencing the Cloud Files driver.
- Non-Microsoft or unsigned processes issuing bursts of DeviceIoControl calls against Cloud Files device objects, especially from low-integrity sessions.
- Sudden creation of SYSTEM-owned processes whose parent is a standard user process shortly after Cloud Files activity.
Detection Strategies
- Hunt for token-swap patterns where a user-mode process gains SYSTEM privileges without a legitimate service transition.
- Correlate Windows Error Reporting entries mentioning cldflt with subsequent privileged process launches on the same endpoint.
- Baseline normal OneDrive and Cloud Files sync engine behavior and alert on anomalous kernel-object handle patterns.
Monitoring Recommendations
- Forward kernel crash telemetry, Sysmon Event ID 1 (process create), and Event ID 10 (process access) to a centralized analytics platform.
- Track patch compliance status for the August 2026 cumulative updates across all Windows 10, Windows 11, and Windows Server assets.
- Alert on new drivers loading alongside cldflt.sys interactions from untrusted binaries.
How to Mitigate CVE-2026-62771
Immediate Actions Required
- Deploy the August 2026 Microsoft security updates that address CVE-2026-62771 to all affected Windows 10, Windows 11, and Windows Server systems.
- Prioritize patching multi-user systems, developer workstations, and jump hosts where local users are more likely to exist.
- Audit local account inventory and remove unnecessary interactive logon rights to reduce the local attack surface.
Patch Information
Microsoft has published fixes through the Security Update Guide. Consult the Microsoft Security Update for CVE-2026-62771 for the specific KB article corresponding to each Windows build and install it through Windows Update, WSUS, Microsoft Update Catalog, or your configured patch management tooling.
Workarounds
- No official workaround is documented by Microsoft; patching is the required remediation.
- Where patching must be delayed, restrict local logon rights and enforce application allowlisting to limit which binaries can invoke Cloud Files interfaces.
- Disable the Cloud Files feature or the OneDrive sync client on systems that do not require cloud file on-demand functionality, understanding that this may impact user workflows.
# Verify cldflt driver version after patching (PowerShell)
Get-Item C:\Windows\System32\drivers\cldflt.sys | Select-Object VersionInfo
# Confirm installed security updates include the August 2026 rollup
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 10
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
