CVE-2025-21271: Windows 10 1809 Privilege Escalation Flaw

CVE-2025-21271 Overview

CVE-2025-21271 is an elevation of privilege vulnerability in the Windows Cloud Files Mini Filter Driver (cldflt.sys). The flaw affects multiple supported Windows client and server releases, including Windows 10 (1809, 21H2, 22H2), Windows Server 2019, and Windows Server 2022. An authenticated local attacker can exploit the vulnerability to gain SYSTEM-level privileges on an affected host. Microsoft classifies the issue under [CWE-126: Buffer Over-read], and the CVSS 3.1 base score is 7.8. No public proof-of-concept exploit code is available at time of writing, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
A local, low-privileged user can escalate to SYSTEM on unpatched Windows endpoints and servers, enabling full host compromise, credential theft, and lateral movement staging.

Affected Products

Microsoft Windows 10 versions 1809, 21H2, and 22H2
Microsoft Windows Server 2019
Microsoft Windows Server 2022

Discovery Timeline

2025-01-14 - CVE-2025-21271 published to NVD alongside Microsoft's January 2025 Patch Tuesday advisory
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-21271

Vulnerability Analysis

The vulnerability resides in the Windows Cloud Files Mini Filter Driver, a kernel-mode file system filter that mediates access between user-mode applications and cloud-backed placeholder files used by providers such as OneDrive. The driver exposes I/O control and reparse point handling paths that are reachable from user mode. Improper validation of a length or boundary field during processing of an attacker-controlled buffer allows the driver to read beyond an allocated structure, matching the [CWE-126] buffer over-read classification.

An attacker who has obtained code execution as a standard local user can craft a request that causes cldflt.sys to operate on out-of-bounds memory. The resulting corrupted kernel state can be leveraged to overwrite privileged tokens or function pointers, ultimately yielding execution in the SYSTEM security context. Because the vulnerable component ships and loads by default on affected Windows builds, the attack surface is broad across enterprise fleets.

Root Cause

The root cause is missing or insufficient bounds checking within the Cloud Files driver's parsing logic for user-supplied input. When the driver processes the malformed request, it computes an access offset that exceeds the size of the source buffer, producing a kernel-mode over-read that adversaries can shape into a privilege escalation primitive.

Attack Vector

Exploitation requires local access and low privileges. The attacker must already be authenticated on the target system, either interactively or through a foothold such as malware delivered via phishing, a compromised service account, or an initial remote code execution flaw. No user interaction is required once the malicious binary runs. The attack does not traverse the network, so exploitation is preceded by an initial access step. Consult the Microsoft CVE-2025-21271 Advisory for the vendor-supplied technical description.

No verified public exploit code exists for this issue. Defenders should treat local privilege escalation flaws in shipped kernel drivers as high-value targets for post-exploitation tooling and prioritize patching accordingly.

Detection Methods for CVE-2025-21271

Indicators of Compromise

Unexpected loads or crashes of cldflt.sys recorded in System event logs or Windows Error Reporting (WER) dumps.
Standard user processes spawning children with SYSTEM or elevated integrity levels without a legitimate elevation prompt.
Unsigned or unusual binaries interacting with Cloud Files placeholder paths outside typical OneDrive or sync client activity.

Detection Strategies

Hunt for process creation events where a MediumIL parent produces a SystemIL child, then correlate with prior handles to \Device\CldFlt or the Cloud Files device object.
Alert on kernel bug checks referencing cldflt.sys in the faulting module field, which may indicate failed exploitation attempts.
Baseline Cloud Files driver interactions per host and flag processes that open the filter's control device without an associated sync-provider parent chain.

Monitoring Recommendations

Ingest Sysmon Event IDs 1, 7, and 11 along with Windows Security Event ID 4688 into a centralized log platform to enable cross-host hunting.
Track patch compliance for the January 2025 Windows security updates across all affected SKUs and alert on drift.
Monitor for the introduction of new local user accounts, scheduled tasks, or services immediately following suspicious cldflt.sys activity, which often follows successful privilege escalation.

How to Mitigate CVE-2025-21271

Immediate Actions Required

Apply the January 2025 Microsoft security updates to all affected Windows 10, Windows Server 2019, and Windows Server 2022 hosts.
Prioritize patching on multi-user systems such as Remote Desktop Session Hosts, Citrix servers, and developer workstations where local low-privileged users are common.
Audit endpoints for the presence of untrusted local accounts and remove or reset stale credentials that could serve as an initial foothold.
Enforce application control policies such as Windows Defender Application Control (WDAC) or AppLocker to constrain execution of unsigned binaries by standard users.

Patch Information

Microsoft addressed CVE-2025-21271 in its January 14, 2025 security update cycle. The vendor advisory and per-SKU KB references are available in the Microsoft CVE-2025-21271 Advisory. Administrators should deploy the corresponding cumulative update for each affected build and reboot to complete driver replacement.

Workarounds

No official workaround is documented by Microsoft; patching is the supported remediation path.
Where immediate patching is not feasible, restrict interactive and remote local logon rights to trusted administrators to reduce the pool of users capable of local exploitation.
Increase kernel telemetry collection and reduce dwell time by shortening detection and response cycles on unpatched systems until the update can be applied.

bash

# Verify installation status of the January 2025 cumulative update on a Windows host
# Replace KB5049981 with the KB identifier applicable to the target build
wmic qfe get HotFixID,InstalledOn | findstr /I "KB5049981"

# PowerShell equivalent
Get-HotFix | Where-Object { $_.HotFixID -eq 'KB5049981' }