check before: 2025-10-28
Product:
Windows, Windows Server
Platform:
Online, Windows Desktop, World tenant
Status:
Change type:
Admin impact
Links:
Details:
A new hardening authentication mitigation has been introduced for the Common Log File System (CLFS) driver. Windows updates that include this new version of CLFS will initiate a 90 day "learning mode" period during which authentication codes will be added to log files automatically. Device behavior will change after this period. For more information, see Common Log File System (CLFS) Authentication Mitigation.
When will this happen:
Windows 11, version 25H2 and Windows Server 2025 updates released on or after October 28, 2025 include this change. A mitigation adoption period, referred to as "learning mode" will be in place for 90 days following installation of updates. During this time, authentication codes are automatically added to existing logfiles when they are opened. After this period ends, the CLFS driver will enter enforcement mode, requiring all logfiles to contain valid authentication codes.
Change Category:
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Scope:
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Release Phase:
Created:
2026-01-10
updated:
2026-01-10
Task Type
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MS How does it affect me
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summary for non-techies**
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Direct effects for Operations**
Logfile Access Issues
After the 90-day learning mode, any logfile without a valid authentication code will fail to open, potentially leading to data access issues.
- roles: System Administrators, Database Administrators
- references: https://support.microsoft.com/topic/common-log-file-system-clfs-authentication-mitigation-af903a7e-ceca-410e-a5bf-58b1c79e861d
Increased Administrative Overhead
Logfiles not accessed during the learning mode will require manual authentication, increasing the workload for administrators.
- roles: System Administrators, IT Support Staff
- references: https://learn.microsoft.com/windows-server/administration/windows-commands/fsutil
User Experience Disruption
Users may experience disruptions in services relying on CLFS logfiles if those files are not authenticated, leading to potential downtime.
- roles: End Users, Application Support Specialists
- references: https://support.microsoft.com/help/5056852
Potential Data Loss
Failure to authenticate logfiles may result in data loss if critical logs cannot be accessed post-enforcement mode.
- roles: Data Analysts, System Administrators
- references: https://support.microsoft.com/topic/common-log-file-system-clfs-authentication-mitigation-af903a7e-ceca-410e-a5bf-58b1c79e861d
Compliance Risks
Organizations may face compliance issues if logfiles are not properly authenticated, leading to potential legal and regulatory repercussions.
- roles: Compliance Officers, IT Security Managers
- references: https://support.microsoft.com/topic/common-log-file-system-clfs-authentication-mitigation-af903a7e-ceca-410e-a5bf-58b1c79e861d
Configutation Options**
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Opportunities**
Automated Logfile Management
Implementing automated scripts to check and open CLFS logfiles during the learning mode can ensure that authentication codes are added without manual intervention. This will reduce the risk of logfiles being left unauthenticated when enforcement mode begins.
- next-steps: Develop and deploy a PowerShell script that opens all CLFS logfiles on the system during the 90-day learning period. Schedule the script to run periodically.
- roles: System Administrators, IT Operations Managers
- references: https://learn.microsoft.com/windows-server/administration/windows-commands/fsutil, https://support.microsoft.com/topic/common-log-file-system-clfs-authentication-mitigation-af903a7e-ceca-410e-a5bf-58b1c79e861d
Training and Awareness Programs
Conduct training sessions for IT staff and administrators on the new CLFS authentication changes, focusing on the importance of the learning mode and how to manually authenticate logfiles if needed. This will enhance the overall preparedness of the IT team.
- next-steps: Create a training schedule and materials covering the CLFS changes, including hands-on sessions for practical understanding of the fsutil command.
- roles: IT Trainers, System Administrators, Compliance Officers
- references: https://support.microsoft.com/help/5056852, https://support.microsoft.com/topic/common-log-file-system-clfs-authentication-mitigation-af903a7e-ceca-410e-a5bf-58b1c79e861d
Monitoring and Reporting Tools
Develop or enhance monitoring tools to track the status of CLFS logfiles and alert administrators about logfiles that have not been opened during the learning mode. This proactive approach can prevent issues when enforcement mode begins.
- next-steps: Evaluate existing monitoring solutions or create a custom monitoring dashboard that integrates with Windows Event Logs to track CLFS logfile activity.
- roles: IT Operations Managers, System Administrators, Security Analysts
- references: https://learn.microsoft.com/windows-server/administration/windows-commands/fsutil, https://support.microsoft.com/topic/common-log-file-system-clfs-authentication-mitigation-af903a7e-ceca-410e-a5bf-58b1c79e861d
Potentional Risks**
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IT Security**
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explanation for non-techies**
Imagine you have a security guard who checks the ID of everyone entering a building. This guard ensures that only authorized people get inside. In the world of computers, a similar process is being introduced for a system called the Common Log File System (CLFS) on Windows. This system keeps track of various activities, much like a logbook in a security office.
Starting with updates for Windows 11 and Windows Server 2025, a new security measure will be added to CLFS. This measure is like giving each entry in the logbook a special stamp of approval, ensuring that it hasn’t been tampered with. This stamp is called an authentication code, and it’s created using a special key that only the system and administrators can access.
When the update is first installed, there will be a 90-day period called "learning mode." During this time, every time a log file is opened, it automatically gets this special stamp. Think of it as the security guard giving everyone a temporary pass to make the transition smoother.
After 90 days, the system will switch to "enforcement mode." At this point, any log file that doesn’t have the special stamp won’t be allowed to open, similar to how someone without proper ID can’t enter the building. If there are any log files that weren’t opened during the learning mode, they’ll need to be manually stamped by an administrator to ensure they can still be accessed.
To prepare, it’s important to review which systems use these log files and make sure they’re accessed during the learning period. This way, they’ll automatically receive their authentication codes. For any files that aren’t accessed, plan to manually authenticate them before the enforcement mode begins. This proactive approach will help avoid any disruptions in accessing important log files.
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