GoKawiilMicrosoft resolves Exchange sign-in bug tied to October Windows Server updates
Microsoft confirmed it has fixed an issue where Outlook web sign-in and mail delivery failed on Exchange Server systems after installing October 2026 Windows Server security updates. The problem stemmed from a new default memory limit on Endpoint Mapper's handling of Endpoint Lookup calls, and affected servers running Windows Server 2019 and later to varying degrees. Microsoft says the November 2026 Patch Tuesday updates, KB5131080 and KB5131090, fix the issue, and it has also published a registry-based workaround for administrators who cannot immediately install them.
GoKawiil's interpretation of the reporting above, not reported fact.
The episode highlights how a routine security patch can ripple into unrelated enterprise systems like Exchange, forcing administrators to weigh immediate security protection against operational disruption. Microsoft's advice not to uninstall the October updates suggests it prioritized maintaining security coverage while a proper fix was developed, rather than rolling back protections. Organizations running on-premises Exchange may now face a two-step patching process, underscoring ongoing complexity in managing interdependent Windows and Exchange update cycles.
- November 2026 updates KB5131080 and KB5131090 fix the Outlook sign-in and mail delivery bug.
- The issue was caused by a new memory limit on Endpoint Mapper introduced in October 2026 updates.
- Microsoft offered a registry-key workaround for systems that can't yet install the November fixes.
Source: bleepingcomputer.com, 2026-11-11
Published there as: “Microsoft fixes Exchange sign-ins broken by Windows Server updates”
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