Patch tracker / KB5126655
KB5126655: Windows Server 2022 Security Update (October 2026)
KB5126655 is a Critical security update for Windows Server 2022, released 2026-10-13. It closes 977 CVEs with a maximum CVSS of 9.8. At least one of them is actively exploited (CISA KEV), so it is urgent, and Microsoft lists 2 known issues: test it before broad deployment. The most likely of them to be attacked is CVE-2026-96951, which FIRST.org rates at 10% probability of exploitation in the next 30 days.
Before you deploy: Microsoft lists 2 known issues for this update.
Our data comes from Microsoft, CISA and other public sources, and it can be wrong or out of date. If you are unsure, check the linked source before you act, and tell us when we are wrong so we can fix it.
Update summary
Fixes 977 CVEs. At least one is actively exploited (CISA KEV). Most severe CVSS 9.8 (Critical). EPSS exploit probability up to 9.8%. Among the top half of tracked Microsoft updates by EPSS exploit probability.
Microsoft documents 2 known issues with this update.
- Users might be unable to sign in to OWA or send emails on Exchange Server
- Windows Server Update Services (WSUS) does not display error details
Senserva AI Opinion and rich prompt for KB5126655
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5126655
- Released
- 2026-10-13
- Last changed
- 2026-10-25
- Changes tracked
- 4
Senserva began recording day-to-day change history on 2026-07-07, and refreshes it several times a day. Release dates are historical where Microsoft publishes them; revision history is tracked from that date forward, so it does not include changes made before then.
Check if KB5126655 is installed
Elevated PowerShell. No output means it is not installed.
Known issues, from Microsoft
Quoted from Microsoft's support article for KB5126655 (checked 2026-11-03). Microsoft lists 2 known issues. Verify against the article before acting.
1. Users might be unable to sign in to OWA or send emails on Exchange Server
Symptoms:
After installing this update, some on-premises Exchange servers might experience issues accessing or sending email. Users might be unable to sign in to Outlook on the web (OWA), or email messages sent from Outlook might remain in the Outbox and not be delivered. Additionally, Exchange might experience issues accessing mailboxes during mailbox replication.
In some affected configurations, Exchange Server OWA might fail with an error MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142) with Event ID 1006 logged in the application log. Another reported symptom is growing email queues. Installing the October 2026 Exchange server security updates isn’t required for these issues to occur.
Workaround
This issue can be temporarily mitigated by modifying the registry key as described in Exchange servers may become unstable after installation of October 2026 Windows updates .
The October 2026 update introduced a default limit on the amount of memory used by Endpoint Mapper when handling Endpoint Lookup calls.
For workloads such as Exchange Server, the default limit of 512 simultaneous Endpoint Lookup calls might not be sufficient under heavy load. Administrators can use this registry setting to increase the maximum number of simultaneous Endpoint Lookup calls, which increases the amount of memory that Endpoint Mapper can use.
Microsoft Support: An additional mitigation is available for affected devices through Microsoft Support. To apply this mitigation in your organization, please contact Microsoft Support for business .
Next steps: Microsoft investigating this issue and will provide more information when it is available.
2. Windows Server Update Services (WSUS) does not display error details
After installing KB5070884 or later updates, Windows Server Update Services (WSUS) does not display synchronization error details within its error reporting. This functionality is temporarily removed to address the Remote Code Execution Vulnerability, CVE-2025-59287 .
Windows release health for this update
From Microsoft's Windows release health for Windows Server 2022, checked 2026-11-03. Microsoft's issue text is linked from each title.
Issues Microsoft traces to this update
- Users might be unable to sign in to OWA or send emails on Exchange Server
Mitigated. Microsoft last updated it 2026-10-16 21:48 PT.
From the Senserva blog
- An Ivanti VPN deadline lands today, Exchange's OWA problem hits day 20, and PHP2Shell reaches the firewalls
November 2, 2026 · Clay, Clay in the Morning - Will This Patch Break Something? Read Past Known Issues
October 17, 2026 · Mark Shavlik - Patch and Vulnerability Intelligence
July 10, 2026 · Kyle, Senserva Dev Team
CVEs fixed by this update
Sorted exploited-first, then by CVSS. Every CVE links to its Senserva page with the full risk facts, references, and a ready-to-paste AI prompt.
Scope and sources
- Affected products
- Also released in October 2026
KB5126646
1041 CVEs · Critical · exploitedKB5126651
918 CVEs · Critical · exploitedKB5126653
965 CVEs · Critical · exploitedKB5126657
899 CVEs · Critical · exploitedKB5126836
649 CVEs · Critical · exploitedKB5126868
769 CVEs · Critical · exploitedKB5127646
997 CVEs · Critical · exploitedKB5127648
1009 CVEs · Critical · exploitedThe full month, summarized: Microsoft Patch Tuesday.
- Authoritative references
See this and every Microsoft update ranked by real-world risk on the Microsoft Patch Tracker.