Controller actions are protected from Cross-Site Request Forgery (CSRF) attacks by including a token in the rendered html for your application. This token is stored as a random string in the session, to which an attacker does not have access. When a request reaches your application, Rails verifies the received token with the token in the session. Only HTML and JavaScript requests are checked, so this will not protect your XML API (presumably you’ll have a different authentication scheme there anyway). Also, GET requests are not protected as these should be idempotent.

CSRF protection is turned on with the protect_from_forgery method, which checks the token and resets the session if it doesn’t match what was expected. A call to this method is generated for new Rails applications by default. You can customize the error message by editing public/422.html.

The token parameter is named authenticity_token by default. The name and value of this token must be added to every layout that renders forms by including csrf_meta_tags in the html head.

Learn more about CSRF attacks and securing your application in the Ruby on Rails Security Guide.

Methods
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Included Modules
Classes and Modules
Instance Protected methods
form_authenticity_param()

The form’s authenticity parameter. Override to provide your own.

Source: show | on GitHub 

     # File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 105
105:       def form_authenticity_param
106:         params[request_forgery_protection_token]
107:       end
form_authenticity_token()

Sets the token value for the current session.

Source: show | on GitHub 

     # File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 100
100:       def form_authenticity_token
101:         session[:_csrf_token] ||= SecureRandom.base64(32)
102:       end
handle_unverified_request()

This is the method that defines the application behavior when a request is found to be unverified. By default, Rails resets the session when it finds an unverified request.

Source: show | on GitHub 

    # File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 84
84:       def handle_unverified_request
85:         reset_session
86:       end
protect_against_forgery?()

Source: show | on GitHub 

     # File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 109
109:       def protect_against_forgery?
110:         allow_forgery_protection
111:       end
verified_request?()

Returns true or false if a request is verified. Checks:

Source: show | on GitHub 

    # File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 93
93:       def verified_request?
94:         !protect_against_forgery? || request.get? ||
95:           form_authenticity_token == params[request_forgery_protection_token] ||
96:           form_authenticity_token == request.headers['X-CSRF-Token']
97:       end
verify_authenticity_token()

The actual before_filter that is used. Modify this to change how you handle unverified requests.

Source: show | on GitHub 

    # File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 75
75:       def verify_authenticity_token
76:         unless verified_request?
77:           logger.warn "WARNING: Can't verify CSRF token authenticity" if logger
78:           handle_unverified_request
79:         end
80:       end