After installing recent security updates on Windows Server 2025, RDP sessions randomly freeze. Mouse and keyboard stop responding, but the session doesn't disconnect. You have to close the RDP window and reconnect, only for it to happen again minutes later.

This affects both direct RDP connections and Remote Desktop Services sessions. Microsoft confirmed the issue is tied to an interaction between UDP transport and bitmap caching in the updated RDP stack.

Fix 1: Disable UDP Transport via GPO

This is the most reliable fix. Open Group Policy Editor on the server:

Computer Configuration > Administrative Templates > Windows Components >
    Remote Desktop Services > Remote Desktop Session Host > Connections

Set Select RDP transport protocols to Enabled, then choose Use only TCP.

Or apply it via registry:

New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services" `
    -Name "SelectTransport" -Value 1 -PropertyType DWORD -Force

Value 1 = TCP only. Value 0 = UDP and TCP. Value 2 = default behavior.

Fix 2: Disable Bitmap Caching

Bitmap caching is the other half of this bug. Disable it on the server side:

New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services" `
    -Name "MaxDisconnectionTime" -Value 1 -PropertyType DWORD -Force

Or disable it on the client side in the .rdp file:

bitmapcachepersistenable:i:0
bitmapcachesize:i:0

You can also uncheck "Persistent bitmap caching" in the Remote Desktop Connection client under the Experience tab.

Fix 3: Disable Continuous Network Detection

Microsoft identified that continuous network detection in the RDP session can trigger the freeze. Disable it via GPO:

Computer Configuration > Administrative Templates > Windows Components >
    Remote Desktop Services > Remote Desktop Session Host > Connections

Set Turn off Continuous Network Detect to Enabled.

Registry equivalent:

New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services" `
    -Name "fDisableNetworkDetect" -Value 1 -PropertyType DWORD -Force

Permanent Fix: Install KB5055523

Microsoft released KB5055523 to address this. Check your current state:

Get-HotFix | Where-Object HotFixID -eq "KB5055523"

If it's not installed, check Windows Update or the Microsoft Update Catalog.

Verify

After applying fixes, restart the Remote Desktop Services service:

Restart-Service TermService -Force

Reconnect via RDP and monitor for 15-20 minutes. The freezes typically happened within the first 10 minutes under the old configuration.

Check which transport protocol is in use from the client:

Get-NetTCPConnection | Where-Object { $_.RemotePort -eq 3389 -and $_.State -eq "Established" }

If you see an active TCP connection on port 3389 and no UDP freezes, the fix is working.

Remote Desktop issues across your server fleet? Contact Rain City Techworks.