The Web Application Hacker’s Handbook Discovering and Exploiting Security Flaws

503 Service Unavailable — 

This normally indicates that, although 

the web server itself is functioning and able to respond to requests, the

application accessed via the server is not responding. You should verify

whether this is the result of any action that you have performed.

HTTPS

The HTTP protocol uses plain TCP as its transport mechanism, which is unen-

crypted and so can be intercepted by an attacker who is suitably positioned on

the network. HTTPS is essentially the same application-layer protocol as