Networking problems after the October 2026 Windows update? NTLM might be to blame

The October 2026 Windows security update may break networking for some customers who are still clinging to long-deprecated NTLMv1 security. A seemingly minor change in the update could have major implications for administrators and enterprises that haven't fully moved away from the legacy technology. Before the October 2026 update, Windows permitted NTLMv1-derived credentials for Single Sign-On and generated warning events. After the update, requests to use NTLMv1-derived credentials are blocked. It comes down to the BlockNtlmSSO registry value. If it hasn't been set, Windows will now use Enforce mode (1) instead of Audit mode (0), thus blocking requests and potentially impacting Wi-Fi, Ethernet, and VPN authentication, and therefore connections. The update does not set the value of BlockNtlmSSO, and administrators can set it to 0 to temporarily allow NTLMv1-derived Single Sign-On credentials. However, Microsoft noted, "Using Audit mode reduces the protection provided by Enforce mode and should only be considered as a scoped, temporary security exception." …