_________________________/\\\\\\\\\\\______________________________________________________________________________________________________________________________________ _______________________/\\\/////////\\\____________________________________________________________________________________________________________________________________ ______________________\//\\\______\///____________________________________________________________/\\\_____/\\\_________/\\\__/\\\_________________________________________ _______________________\////\\\_____________/\\\\\\\\______/\\\\\\\\__/\\\____/\\\__/\\/\\\\\\\__\///___/\\\\\\\\\\\___\//\\\/\\\__________________________________________ __________________________\////\\\________/\\\/////\\\___/\\\//////__\/\\\___\/\\\_\/\\\/////\\\__/\\\_\////\\\////_____\//\\\\\___________________________________________ _____________________________\////\\\____/\\\\\\\\\\\___/\\\_________\/\\\___\/\\\_\/\\\___\///__\/\\\____\/\\\__________\//\\\____________________________________________ ______________________/\\\______\//\\\__\//\\///////___\//\\\________\/\\\___\/\\\_\/\\\_________\/\\\____\/\\\_/\\___/\\_/\\\_____________________________________________ _____________________\///\\\\\\\\\\\/____\//\\\\\\\\\\__\///\\\\\\\\_\//\\\\\\\\\__\/\\\_________\/\\\____\//\\\\\___\//\\\\/______________________________________________ _______________________\///////////_______\//////////_____\////////___\/////////___\///__________\///______\/////_____\////________________________________________________ _____________________________________________/\\\\\\\\\\\__________________/\\\_______________________________________________________________________________________________________ ___________________________________________/\\\/////////\\\_______________\/\\\_______________________________________________________________________________________________________ __________________________________________\//\\\______\///________________\/\\\_________________________/\\\_______________________________________/\\\_______________________________ ___________________________________________\////\\\__________/\\\____/\\\_\/\\\_________/\\\\\\\\\\__/\\\\\\\\\\\__/\\/\\\\\\\___/\\\\\\\\\_____/\\\\\\\\\\\_____/\\\\\\\\____________ ______________________________________________\////\\\______\/\\\___\/\\\_\/\\\\\\\\\__\/\\\//////__\////\\\////__\/\\\/////\\\_\////////\\\___\////\\\////____/\\\/////\\\___________ _________________________________________________\////\\\___\/\\\___\/\\\_\/\\\////\\\_\/\\\\\\\\\\____\/\\\______\/\\\___\///____/\\\\\\\\\\_____\/\\\_______/\\\\\\\\\\\____________ __________________________________________/\\\______\//\\\__\/\\\___\/\\\_\/\\\__\/\\\_\////////\\\____\/\\\_/\\__\/\\\__________/\\\/////\\\_____\/\\\_/\\__\//\\///////_____________ _________________________________________\///\\\\\\\\\\\/___\//\\\\\\\\\__\/\\\\\\\\\___/\\\\\\\\\\____\//\\\\\___\/\\\_________\//\\\\\\\\/\\____\//\\\\\____\//\\\\\\\\\\___________ ___________________________________________\///////////______\/////////___\/////////___\//////////______\/////____\///___________\////////\//______\/////______\//////////____________
_________________________/\\\\\\\\\\\______________________________________________________________________________________________________________________________________ _______________________/\\\/////////\\\____________________________________________________________________________________________________________________________________ ______________________\//\\\______\///____________________________________________________________/\\\_____/\\\_________/\\\__/\\\_________________________________________ _______________________\////\\\_____________/\\\\\\\\______/\\\\\\\\__/\\\____/\\\__/\\/\\\\\\\__\///___/\\\\\\\\\\\___\//\\\/\\\__________________________________________ __________________________\////\\\________/\\\/////\\\___/\\\//////__\/\\\___\/\\\_\/\\\/////\\\__/\\\_\////\\\////_____\//\\\\\___________________________________________ _____________________________\////\\\____/\\\\\\\\\\\___/\\\_________\/\\\___\/\\\_\/\\\___\///__\/\\\____\/\\\__________\//\\\____________________________________________ ______________________/\\\______\//\\\__\//\\///////___\//\\\________\/\\\___\/\\\_\/\\\_________\/\\\____\/\\\_/\\___/\\_/\\\_____________________________________________ _____________________\///\\\\\\\\\\\/____\//\\\\\\\\\\__\///\\\\\\\\_\//\\\\\\\\\__\/\\\_________\/\\\____\//\\\\\___\//\\\\/______________________________________________ _______________________\///////////_______\//////////_____\////////___\/////////___\///__________\///______\/////_____\////________________________________________________ _____________________________________________/\\\\\\\\\\\__________________/\\\_______________________________________________________________________________________________________ ___________________________________________/\\\/////////\\\_______________\/\\\_______________________________________________________________________________________________________ __________________________________________\//\\\______\///________________\/\\\_________________________/\\\_______________________________________/\\\_______________________________ ___________________________________________\////\\\__________/\\\____/\\\_\/\\\_________/\\\\\\\\\\__/\\\\\\\\\\\__/\\/\\\\\\\___/\\\\\\\\\_____/\\\\\\\\\\\_____/\\\\\\\\____________ ______________________________________________\////\\\______\/\\\___\/\\\_\/\\\\\\\\\__\/\\\//////__\////\\\////__\/\\\/////\\\_\////////\\\___\////\\\////____/\\\/////\\\___________ _________________________________________________\////\\\___\/\\\___\/\\\_\/\\\////\\\_\/\\\\\\\\\\____\/\\\______\/\\\___\///____/\\\\\\\\\\_____\/\\\_______/\\\\\\\\\\\____________ __________________________________________/\\\______\//\\\__\/\\\___\/\\\_\/\\\__\/\\\_\////////\\\____\/\\\_/\\__\/\\\__________/\\\/////\\\_____\/\\\_/\\__\//\\///////_____________ _________________________________________\///\\\\\\\\\\\/___\//\\\\\\\\\__\/\\\\\\\\\___/\\\\\\\\\\____\//\\\\\___\/\\\_________\//\\\\\\\\/\\____\//\\\\\____\//\\\\\\\\\\___________ ___________________________________________\///////////______\/////////___\/////////___\//////////______\/////____\///___________\////////\//______\/////______\//////////____________
cve

CVE-2025-62221: Use-After-Free Vulnerability in Windows Cloud Files Driver Enables Privilege Escalation

Security Substrate

— 6 min read

CVE ID: CVE-2025-62221

Severity: HIGH | CVSS Score: 7.8

Attack Vector: LOCAL | CWE: CWE-416 (Use After Free)

Sources: 2 different security sources

Published: December 9, 2025

Let Me Explain What Happened

My friends, we need to talk about an important security issue that Microsoft addressed in their December 2025 Patch Tuesday. This vulnerability affects the Windows Cloud Files Mini Filter Driver—a component that helps Windows manage cloud-connected files like those in OneDrive. The problem is what we call a "use after free" vulnerability, which is like someone trying to use a key after the lock has already been changed. When an attacker exploits this flaw, they can elevate their privileges on a Windows system, potentially gaining complete control over your computer.

This is part of Microsoft's final Patch Tuesday release for 2025, which addressed 56 to 57 security vulnerabilities across Windows and related software. While this particular vulnerability hasn't been reported as actively exploited in the wild yet, it's serious enough that both Talos Intelligence and security researcher Brian Krebs highlighted it in their coverage of this month's patches.

A Bit More Detail

Let me walk you through what makes this vulnerability concerning. The Windows Cloud Files Mini Filter Driver operates at a very privileged level in your operating system—it's part of the kernel, which is the core of Windows. When software has a "use after free" vulnerability, it means the program tries to access memory that has already been freed up for other uses. Think of it like trying to retrieve something from a recycling bin that's already been emptied and refilled with something else. An attacker who already has limited access to your system can manipulate this flaw to trick Windows into giving them administrator-level privileges.

According to the National Vulnerability Database, this vulnerability received a CVSS score of 7.8 out of 10, placing it firmly in the "HIGH" severity category. What's particularly noteworthy is that the vulnerability requires an attacker to already have local access to the system with low-level privileges. This means it's typically used as a second-stage attack—the attacker gets in through one door, then uses this vulnerability to unlock all the other doors in the house.

The Technical Specifics

  • Attack Vector: LOCAL (AV:L) - The attacker needs local access to the system
  • Attack Complexity: LOW (AC:L) - Once local access is obtained, exploitation is straightforward
  • Privileges Required: LOW (PR:L) - Only basic user privileges are needed to exploit
  • User Interaction: NONE (UI:N) - No victim interaction required
  • Scope: UNCHANGED (S:U) - Impact is limited to the vulnerable component
  • Impact: HIGH across Confidentiality, Integrity, and Availability (C:H/I:H/A:H)
  • CWE Classification: CWE-416 (Use After Free)
  • Affected Component: Windows Cloud Files Mini Filter Driver
  • CVSS Vector String: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Understanding Use-After-Free Vulnerabilities

Let me take a moment to explain what "use after free" means in simpler terms. When programs run, they request memory from the operating system to store information temporarily. When they're done with that memory, they're supposed to "free" it—giving it back to the system so other programs can use it. A use-after-free vulnerability happens when a program frees memory but then mistakenly tries to use it again later.

Here's why this is dangerous: After memory is freed, the operating system might give that same space to another program. If an attacker can control what gets written into that memory space, and then trigger the original program to use it again, they can potentially trick the program into executing malicious code or accessing data it shouldn't. In the case of a kernel driver like the Windows Cloud Files Mini Filter Driver, this can lead to complete system compromise.

The December 2025 Patch Tuesday Context

This vulnerability was addressed as part of Microsoft's December 2025 Patch Tuesday, which Talos Intelligence reported included 57 vulnerabilities total. Brian Krebs, in his coverage on Krebs on Security, noted that Microsoft fixed at least 56 security flaws, with the discrepancy likely due to different counting methodologies. The patch release included two vulnerabilities rated as "critical" and one zero-day vulnerability that was already being actively exploited—though CVE-2025-62221 was not among those most urgent cases.

It's worth noting that CISA (the Cybersecurity and Infrastructure Security Agency) has included this CVE in their Known Exploited Vulnerabilities catalog, which suggests they consider it significant enough to warrant special attention, even if active exploitation hasn't been publicly documented yet.

What You Should Do About This

  • Right Now - Immediate Actions:
    • Check your Windows Update status: Open Windows Settings, navigate to Update & Security, and check for available updates. The December 2025 Patch Tuesday updates should be available for all supported Windows versions.
    • Prioritize systems with cloud file integration: If you use OneDrive, SharePoint, or other cloud storage solutions integrated with Windows, prioritize patching those systems first, as they're most likely to have the vulnerable driver active.
    • Review user privileges: While you're addressing this, take a moment to ensure users on your systems have only the minimum privileges they need. This vulnerability requires an attacker to already have local access, so limiting who can log in locally reduces your exposure.
    • Monitor for unusual privilege escalation attempts: Watch your security logs for unexpected elevation of privileges, especially from accounts that normally operate with standard user rights.
  • For the Long Term - Sustained Protection:
    • Enable automatic updates: If you haven't already, configure Windows to install security updates automatically. This ensures you won't miss critical patches in the future.
    • Implement least privilege principles: Ensure users operate with standard accounts for daily work and only elevate to administrator privileges when absolutely necessary.
    • Deploy endpoint detection and response (EDR) solutions: These tools can help detect exploitation attempts even for vulnerabilities you haven't patched yet.
    • Maintain an asset inventory: Know which systems are running Windows and which versions, so you can quickly identify what needs patching when vulnerabilities like this are announced.
    • Test patches in a controlled environment: Before deploying patches widely, test them on representative systems to ensure they don't cause operational issues in your specific environment.

Detection and Hunting Guidance

While the sources provided don't include specific indicators of compromise or detection signatures for this particular vulnerability, here are some general approaches for detecting potential exploitation attempts:

  • Monitor for unexpected privilege escalation: Look for events where standard user accounts suddenly gain administrative privileges, especially if those events involve the Cloud Files Mini Filter Driver or related components.
  • Watch for unusual kernel-mode activity: Since this affects a kernel driver, exploitation attempts might generate unusual kernel-mode exceptions or crashes. Review your Windows Event Logs for unexpected system errors related to file system drivers.
  • Audit cloud file access patterns: Unusual access patterns to cloud-synced files, especially when combined with privilege changes, could indicate exploitation attempts.
  • Review security event logs: Pay particular attention to Event ID 4672 (Special privileges assigned to new logon) and Event ID 4673 (A privileged service was called) for accounts that shouldn't be receiving elevated privileges.

Going Deeper: MITRE ATT&CK Context

From a threat intelligence perspective, this vulnerability maps to several MITRE ATT&CK techniques:

  • TA0004 - Privilege Escalation: This is the primary tactic enabled by this vulnerability. Specifically, it aligns with technique T1068 (Exploitation for Privilege Escalation), where adversaries exploit software vulnerabilities to elevate their privileges on a system.
  • Defense Evasion considerations: Once an attacker has elevated privileges through this vulnerability, they can potentially disable security tools, modify logs, or take other actions to hide their presence.

The use-after-free vulnerability class (CWE-416) is particularly concerning because these flaws can be difficult to detect through standard code review and often require sophisticated memory analysis tools to identify. They're also notoriously tricky to exploit reliably, which means that if someone has developed a working exploit, they likely have significant technical capabilities.

Why This Matters for Your Security Posture

You might be wondering why you should prioritize this particular vulnerability when there were dozens of others patched in the same release. Here's my thinking: While this isn't being actively exploited yet (as far as we know), it has several characteristics that make it attractive to attackers:

  • It affects a widely deployed component (most Windows systems use cloud file integration)
  • The attack complexity is low once an attacker has initial access
  • It provides complete system compromise (high impact on confidentiality, integrity, and availability)
  • CISA's inclusion of it in their Known Exploited Vulnerabilities catalog suggests government agencies consider it a significant risk

In my experience, vulnerabilities like this often become part of attacker toolkits once proof-of-concept code becomes available. It's much better to patch now, while you're ahead of the curve, than to scramble later when exploitation becomes widespread.

Where I Found This Information

Note: This analysis is based on publicly available security intelligence from multiple trusted sources. The technical details are drawn from official vulnerability databases and respected security research organizations. Always test security updates in a controlled environment before deploying them to production systems, and consult your organization's change management procedures. Stay safe out there, and remember—keeping your systems patched is one of the most effective security measures you can take.

Read more