Not sure how you tried to reproduce this error. You would have had to create a WCF web service and within the web service try and connect to an SFTP server that wants to check a fingerprint. Even though you setup the HostKeyVerifying event handler, in a web service that event handler does not get called. If you did that and the event handler gets called then I must be doing something wrong on my side. If you could add a way to specify a callback function for the HostKeyVerifying in CPRO (in addition to even handler) it may work.