On September 29, 2025, Microsoft released the preview update KB5065789 for Windows 11 OS Builds 26200.6725 and 26100.6725, which included fixes for browser print preview freezing, Gamepad sign-in responsiveness issues, PowerShell Remoting timeouts, and Windows Hello USB infrared camera setup errors. The subsequent October update caused connectivity failures amongst users.
On October 14, 2025, Microsoft rolled out the security cumulative update KB5066835 for OS Builds 26200.6899 and 26100.6899. While the update was intended to address security vulnerabilities and integrate the earlier fixes, it unintentionally broke localhost connectivity for developers and IT environments. Immediately after installation, users began reporting failures connecting to services via 127.0.0.1 or localhost, including timeouts for local web servers, MySQL and PostgreSQL databases, Docker containers, and API testing tools.
The disruption gained widespread recognition shortly after October 14, as reports surfaced across development communities and cybersecurity outlets, including GBHackers. By October 18, 2025, coverage confirmed that Microsoft had not documented the issue in its official notes, despite using rollback instructions via DISM as a temporary workaround. As of that date, Microsoft had not issued a statement or patch to resolve the localhost regression introduced in KB5066835.
Localhost, represented by the IP address 127.0.0.1, is a loopback network address that allows a computer to communicate with itself. Instead of routing traffic over the internet or a physical network, localhost sends data back into the same device, making it necessary for testing, development, and running local services.
Developers use it to host web servers, databases, APIs, and applications in a controlled environment without exposing them externally. The address belongs to the reserved IPv4 loopback range (127.0.0.0 to 127.255.255.255), but 127.0.0.1 is the standard default. When a browser, application, or command tries to access “localhost” or “127.0.0.1,” the operating system reroutes that request internally to the local machine’s networking stack.
One user on the Microsoft forum noted, “Since the last cumulative update of 2025-10 I can not access applications via localhost anymore. It looks like I am not the only one with this issue: https://serverfault.com/questions/1193405/http-2-protocol-error-after-windows-update and https://stackoverflow.com/questions/79790827/localhost-applications-failing-after-installing-2025-10-cumulative-update-for-w. When the update (KB5066835) is removed, everyhting works again. This is affecting local development as well as business applications”
Hospitals, clinics, and health IT vendors often run local services, such as EHR testing environments, API integrations, billing applications, authentication tools, and medical device interfaces, on 127.0.0.1 to ensure protected health information (PHI) stays internal and compliant with HIPAA. When localhost fails, workflows like software updates, API debugging, claims processing, and lab system integrations can stall, impacting both patient care and back-office operations.
The issue also forces IT teams into a dangerous trade-off: roll back the update and lose security patches, or keep the update and risk operational outages. Healthcare IT environments already face high cybersecurity stakes, and any interruption increases the risk of delayed treatment, billing errors, compliance violations, or manual workarounds that introduce new vulnerabilities.
See also: HIPAA Compliant Email: The Definitive Guide (2025 Update)
IT and security teams within covered entities (like hospitals, clinics, and health systems) are responsible for evaluating, testing, and deploying patches. Business associates, such as billing services, cloud vendors, and EHR providers, must also maintain secure, patched systems under HIPAA requirements.
Yes. The HIPAA Security Rule requires covered entities and business associates to protect against known vulnerabilities. Applying timely software updates is considered part of the Security Management Process and Technical Safeguards. Failure to patch systems has been cited in OCR enforcement actions.
Healthcare organizations are expected to test patches in controlled environments before deployment. If a patch disrupts clinical systems, IT teams may temporarily roll it back—but they must document the issue, implement alternative safeguards, and apply the fix once stable.