Windows 10 KB5009543 Update causes the L2TP encrypted tunnel to fail to connect

This week, Microsoft released the latest cumulative update to all supported versions of Windows 10, and it is no surprise that these cumulative updates have brought new problems that affect users. The problem with Windows 10 20H1/20H2/21H1/21H2 this time is that the L2TP VPN cannot be connected normally. This problem mainly affects enterprise users. Including encrypted tunnels deployed by enterprises internally through Cisco, etc., cannot be connected.

Microsoft has confirmed the issue:

“After installing KB5009543, IP Security (IPSEC) connections which contain a Vendor ID might fail. VPN connections using Layer 2 Tunneling Protocol (L2TP) or IP security Internet Key Exchange (IPSEC IKE) might also be affected.”

Affected platforms:

• Client: Windows 11, version 21H2; Windows 10, version 21H2; Windows 10, version 21H1; Windows 10, version 20H2; Windows 10, version 1909; Windows 10, version 1809; Windows 10 Enterprise LTSC 2019; Windows 10 Enterprise LTSC 2016; Windows 10, version 1607; Windows 10 Enterprise 2015 LTSB
• Server: Windows Server 2022; Windows Server, version 20H2; Windows Server 2019; Windows Server 2016

The temporarily available method is to uninstall the cumulative update KB5009543.

Microsoft saying:

“To mitigate the issue for some VPNs, you can disable Vendor ID within the server-side settings. Note: Not all VPN servers have the option to disable Vendor ID from being used.”

Via: BleepingComputer