When using Remmina to connect to some of our older Windows systems we’re seeing a certificate problem that prevents it from connecting. Remmina pretty much says you can’t connect, but you can see the error message if you run remmina from a terminal and try to connect.

connected to server:3389
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@           WARNING: CERTIFICATE NAME MISMATCH!           @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
The hostname used for this connection (m3app) does not match any of the names given in the certificate:
Common Name (CN): no CN found in certificate
A valid certificate for the wrong name should NOT be trusted!
tls_connect: certificate not trusted, aborting.
Error: protocol security negotiation or connection failure

This is often the case with older certificate templates not having a CN. But you’ll also see a similar error if the server you are trying to connect to has a CN that is a different case than you are trying to connect to. So watch out for SERVER.domain.local instead of server.domain.local

Now the tricky bit is getting to a remote server to change the certificate that is stopping you from connecting remotely. I got around this by install freerdp-X11 and using a terminal to connect whilst ignoring the certificate errors.

$ xfreerdp /v:[SERVER] /u:[USER] /cert-ignore

Then just get into mmc and change the certificate as necessary.