Preventing a SYSVOL Horror Story

By Tammy Mindel October 20, 2022 | Active Directory

October is Cybersecurity Awareness Month, and an excellent time to bust the ghosts of configurations past. One of the actions that the Cybersecurity & Infrastructure Security Agency (CISA) and National Cybersecurity Alliance (NCA) recommend taking is “Update your software.” A perfect place to start: Rid your domains of the outdated SYSVOL replication protocol, File Replication Service (FRS).

Once upon a time…

SYSVOL is a special directory that resides on each domain controller (DC) within a domain. The directory comprises folders that store Group Policy objects (GPOs) and logon scripts that clients need to access and synchronize between DCs.

For these logon scripts and GPOs to function properly, SYSVOL should be copied accurately and rapidly throughout the domain. This process, referenced as SYSVOL replication, ensures the identical duplication of relevant policies of a domain to another DC in that domain.

So, how does SYSVOL replication occur?

Initially, with Windows 2000 Server, replication was handled by the FRS protocol. FRS replaced the LAN Manager Replication Service of Windows NT Server. However, Microsoft deprecated FRS in Windows Server 2008. In its place, the Distributed File System (DFS) protocol now manages SYSVOL replications. DFS Replication (DFSR) improves replication performance, reliability—and cybersecurity.

The problem? Despite being replaced with the newer, better-functioning DFSR, FRS is still widely used, especially in domains that had a Windows Server 2003 DC at any point. But because it’s deprecated, FRS no longer receives any bug or security fixes.

That could spell big trouble for environments where FRS is still hanging around. Continued use of an old protocol interfacing with DCs is risky. Potentially, attackers can manipulate FRS vulnerabilities to compromise SYSVOL and change GPOs or logon scripts to propagate malware and move laterally across the environment. Spooky stuff, indeed.

Banishing FRS replication

First, determine whether FRS is still in use on a DC.

  1. Locate the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\DFSR\Parameters\SysVols\Migrating Sysvols\LocalState registry subkey.
  2. Check the subkey value.
    • A value of “3” indicates that DFSR is in use.
    • A different value or missing subkey indicates that FRS is still in use.
  3. See the Microsoft article “Backing Up and Restoring an FRS-Replicated SYSVOL Folder” for more information.

If you find FRS in use on a DC, migrate to DFSR and disable FRS as soon as possible. The migration process will upgrade the replication process will correct related health issues in your environment.

For a detailed guide to migrating from FRS to DFSR, see the Microsoft article “Migrate SYSVOL Replication to DFS Replication” or the SYSVOL Replication Migration Guide, available for download from Microsoft.

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About the author
Tammy Mindel
Tammy Mindel Security Product Manager
Tammy Mindel, Semperis Security Product Manager, has held customer and product-centric roles in the cybersecurity sector and has experience in application security, infrastructure/network security, incident response and forensics, and security best practices and standards. Linkedin
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