This is intended to be an introduction for implementation of stronger authentication and server security focused around the CGI web interface.
There are many ways to enhance the security of your monitoring server and Icinga environment. This should not be taken as the end all approach to security. Instead, think of it as an introduction to some of the techniques you can use to tighten the security of your system. As always, you should do your research and use the best techniques available. Treat your monitoring server as it were the most important server in your network and you shall be rewarded.
Stronger Authentication using Digest Authentication . If you have followed the quickstart guides, chances are that you are using Apache's Basic Authentication. Basic Authentication will send your username and password in "clear text" with every http request. Consider using a more secure method of authentication such as Digest Authentication which creates a MD5 Hash of your username and password to send with each request.
Forcing TLS/SSL for all Web Communication . Apache provides TLS/SSL through the mod_ssl module. TLS/SSL provides a secure tunnel between the client and server that prevents eavesdropping and tampering using strong publickey/privatekey cryptography.
Locking Down Apache Using Access Controls . Consider locking down access to the Icinga box to your IP address, IP address range, or IP subnet. If you require access outside your network you could use VPN or SSH Tunnels. This is a easy and strong to limit access to HTTP/HTTPS on your system.