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Layers of abstraction
HTTP protocol (stateless API between client & server)
Server: retrieves files off the hard drive, executes a program using the CGI API, or interprets a script using an internal module (e.g. PHP, ASP)
- file access limited to the web root, possibly restricted by .htaccess, etc - these restrictions are enforced by the web server!
- CGI or internal scripts are not limited to the web root, auth restrictions, etc. CGI/scripts have access to the OS, execute programs, access databases, etc.