The January Windows 10 Patch Tuesday is causing “Can’t connect to VPN. The L2TP connection attempt failed” error

by Surur
January 13, 2022

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Microsoft’s first Patch Tuesday of 2022 appears to be cursed with issues. Users are reporting that KB5009543 for Windows 10 2004, 20H1, and 21H1 is causing issues connecting to VPN for a number of clients and servers over the L2TP VPN protocol.

Users are reporting running into the  “Can’t connect to VPN. The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer,” error, and is mainly affecting the built-in Windows VPN client when connecting to Ubiquiti Client-to-Site VPN, SonicWall, Cisco Meraki, and WatchGuard Firewalls.

Microsoft has confirmed the issue, saying:

“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.”

The main workaround is to uninstall KB5009543 by going to Control Panel> Programs -> View installed updates.

Server-side mitigation is possible, with 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.”

Are any of our readers running into this issue? Let us know below.

via BleepingComputer

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