KB5121003 Breaks RDP Multi-Monitor setup.

Question

KB5121003 Breaks RDP Multi-Monitor setup.

romibi 0

I'm not sure if this is the right place …
Since KB5121003 some multi monitor setups with RDP using multiple (but not all of the) monitors for RDP breaks.

Locally I have 3 Displays.

mstsc.exe /l gives me this list:

0: 2560 x 1440; (-2560, 0, -1, 1439)
1: 2560 x 1440; (0, 0, 2559, 1439)
2: 3200 x 1800; (2560, -195, 5759, 1604)

So my main display (1) in the middle and two side displays to both sides.
Display 0 and 1 are scaled 100% and display 2 is scaled 150%.

My .rdp File has the following lines (among others):
screen mode id:i:2
use multimon:i:1
selectedmonitors:s:1,2

Up until KB5121003 this setup worked to display a remote connection on my main and on my right display.
It was a bit wonky, after connecting the RDP content was shown on display 0 and 1, overlapping a little bit onto display 3.
But after leaving fullscreen, moving the small RDP window to the middle of display 1 and reentering fullscreen fixed the issue and RDP would show nicely on Display 1 and 2.

Since KB5121003 that workaround does not work anymore. Content is always on display 0 and 1 even though the contents are sized for display 1 and 2.

I validated that KB5121003 is the culprit by uninstalling/reinstalling and comparing behavior before and after.

This is the screen layout before a connection:
KB5121003 RDP multimon before connection

This is how it looks directly after the connection both with and without KB5121003:
KB5121003 RDP multimon after connection

This is how it should look which is doable without KB5121003 by leaving fullscreen and enter fullscreen on one of the correct screens but not anymore with KB5121003 :
KB5121003 RDP multimon how it should look

I would be glad if the relevant people who can fix this get the information that this broke …
Thank you very much!

0 comments

1 answer

Your answer

Answer 1

Hello @romibi

The behavior does appear consistent with a regression introduced after KB5121003, especially if multi-monitor RDP works normally immediately after uninstalling the update and fails again when it is reinstalled.

KB5121003 is the August 11, 2026 cumulative security update for Windows 11 24H2/25H2 (OS Builds 26100.9168 / 26200.9168).

However, as of Microsoft's current documentation, RDP multi-monitor failure is not listed as a confirmed known issue for KB5121003. The currently documented issue involves certain games and RGB-related drivers.

Since you've already isolated the problem to the update, I would recommend:

Confirm the affected client build with winver and test mstsc /multimon after installing and removing KB5121003.
Check Event Viewer > Applications and Services Logs > Microsoft > Windows > TerminalServices-ClientActiveXCore for errors during the failed connection.
Update the display/GPU drivers and test with different monitor arrangements/scaling values to rule out a display-driver interaction.
Submit the reproduction through Feedback Hub (Win+F) and include the KB number, monitor topology/resolutions, GPU/driver version, RDP logs, and the fact that uninstalling KB5121003 restores functionality.
In an enterprise environment, I'd also open a Microsoft support case so they can investigate the issue as a possible KB5121003 regression.

I would not recommend uninstalling KB5121003 as a permanent solution, since it's a security cumulative update. If removing it is the only working mitigation, use that only temporarily while Microsoft investigates or until a subsequent cumulative update resolves the behavior.

Sharing these references with you:

KB5121003 - August 11, 2026 Windows 11 update

Windows 11 24H2 known issues and notifications

Please "Accept the Answer" if this information helped you. This will help us and others in the community.