The October 2026 Windows security update is poised to disrupt network connectivity for some users who continue to rely on the deprecated NTLMv1 authentication protocol. The change, which enforces stricter controls by default, could affect Wi-Fi, Ethernet and virtual private network connections in environments that have not fully transitioned away from legacy authentication methods.

The update modifies the BlockNtlmSSO registry setting, shifting its default behavior from Audit mode to Enforce mode. When enabled in Enforce mode, Windows blocks authentication attempts that use NTLMv1-derived credentials, potentially preventing devices from joining corporate networks. Administrators who wish to maintain temporary compatibility can manually configure the setting to Audit mode, but Microsoft cautions that this reduces security protections and should only be used as a short-term measure.

Microsoft has long planned to phase out NTLMv1, a protocol introduced in 1993 with Windows NT 3.1. Although the technology has been deemed insecure by modern standards, its deep integration into enterprise identity systems has made complete removal challenging. The company previously removed NTLM support from Windows 11 24H2 and Windows Server 2025, yet remnants persist in certain connection types, including PEAP-MSCHAPv2 and EAP-MSCHAPv2 used in domain-joined systems.

To assist with migration, Microsoft introduced audit logging for NTLMv1 usage in late 2025. The October 2026 update represents the next phase in this transition, moving toward full enforcement of NTLMv1 deprecation. While in-market versions of Windows will continue to support the protocol for the foreseeable future, Microsoft has stated its long-term objective is a platform that does not require NTLM at all.

The upcoming changes have prompted concern among IT administrators who may find their network access policies unexpectedly altered. Microsoft has not indicated a timeline for when the BlockNtlmSSO workaround will cease to function, though it is expected to remain viable only until the next major Windows release cycle. Organizations are advised to review their authentication configurations and begin planning for full migration to more secure alternatives.