May 2021 cumulative update: KB5003173/build 19042(3).985 #KBAlert


Hi all,

Although not released yet, starting today, cumulative update announcements will include Version 21H1 (build 19043/May 2021 Update) as Win10 forum cluster is in 21H1 era.

Mostly for new family members: those reading the subject line may have noticed an odd text: build 19042(3).revision. This is a standard practice when announcing cumulative updates for two Windows 10 releases linked by a common core i.e. enablement packages. As the subject line points out, build 19043 (Version 21H1/May 2021 Update) is an enablement package on top of build 19042 (Version 20H2/October 2020 Update). Therefore for the next few weeks, cumulative updates for 20H2 and 21H1 will be denoted as 19042(3).revision. You may occasionally see links that point out that these updates are also applicable to build 19041 (Version 2004/May 2020 Update) – in fact, 2004, 20H2, and 21H1 share same system files and are updated at the same time, hence the same KB number and revision number at the end. Eventually cumulative update announcements will focus on Version 21H1 but that won’t happen until northern summer (late July at the earliest).

The above practice was established when cumulative updates for Version 1909 were released alongside 1903, as these two feature updates (1903/build 18362 and 1909/build 18363( are really a single release with an enablement package not applied (1903) and applied (1909), respectively. Speaking of Version 1909, see an accompanying KB alert, as today marks end of life for this feature update for consumers.


May 2021 cumulative update is now available:

May 11, 2021—KB5003173 (OS Builds 19041.985 and 19042.985) (



Updates to improve security when Windows performs basic operations.

Updates to improve Windows OLE (compound documents) security.

Updates security for Bluetooth drivers.



Version 20H2:

Security updates to Windows App Platform and Frameworks, the Windows Kernel, Windows Media, the Microsoft Scripting Engine, and the Windows Silicon Platform.


Version 21H1: same as 20H2.