The HTTP XML API requires that you pass across the admin username and password in order for the API to validate and run the API call.
If you are worried about security you could install an SSL certificate on your website and then use HTTPS to access the API.
If you are also concerned about Brute Force, Dictionary or DDOS attack then you could look at installing Microsoft's IIS Dynamic IP Restrictions that can block these types of attacks.
Attempting to prevent DDoS attacks at application level within Web Wiz Forums would be pointless as any calls to the application require database hits and so you would not be able to sufficiently stop DDoS at this level. It would be much better using Microsoft's IIS Dynamic IP Restrictions or better still using a hardware firewall, or some Switches from companies like Cisco also have this ability built in.