Windows 11 KB5066835 Causes WinRE USB and Localhost HTTP/2 Failures (KIR and KB5070773 Fix)

Microsoft’s October cumulative update for Windows 11 — identified as KB5066835 — was intended to harden the platform with security fixes, but it instead introduced cascading regressions that left developers, IT admins, and new Windows users scrambling. The update disabled USB input inside the Windows Recovery Environment (WinRE) for some systems and broke loopback HTTP/2 connections handled by HTTP.sys, causing IIS and many developer tooling scenarios to fail with “connection reset” and HTTP/2 protocol errors. Microsoft moved quickly to patch the first issue with an out‑of‑band update (KB5070773) and to mitigate the network regression via Known Issue Rollback (KIR), but the incident exposed a fragile interaction between urgent security servicing and critical kernel‑mode networking components — and it landed at the worst possible time as Windows 10 reached end‑of‑support.

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Background​

Microsoft ships cumulative updates each month to deliver security fixes and quality improvements across Windows client and server builds. October’s bundle for Windows 11 — KB5066835 — was published on October 14 and became broadly available via Windows Update. Within days, multiple high‑impact problems appeared in the field: USB input stopped working inside WinRE (so recovery menus were unusable for affected machines), and HTTP.sys — the kernel‑mode HTTP stack that accepts requests on behalf of IIS and other local servers — started terminating loopback HTTP/2 sessions prematurely, leaving localhost and developer workflows unable to connect. Microsoft documented both symptoms on its Windows Release Health pages and issued emergency remediations.

Why this matters now​

• WinRE is the last line of defense when Windows won’t boot; if keyboard and mouse are dead there, recovery and reset flows become difficult or impossible for affected PCs.
• HTTP.sys sits in the kernel and mediates HTTP/2 connections for user‑mode servers. A regression at this layer can render working services silently unreachable despite the server process itself being healthy. That’s what happened with connections to 127.0.0.1 (localhost) and services relying on HTTP.sys.
• The timing is awkward: Microsoft had just ended mainstream Windows 10 support, putting added pressure on migrations to Windows 11 and on administrators to keep systems patched and secure. That widening user base increased the number of people who could run into the regression.

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What happened: a technical breakdown​

KB5066835 introduced two distinct but high‑impact failures​

• USB input disabled in WinRE
  After installing KB5066835, some machines reported that USB keyboards and mice stopped working inside the Windows Recovery Environment. The symptom prevented users from navigating WinRE menus and executing repair workflows even though the same devices worked normally after booting into the main OS. Microsoft acknowledged the problem and added it to the release health dashboard.
• HTTP.sys regression broke local HTTP/2 sessions
  The update changed behavior in HTTP.sys such that the kernel layer could terminate or reset HTTP/2 sessions before user‑mode listeners (IIS, IIS Express, ASP.NET Core apps) saw the request. Browsers and tools displayed ERR_CONNECTION_RESET or ERR_HTTP2_PROTOCOL_ERROR when attempting to talk to www‑style local endpoints (http://localhost or 127.0.0.1). Because HTTP.sys operates below the server process, this looked indistinguishable from the server not listening at all. Microsoft described the issue on the Windows Server and Windows client release health pages.

Root cause and why loopback is fragile​

HTTP.sys handles loopback (localhost) traffic differently than normal remote traffic, especially when HTTP/2 is negotiated. A small kernel change that affects frame handling or session lifetimes can produce immediate, widespread symptoms across development tools, administration consoles, authentication callbacks, and local embedded servers — because so many components rely on localhost as a simple IPC or development surface. The result is not a localized application bug but a systemic failure that undermines developer productivity and server availability.

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The immediate fallout​

Developers and local servers​

• Visual Studio debugging, IIS Express, ASP.NET Core development servers, Electron apps that use loopback for OAuth flows, local Docker setups, and many vendor tools stopped working for engineers and testers. Community forums lit up with reports that uninstalling KB5066835 restored functionality in many environments, but rolling back a security update has obvious risks.

Administrators and production sites​

• On servers and appliances that use HTTP.sys (including IIS and other third‑party services using the kernel HTTP API), incoming connections could fail with ‘connection reset’ errors — in some cases affecting admin GUIs hosted on localhost or even public endpoints. Microsoft labeled the problem “Confirmed” on its release‑health pages and provided mitigation guidance.

Ordinary users and recovery scenarios​

• Users who need to use WinRE (to reset their PC, repair startup, or recover from a failed update) were at risk of being unable to navigate the environment if their only input devices were USB. For many laptops and modern desktops, USB is the default, making WinRE unusable until the condition was fixed. Microsoft released an out‑of‑band update to address this particular symptom.

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Microsoft’s response: KIR and a hotfix​

Microsoft used a multi‑pronged approach to contain the damage:

• Known Issue Rollback (KIR) and Group Policy KIR packages
  For the HTTP.sys/localhost regression, Microsoft provided a Known Issue Rollback (KIR) mitigation that could be pushed via special Group Policy MSI or would be applied automatically in many unmanaged environments. The release‑health guidance walked admins through obtaining and deploying the KIR so that devices would ignore the problematic code paths introduced by the update. This approach is designed to be reversible and fast to deploy compared with a replacement cumulative update.
• Out‑of‑band cumulative update (KB5070773) to restore WinRE USB input
  The WinRE USB input failure could not be remedied via server‑side rollbacks, because it required replacing or refreshing Safe OS components used by the recovery environment. Microsoft released an out‑of‑band cumulative update, KB5070773, on October 20 to restore USB device functionality inside WinRE. The update is cumulative and includes the October security fixes plus the corrective Safe OS components.
• Ongoing investigation and staged fixes
  Release health notes showed Microsoft documenting the conditions, publishing workarounds (including registry toggles and KIR guidance), and updating pages as fixes rolled out. In some cases, Microsoft recommended checking Windows Update, installing the KIR, and restarting to accelerate the mitigation delivery.

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Practical mitigation and recovery steps for admins and developers​

The following are practical steps — ranked from least to most invasive — to protect production services and restore developer workflows if you were affected.

• Check the Windows Release Health dashboard and your update catalog first. Confirm whether your device shows the affected OS build and whether Microsoft has published a KIR or OOB fix for your platform.
• If you manage fleets, deploy the Microsoft KIR package via Group Policy or your management stack. This can often resolve the HTTP.sys regression without uninstalling security updates. The MSI and Group Policy guidance are published on the release‑health page.
• If you’re a developer with a broken localhost workflow and cannot yet apply the KIR, consider these options:
• Temporarily disable HTTP/2 on the machine (registry keys) to force HTTP/1.1 loopback behavior:
• HKLM\SYSTEM\CurrentControlSet\Services\HTTP\Parameters\EnableHttp2Tls = 0
• HKLM\SYSTEM\CurrentControlSet\Services\HTTP\Parameters\EnableHttp2Cleartext = 0
• Restart the HTTP service or reboot.
• Use a local bind to 127.0.0.1ort instead of relying on HTTP/2 loopback if your app supports it.
• If acceptable, uninstall KB5066835 and KB5065789 to restore prior behavior — but coordinate with security teams because uninstalling security patches removes protections.
• If WinRE is unusable and you cannot boot:
• Try touchscreen input (if available), a PS/2 keyboard or mouse (older desktops), or a pre‑created USB recovery drive that was built before the offending update was applied. If the machine can boot, install KB5070773 immediately.
• For production web servers: monitor incoming requests and error rates; if you see ERR_CONNECTION_RESET or HTTP/2 protocol errors, apply the KIR or roll back updates in a controlled maintenance window while compensating with other security controls (network isolation, WAFs).
• Validate vendor‑supplied appliances and specialized software that rely on embedded localhost bindings (medical devices, controllers, or administrative web consoles). Open a vendor support case — many third‑party vendors reported immediate impact and provided guidance. Community forums captured many quick‑fixes for specific agents.

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Why this escalation happened: system and process analysis​

1) Security changes touched deep platform surfaces​

KB5066835 included protections and hardening that touched cryptography stacks and kernel‑level networking components. When a patch reaches into kernel networking (HTTP.sys) or Safe OS images (WinRE), the surface area for regressions grows. Kernel changes are high stakes: they’re broadly impactful and often difficult to test against every combination of installed software, drivers, and previous updates.

2) Cumulative servicing model complicates rollback​

Cumulative updates bundle many fixes; rolling back one change without removing others is nontrivial. Microsoft’s Known Issue Rollback provides a valuable blunt instrument for turning off a daytime‑breaking regression without removing security patches, but KIRs are not a panacea — they must be designed carefully and may not address Safe OS image mismatches (which required KB5070773).

3) Real‑world diversity confounds validation​

Community reports show fresh installs sometimes avoided the bug while upgraded machines reproduced it reliably. That pattern suggests the regression is sensitive to historical state (installed previews, stacks, or driver versions). No test matrix can perfectly simulate the huge diversity of enterprise endpoints, but these incidents underline why thorough staged deployment and broad telemetry are essential.

4) The human factor: speed vs. assurance​

When a security bulletin is urgent, pressure mounts to ship fixes quickly. That operational tempo can increase the risk of regression. Microsoft’s use of out‑of‑band updates and KIRs demonstrates its ability to respond, but the pattern of repeated high‑impact regressions this year raises questions about whether release pipelines are tuned correctly for both speed and systemic risk reduction. Observers have noted increased reliance on automated coding and AI-driven tools in development pipelines; where these are used, manual QA and systems thinking remain critical.

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The broader impact: trust, migration, and the Windows 10 EOL backdrop​

The October patch fallout coincided with a major Windows milestone: Windows 10 reached end of support on October 14, 2025. That move pushed a broader population toward Windows 11 and increased the urgency behind updating machines — exactly the environment in which a misbehaving cumulative update causes maximum friction. For organizations migrating from Windows 10, the incident strengthens the argument for careful, staged rollouts and defensive patch management rather than “install immediately” policies.

• For enterprises, the incident increases the cost of chasing upgrades: testing matrices must expand to cover more diverse legacy configurations and local tooling used by developers.
• For developers, the broken localhost incident was a painful reminder that kernel bugs can wreck productivity faster than any IDE bug.
• For Microsoft, repeated high‑impact regressions erode confidence in Windows Update as an automatic, safe mechanism for keeping endpoints secure and stable.

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Recommendations: a checklist for protecting users and services​

• Delay broad security rollouts to production: Use phased deployments, pilot rings, and canary groups for cumulative updates. Test updates on machines that represent both fresh installs and long‑lived upgrade histories.
• Automate KIR and rollback readiness: Ensure your management stack can deploy Known Issue Rollback packages and that IT runbooks include KIR steps and verification.
• Keep pre‑built recovery media: Maintain USB recovery drives (built before major servicing runs) for critical endpoints and ensure alternate input methods (touch, PS/2) are available in repair kits.
• Monitor kernel‑level telemetry and test localhost paths: For developer workstations and web servers, include loopback HTTP/2 tests in your automated validation suites.
• Coordinate security vs. availability: If a security update must be delayed due to operational risk, pair the delay with compensating security controls (network segmentation, web application firewalls, limited exposure) and a clear timeline for patching.
• Inventory and vendor outreach: Maintain an inventory of third‑party tools that use localhost callbacks (SSO agents, device management portals) and coordinate with vendors to validate compatibility for each monthly rollup.

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What Microsoft must fix in process and communication​

The technical fixes (KIR, KB5070773) were necessary and effective, but the incident reveals several process shortcomings:

• Faster, clearer guidance: Release health entries are useful, but early, clear mitigation advice for developers (including recommended registry settings and the availability of KIR packages) should appear immediately with KBs that touch kernel networking.
• Better pre‑deployment coverage: Test harnesses that simulate long‑lived upgrade histories and complex third‑party stacks could catch stateful regressions earlier.
• Transparent timelines: Communicating expected windows for definitive fixes — not just “we’re investigating” — would help admins make better risk decisions.

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Conclusion​

KB5066835’s twin regressions — broken WinRE input and a kernel‑level HTTP.sys failure that damaged localhost connectivity — were high‑impact, high‑visibility failures that exposed the fragility of tightly coupled platform components. Microsoft’s rapid use of Known Issue Rollback and the out‑of‑band KB5070773 fix mitigated the most immediate pain, but the episode should prompt organizations to rethink their patch management posture: prioritize staged rollouts, readiness for KIR deployment, and robust validation of developer tooling and recovery mechanisms.
For developers and admins, the core takeaway is simple and pragmatic: treat cumulative security updates as critical but not infallible. Validate them in representative environments, maintain recovery options, and be prepared to apply Microsoft’s KIR guidance or hotfixes when the update train produces a broken carriage. The platform fixed what was broken; the question now is how the ecosystem will harden the processes that let those breaks happen in the first place.

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Source: htxt.co.za SPEEDRUN - Broken update welcomes new Windows 11 users - Hypertext