SQL Server linked server bug: Microsoft confirms October Windows update leaves connections open

Microsoft confirmed on 11 November 2026 that Windows updates released on 13 October 2026 and later can leave database connections open on machines running SQL Server, slowing servers down the longer they run. The company says a fix will arrive as an out-of-band update "in the next few days", and that a mitigation is available now through its business support channel.

The problem was added to the Windows Server 2025 release health page with the status "Confirmed". It was reported the next day by The Register and BleepingComputer.

What has Microsoft confirmed?

According to Microsoft's known issue entry, connections created by SQL Server linked server queries "might remain open after the queries complete" once the October update or a later one is installed. As more queries run, connections to external databases pile up instead of closing. Microsoft says this "can slow or degrade server performance after prolonged run time".

The fault is narrow. Microsoft says it affects linked server queries that use OLE DB providers to connect to external databases, and that Oracle and PostgreSQL connections are "among those known to be affected". The wording leaves open the possibility that other data sources are affected too.

The sign to look for, in Microsoft's words, is "an increasing number of open connections to external databases, even when the associated SQL Server linked server queries are no longer running".

What is a linked server?

A linked server lets the SQL Server Database Engine read data from, and run commands against, a data source outside its own instance. Microsoft's linked servers documentation says they are typically set up so that a query can include tables held in another SQL Server instance or in "another database product such as Oracle".

Each linked server definition names an OLE DB provider, which Microsoft describes as a DLL that manages and interacts with a specific data source. That provider layer is where the leftover connections are being created.

Which Windows versions are affected?

The issue is tied to the Windows update, not to a SQL Server patch. Microsoft's entry lists both client and server editions of Windows as affected platforms:

• Client: Windows 11 versions 26H1, 25H2, 24H2 and 23H2; Windows 10 version 1809; Windows 10 Enterprise LTSC 2019
• Server: Windows Server 2025; Windows Server 2022; Windows Server, version 1809; Windows Server 2016; Windows Server 2012 R2; Windows Server 2012

Windows 11 version 26H2 does not appear in that list. The Register noted the same omission.

The originating update differs by Windows version. These are the ones we confirmed on Microsoft's own pages:

Windows version	Originating update (13 October 2026)	OS build	Source
Windows Server 2025	KB5126646	26100.33650	Microsoft
Windows Server 2022	KB5126655	20348.5761	Microsoft
Windows 10 version 1809 and Windows Server 2019	KB5126651	17763.9376	Microsoft

Microsoft says the issue applies to the 13 October updates "and later". BleepingComputer reports it as affecting Windows updates released since the October 2026 Patch Tuesday. Installing a newer monthly update is therefore not described as a fix.

Microsoft has not published the underlying cause, and it has not said how many customers are affected.

What should administrators do now?

Microsoft's entry sets out three options, and each one has limits.

1. Close idle connections by hand. Microsoft says that if idle connections can be identified, closing them can "temporarily relieve server load".
2. Restart the SQL service. Microsoft says this can temporarily restore system stability.
3. Ask Microsoft for the mitigation. Microsoft says a mitigation "is available for affected enterprise devices" and directs organisations to contact Microsoft Support for business to apply it.

Microsoft attaches a warning to the first two. Those steps "can also interrupt active queries or transactions", and the problem "can resurface again after sufficient operation time". They buy time; they do not remove the fault.

The entry does not tell administrators to uninstall the October update, and it does not describe what the support mitigation consists of.

When will the fix arrive?

Microsoft's stated next step is an out-of-band update "that will be released in the next few days", with the documentation to be updated once it ships. No date or KB number had been published on the pages we checked on 13 November 2026.

How does this fit with the other October update problems?

This is the second server workload Microsoft has linked to the same October release. The same release health page records that some on-premises Exchange servers had trouble with Outlook on the web sign-ins and mail delivery after the October 2026 Windows security update. Microsoft marks that issue as resolved by the Windows updates released on 10 November 2026.

The two faults are documented separately, and Microsoft has not said they share a cause. For Exchange, Microsoft pointed to a new default limit on Endpoint Mapper memory. For SQL Server, it has given no cause at all.

BleepingComputer also notes that in September 2026 Microsoft shared a temporary workaround for SQL Server crashes on some Windows Server 2025 systems.

Analysis: a slow fault is harder to spot than a crash

This section is our own analysis, not Microsoft's guidance.

The awkward feature of this bug is its timing. The October update shipped on 13 October 2026 and the known issue was published on 11 November 2026, roughly four weeks later. A server that degrades gradually does not produce an obvious failure on patch night, so it is easy to blame the slowdown on something else.

It also sits in a part of the estate that tends to be old and little watched. Linked servers to Oracle or PostgreSQL are often the glue between a reporting system and a line-of-business database, set up years ago. The connection build-up shows on the remote database as well as on the SQL Server host, so the first complaint may come from the team running the other system.

Three practical points follow from Microsoft's description:

• Check whether you use linked servers with OLE DB providers at all. If you do not, Microsoft's description does not cover you.
• If you do, watching the count of open connections on the external database gives a direct view of the symptom Microsoft describes.
• A restart interrupts running work, by Microsoft's own account, so it is better planned than done in a hurry.

The wider tension is a familiar one. Security updates need to go on quickly, yet each one can break something that only shows under real workloads. We covered that balance in our piece on cybersecurity challenges and solutions.

Sources

• Microsoft: Windows Server 2025 known issues and notifications
• Microsoft: Windows Server 2022 known issues and notifications
• Microsoft: Windows 10, version 1809 and Windows Server 2019 known issues and notifications
• Microsoft: Linked Servers (Database Engine)
• The Register: Microsoft admits SQL Servers might wobble after October update
• BleepingComputer: Microsoft: Recent Windows updates cause SQL Server performance issues