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If you want be sure that there's no attack, you should get a certificate from a trusted Certificate Authority, configure your Windows 7 pc to present this certificate when a RDP session is initiated, and accept this certificate on the other side.
Right now, the pc accepting the RDP session is presenting an automatically generated certificate. If you have a signed and trusted certificate, you can replace it using the steps from Rivald's blog
- open mmc.exe (Microsoft Management Console)
- add the add-in - certificates (for the computer account) (and select local computer)
- navigate to the remote desktop folder -> certificates
- delete the certificate for the name of the server and close the mmc instance
- Go to: administrative tools -> remote desktop services -> remote desktop session host configuration
- Select the instance in the main window - rdp -tcp -> right click and select properties
- on the window that pops up, select default
1If you want to be sure that there's no MiM attack you could view the certificate and see if it matches the certificate you have on the PC you're trying to RDP to. – Bungicasse – 2017-01-06T12:00:05.343