Microsoft's July Patch Tuesday, due 14 July 2026, completes a multi-year rollout that removes support for an older, weaker form of Windows domain authentication.

• What it is: Kerberos RC4 encryption support is moving from "audit" mode to full enforcement on Windows domain controllers. RC4-based logins will be rejected outright rather than just logged.
• Who is affected: Any business running its own Windows Server domain controllers and Active Directory, particularly older servers, legacy line-of-business applications, or devices that have never been updated to support modern Kerberos (AES) encryption.
• What the risk is: Devices or applications still relying on RC4 authentication may suddenly be unable to log in or access network resources once enforcement takes effect.
• What to do: Check Windows Server event logs (Event ID 4768/4769) ahead of July 14 for any RC4 usage, and update or replace legacy systems that depend on it.

If your business runs its own domain controllers, we recommend reviewing authentication logs before Patch Tuesday to avoid unexpected lockouts.