A 0-Day Vulnerability in Windows Allowing SYSTEM Privilege Escalation Has Been Disclosed

On December 9, 2025, Microsoft released emergency security updates addressing a critical 0-day vulnerability — CVE-2025-62221 — discovered in Windows systems. The flaw was found in the Cloud Files Mini Filter Driver (cldflt.sys), a kernel-level driver, and allows attackers to escalate privileges from a regular user account to SYSTEM level.

Microsoft has officially confirmed that this vulnerability “is being actively exploited by attackers”, meaning it is already used in real-world attacks.

What is the vulnerability about?

cldflt.sys is a system driver used by Windows to integrate with cloud storage services such as OneDrive. It enables cloud files to appear as local “placeholders” without being fully downloaded.

The vulnerability is caused by a Use-After-Free memory handling error. An attacker can:

  • log in to the system as a regular local user,
  • trigger the flaw in the driver,
  • create an invalid memory state,
  • and ultimately execute privileged code with SYSTEM permissions.

This is extremely dangerous because SYSTEM is the highest level of privilege in Windows, essentially granting unrestricted control over the machine.

Who is affected?

This vulnerability impacts a wide range of Windows versions, including:

  • Windows 11 (23H2, 24H2, 25H2)
  • Windows 10 (22H2, 21H2, 1809)
  • Windows Server 2019, 2022, 2025

Microsoft has already released the necessary KB patches. System administrators must verify installation based on KB IDs and ensure that updates are applied.

How is the vulnerability exploited?

This flaw does not allow remote exploitation by itself — the attacker must already have some level of access to the system.

However, if:

  • malware is already present,
  • unknown user accounts appear,
  • or an attacker has gained initial access to the network,

then CVE-2025-62221 becomes a perfect second-stage attack vector.

In such cases, attackers use it to:

  • escalate privileges,
  • disable security tools,
  • install backdoors,
  • and maintain long-term persistence.

This makes it highly attractive for APT groups, automated malware, and botnets.

Practical recommendations for administrators

1. Install updates immediately
The only real protection against CVE-2025-62221 is the official patch from Microsoft.
No workaround or temporary fix exists.

2. Restrict local access
Because the attacker needs local privileges:

  • Remove unnecessary user accounts
  • Enforce strong passwords or MFA
  • Limit RDP and other access channels

3. Strengthen EDR/Antivirus monitoring
Watch for signs of:

  • privilege escalation,
  • unknown processes,
  • unusual driver interactions.

4. Review security logs
Inspect Security, System, and Application logs for suspicious:

  • local logins,
  • service activations,
  • or privilege escalation attempts.

5. Prepare for rapid response
Ensure backups, disaster recovery plans, and incident response procedures are up to date.

CVE-2025-62221 is a dangerous vulnerability in a Windows kernel-level driver and is already being used in active attacks. It allows even a regular local user to gain elevated SYSTEM privileges.

Therefore, the most critical actions for all users and IT administrators are:

  • apply updates immediately,
  • restrict local access,
  • strengthen monitoring and log analysis,
  • and be prepared for quick incident response.

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