From 14 July 2026, Microsoft completes its rollout of Kerberos RC4 hardening on Windows domain controllers, removing the "audit" option that has let businesses monitor RC4 use without breaking anything. After this date, RC4 authentication requests will be blocked outright.

• What it is: A security enforcement change affecting how Windows Server domain controllers authenticate devices and applications.
• Who is affected: Businesses running their own Windows Server domain controllers, particularly older or legacy systems still relying on RC4 encryption.
• What the risk is: Legacy applications, older printers, or third-party systems still using RC4 may lose the ability to authenticate, causing login failures or service outages.
• What action is needed: Review Windows Server event logs for RC4 usage warnings before 14 July, and update or replace any system still dependent on RC4 encryption.

This mainly affects businesses with on-premises servers rather than cloud-only environments. If you are unsure whether your network is affected, we recommend a quick check before the enforcement date to avoid unexpected outages.