L2TP VPN connections killed

January 2022’s Patch Tuesday was a double whammy. While KB5009557 was boot-looping domain controllers (see that entry), KB5009543 was simultaneously killing L2TP VPN connections across Windows 10 and 11. Every L2TP/IPsec connection attempt failed with “the security layer encountered a processing error.” Cisco Meraki, SonicWall, Ubiquiti, and WatchGuard VPN appliances were all affected. Remote workers got to explain to their bosses why they couldn’t connect to the corporate network. Microsoft needed a week to ship emergency fixes.