CVE-2025-62221: Microsoft Windows Cloud Files Mini Filter Driver -- Use-After-Free Privilege Escalation

Background

The Windows Cloud Files Mini Filter Driver (cldflt.sys) is a kernel-mode component that implements the Windows Cloud Files API, enabling on-demand file syncing for services like OneDrive. As a kernel-mode driver, vulnerabilities here operate with the highest system privileges and bypass typical user-space security controls.

CVE-2025-62221 describes a use-after-free vulnerability (CWE-416) in this driver that allows an authorized (locally logged-in) attacker to elevate privileges. CISA added this to the KEV catalog on 2025-12-09, indicating it is being actively weaponized — most likely as the privilege escalation component in a multi-stage attack chain following initial access through another vector.

Local privilege escalation (LPE) vulnerabilities in Windows kernel drivers are critical components of advanced attack toolchains. They enable attackers who have compromised a low-privilege account to gain SYSTEM-level access, disable security tools, and establish persistence.

Technical Mechanism

CWE-416 (Use After Free) occurs when a program continues to use a pointer after the memory it references has been freed. In kernel drivers, this can result in arbitrary code execution at the kernel level.

// Conceptual vulnerable pattern in a kernel filter driver
typedef struct _FILE_CONTEXT {
    ULONG refcount;
    PVOID callback;
} FILE_CONTEXT, *PFILE_CONTEXT;

VOID CleanupFileContext(PFILE_CONTEXT ctx) {
    ExFreePool(ctx);  // memory freed here
}

NTSTATUS HandleIoRequest(PFILE_CONTEXT ctx, PIRP irp) {
    // Bug: ctx may have been freed by a racing thread
    ctx->callback(irp);  // use-after-free: attacker controls freed memory
    return STATUS_SUCCESS;
}

An attacker exploits the race condition or trigger condition to free the object, then spray controlled data into the freed memory before the dangling pointer is dereferenced, achieving kernel-mode code execution.

Real-World Exploitation Evidence

CISA’s KEV listing confirms active exploitation. Local privilege escalation vulnerabilities are routinely chained with initial access exploits — for example, a phishing payload or browser exploit provides unprivileged code execution, then an LPE like this one escalates to SYSTEM. From SYSTEM, attackers can disable EDR/AV, dump credentials from LSASS, and deploy ransomware or persistent implants. Kernel LPE exploits are hallmarks of advanced persistent threat toolkits.

Impact Assessment

• Escalation from standard user to SYSTEM privileges on the local machine
• Ability to disable endpoint security software running at lower privilege levels
• Credential dumping from LSASS and the Windows credential store
• Persistent kernel-level implant installation
• Enablement of further lateral movement from a compromised endpoint

Affected Versions

Remediation Steps

1. Apply Microsoft’s December 2025 Patch Tuesday update immediately.
2. Enable automatic Windows Update for all endpoints and servers.
3. Enforce least-privilege principles — minimize the number of accounts with interactive login access.
4. Monitor for suspicious privilege escalation patterns in EDR telemetry.
5. Deploy Windows Defender Credential Guard to protect against post-escalation credential theft.

Detection Guidance

Log Sources: Windows Event Logs, Sysmon, EDR kernel telemetry.

IOCs: Unexpected SYSTEM-level processes spawned from user-context parents; cldflt.sys exceptions in crash dumps; process privilege escalation events (Event ID 4672).

Sigma rule:

title: Suspicious Privilege Escalation via Cloud Files Driver CVE-2025-62221
logsource:
  product: windows
  category: process_creation
detection:
  selection:
    IntegrityLevel: "System"
    ParentIntegrityLevel: "Medium"
    ParentImage|endswith:
      - '\explorer.exe'
      - '\cmd.exe'
      - '\powershell.exe'
  filter:
    Image|endswith: '\conhost.exe'
  condition: selection and not filter
level: high

Timeline

References

• NVD — CVE-2025-62221
• CISA KEV — CVE-2025-62221
• Microsoft MSRC — CVE-2025-62221