Impending Changes to NTLMv1 Security Protocol
The upcoming Windows security update scheduled for October 2026 is poised to disrupt networking for organizations that continue to rely on the outdated NTLMv1 security protocol. This seemingly minor adjustment could have significant ramifications for system administrators and enterprises that have yet to fully transition away from this legacy technology.
Historically, Windows has allowed NTLMv1-derived credentials for Single Sign-On, albeit with warning notifications. However, post-update, any requests utilizing NTLMv1-derived credentials will be outright blocked. This shift is primarily governed by the BlockNtlmSSO registry value. If this value remains unset, Windows will default to Enforce mode (1), effectively blocking requests and potentially disrupting Wi-Fi, Ethernet, and VPN authentication processes.
While the update does not automatically configure the BlockNtlmSSO setting, administrators have the option to temporarily adjust it to 0, allowing NTLMv1-derived Single Sign-On credentials. However, Microsoft cautions that “Using Audit mode reduces the protection provided by Enforce mode and should only be considered as a scoped, temporary security exception.” It is important to note that this change is intentional and not a bug. The deprecation of NTLM was first announced in 2024, with audit logs for NTLMv1 usage introduced in late August 2025. Microsoft has been transparent about its goal to phase out this legacy technology, stating as early as 2023, “Our end goal is eliminating the need to use NTLM at all.”
The challenge lies in NTLM’s long-standing presence; it was first introduced in 1993 with Windows NT 3.1. Despite its somewhat insecure nature by modern standards, removing it from enterprise environments has proven to be a complex task. While Microsoft has already eliminated NTLM from Windows 11 24H2 and Windows Server 2025, traces of it remain, particularly in configurations utilizing MS-CHAPv2-based connections, such as PEAP-MSCHAPv2 and EAP-MSCHAPv2, within domain-joined environments.
The introduction of audit mode in 2025 was a strategic move to assist administrators in identifying lingering uses of the technology. Now, as we approach 2026, Microsoft is taking decisive action. If Credential Guard is not enabled and the BlockNtlmSSO value is not set to Audit mode (0), the use of NTLMv1-derived credentials will be blocked as part of Microsoft’s initiative to “finalize the disablement of NTLMv1.”
There is currently no indication of how long the workaround involving the BlockNtlmSSO setting will remain viable, likely lasting only until the next major Windows release. In September, Microsoft confirmed that “In-market versions of Windows client and Windows Server will continue to support NTLM,” but reiterated its long-term vision for a Windows platform devoid of NTLM altogether.
As such, the shift to enforce NTLMv1 blocking by default serves as a clear signal that organizations must accelerate their migration efforts. Networking issues following the October 2026 update may reveal that many administrators have not progressed as far in their transition plans as they had anticipated.
