Microsoft's October Windows Server Updates: The Exchange Sign-In Surprise

Emily Davis · 2026-10-19

Microsoft has confirmed that this month's Windows Server security updates are causing OWA sign-in and mail delivery issues on some Exchange Server systems. If you're running Exchange, you might have noticed users struggling to log in or emails piling up in queues. It's a headache, but you're not alone. ### What's Going On? The October security updates, specifically KB5031361 and KB5031356, are the culprits. According to Microsoft, these updates can break Outlook on the web (OWA) sign-in and disrupt mail flow. The issue seems to stem from how the updates handle authentication certificates. In some cases, the updates cause the Exchange server to use an invalid certificate, leading to sign-in failures. ### Who's Affected? Not everyone will see problems. Microsoft says the issue affects Exchange Server 2019 and 2016 on Windows Server 2019 and 2016. If you're on Exchange Online or a newer version, you might be in the clear. But for those on-premises setups, it's worth checking. ### What Can You Do? First, don't panic. Microsoft is aware and working on a fix. In the meantime, here are some steps you can take: - **Check your certificates**: Ensure your Exchange server has a valid certificate for OWA and SMTP. If it's expired or misconfigured, renew or fix it. - **Temporarily uninstall the update**: If the issue is severe, you can uninstall the problematic update (KB5031361 or KB5031356) as a temporary workaround. But be aware this leaves your server vulnerable to the security flaws the update patched. - **Monitor mail flow**: Keep an eye on your mail queues. If messages are stuck, you might need to restart the Transport service. - **Stay updated**: Keep an eye on Microsoft's support channels for an official fix. ### Why This Matters Security updates are crucial, but they can sometimes cause unintended side effects. This incident highlights the importance of testing updates in a staging environment before rolling them out to production. It also reminds us that even tech giants like Microsoft aren't immune to bugs. For IT pros managing Exchange servers, this is a familiar dance: apply updates, cross your fingers, and hope nothing breaks. But with a bit of preparation and a solid rollback plan, you can minimize downtime and keep your users happy. ### The Bigger Picture While this specific issue is about Exchange, it's a good reminder to review your update strategy. Are you testing updates before deploying? Do you have a rollback plan? And most importantly, are you communicating with your users about potential disruptions? A little transparency goes a long way. We'll keep you posted as Microsoft releases more information. In the meantime, if you've been affected, hang in there. You're not the only one Googling error codes at 2 AM.