The primary mechanism by which the SP makes attribute and other session information available to applications is by "exporting" the data to a set of server variables or HTTP request headers that are generally exposed to web applications using the CGI (Common Gateway Interface) defined in the early days of the web.
Always use Server Variables
The safest mechanism, and the default for servers that allow for it, is the use of server variables. The term refers to a set of controlled data elements that the web server supplies to applications and that cannot be manipulated in any way from outside the web server. Specifically, the client has no say in them.
Ruby on Rails Environment Access
Ruby on Rails Header Access