Ну не даром проверка сертификата клиента у них опциональная в их же стандарте. 🙃
Подсуетились, не подкопаешься 😆
[MS-SSTP] - version major 18
3.2.5.2 Crypto Binding
During the SSL/TLS handshake, as part of establishing an HTTPS connection, the SSTP client authenticates the SSTP server. However, it is optional for the SSTP server to authenticate the client. The client is authenticated by the server during the higher-layer authentication (that is, PPP authentication). Therefore, it is possible for a man-in-the-middle to establish the HTTPS connection to the SSTP server and forward the PPP packets that it received from a client for a communication other than SSTP communications (for example, wireless communications). To prevent such attacks, it is important to cryptographically bind the two authentications.