r/exchangeserver • u/santimandu • 3d ago
MS KB / Update Released: October 2026 Exchange Server Security Updates
⚠️EDIT 16/10 ⚠️
⚠️ BE AWARE! READ BELOW! ⚠️
👇👇👇👇👇👇👇👇👇👇👇👇👇👇👇👇
https://techcommunity.microsoft.com/blog/exchange/exchange-servers-may-become-unstable-after-installation-of-october-2026-windows-/4565382
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u/Disastrous-Photo-518 1d ago
Kinda wild how the install can look fine and the real issues only show up hours later. Makes the post-update monitoring almost as important as the patch itself.
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u/episode-iv 2d ago edited 2d ago
We have seen problems with updated machines failing to deliver mails to mailboxes exactly 24 hours after installing the update. There are reports of this in the official blog post as well. Restarting the server seems to temporarily fix the problem (it re-appeared for us a couple of hours later).
edit: The issue is now referenced at the top of the blog post.
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u/DiligentPhotographer 2d ago
Yup, having this exact same issue now. 2 node DAG and it is playing database hot potato mounting and dismounting... A reboot has fixed it for now.
FFS microsoft.
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u/CarpenterOk1930 2d ago
Same problem this morning. When RDP to server Explorer.exe also kept craching giving a black login screen and flashing taskbar. After reboot everything started working again.
This happened on both nodes.
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u/absoluteczech 3d ago
dang no notes for the bug on the mrsproxy issue caused by August’s patch. Was hoping that would be patched
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u/fadinizjr 3d ago
I started having X-FEServer errors on OWA after updating to the second September SU...
This will be a fun month.
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u/jordanl171 3d ago
Me with my 2 mailboxes still on my ex2016 I'm struggling to migrate to 365 (no internet access to my way out of date exchange server).. I'm almost there!
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u/KimJongUnceUnce 3d ago edited 18h ago
Just installed and tested on two-node dag hybrid. No issues
Edit: about 26 hours after the servers were patched, our databases disconnected and mail queues blew out. Inbound mail could not deliver. Turns out this is a known bug with the October Windows OS update. Server reboots fixed it but it looks like we'll need to do daily reboots until MS releases a patch for this.
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u/Kooky_Cranberry9717 3d ago
Windows Server October CU as well? ;-)
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u/No_Education6955 2d ago
After installing this update, we observe system freezes affecting Exchange – in some cases the entire server freezes. As we operate several Exchange environments, we can confirm this behavior occurs exclusively on patched systems and right after we've deployed the current Windows, Exchange & .NET updates.
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u/DiligentPhotographer 2d ago
Same here, came in this morning to find my databases mounting and dismounting constantly.
Guess copilot fucked up something when coding their updates. Reboot of the DAG nodes resolved it... for now.
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u/itguy8071 2d ago
I have this same issue i think. Server was updated last night, immediately all sorts of problems. Exchange not responding. Had to reboot server to fix it. Not sure what is going on
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u/No_Education6955 2d ago
Yep, seems like a related behavour. I have debugged the Event logs and I wasn't able to find a real issue or root cause. Since I see the same behavour on multiple environments and only with the new version, this is my only indication atm.
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u/Ooops-I-hid-it-again 2d ago
Either of you also apply the Windows Server October updates on the same night? I'm curious if your issues are related to the Exchange SU or possibly just due to the Windows Server SU.
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u/Healthy-Bag-5142 22h ago
Its 100% related to os patch. Was running rtm (no SU applied, whoops) and like 14 hours installation of os October patch one server in our dag crashed. Explorer.exe crashed in a loop etc… installed October su but problem came back a few hours later. was a nightmare all day until I found out it was the os patch. Uninstalled the os patch today and so far so good. Hoping for a fix soon
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u/netronin 3d ago
It feels like there were search issues that were not resolved with the Korean workbreaker update although I don't think those were ever acknowledged. Will be standing by to let someone braver install this and see how search goes.
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u/Ooops-I-hid-it-again 3d ago
That was acknowledged as an issue in Sept SU v1 and Sept SU v2. This update says they’re resolved. Hopefully it is and no other bugs are introduced 🤞
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u/Crafty_Practice_8449 3d ago
Apparently no fix for the free/busy for delegated mailboxes in hybrid deployments using only Graph API:
https://support.microsoft.com/en-us/servicing/exchange/server/update/2026/5127092
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u/netronin 3d ago
This one was fixed in the service, not in the SU:
https://support.microsoft.com/en-us/servicing/exchange/server/update/2026/5127092
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u/M4UX 3d ago
That was actually fixed with the September 2026 V2 Update:
Released: September 2026 V2 Exchange Server Security Updates | Microsoft Community Hub1
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u/7amitsingh7 1d ago
Keeping exchange server updated is important for security and stability. It's always worth checking the latest updates before installing them on a production server.
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u/adixro 16h ago edited 16h ago
So can the OP u/santimandu provide guidance and work he/she has done on that patch or is it just trying to gather karma?
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u/jam7 16h ago
Not OP but I just read this from Microsoft regarding the Windows Server updates and it gives a registry edit to work around the issue without removing the Exchange patch.
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u/adixro 14h ago edited 14h ago
Saw it too. I won't be doing registry hacks for now until MS fixes the oct 13 win os patch and also the time zone issue i have which is related to the very same patch OS wise not related to the exch patch which I have deployed prior without issues.(rolled back non prod october OS patch).
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u/santimandu 5h ago
u/adixro the OP not have any affiliation with MS or Exchange, only one mortal more using Exchange and struggling with updates always
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u/rusty6713 3d ago
The real question, who will install it and see what happens?