Hello everyone,

January Patch Tuesday updates are out and Windows administrators have been seeing issues with the Windows 10 KB5009543 and Windows 11 KB5009566 cumulative updates. These updates seemingly break L2TP VPN connections.

The issue:

Users who have installed the above mentioned updates, find their L2TP VPN connections, broken when attempting to connect using the Windows VPN client. When attempting to connect to a VPN device, they are shown an error stating, "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.

The Event Log will also log entries with error code 789, stating that the connection to the VPN failed.

Affected versions:

The bug is not affecting all VPN devices and seems only to be affecting users using the built-in Windows VPN client to make the connection.

Many Reddit reports also mention connection failures to SonicWall, Cisco Meraki, and WatchGuard Firewalls.

With many users working remotely and using VPN, this breakage has cost a lot and Windows admins have had to remove it.

Affected patches:

As Microsoft bundles all security updates in a single Windows cumulative update, removing the update will remove all fixes for vulnerabilities patched during the January Patch Tuesday.