edge badge

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. All requests are checked except GET requests as these should be idempotent. Keep in mind that all session-oriented requests should be CSRF protected, including JavaScript and HTML requests.

Since HTML and JavaScript requests are typically made from the browser, we need to ensure to verify request authenticity for the web browser. We can use session-oriented authentication for these types of requests, by using the protect_from_forgery method in our controllers.

GET requests are not protected since they don't have side effects like writing to the database and don't leak sensitive information. JavaScript requests are an exception: a third-party site can use a <script> tag to reference a JavaScript URL on your site. When your JavaScript response loads on their site, it executes. With carefully crafted JavaScript on their end, sensitive data in your JavaScript response may be extracted. To prevent this, only XmlHttpRequest (known as XHR or Ajax) requests are allowed to make GET requests for JavaScript responses.

It's important to remember that XML or JSON requests are also affected and if you're building an API you should change forgery protection method in ApplicationController (by default: :exception):

class ApplicationController < ActionController::Base
  protect_from_forgery unless: -> { request.format.json? }
end

CSRF protection is turned on with the protect_from_forgery method. By default protect_from_forgery protects your session with :null_session method, which provides an empty session during request.

We may want to disable CSRF protection for APIs since they are typically designed to be state-less. That is, the request API client will handle the session for you instead of Rails.

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.

Namespace
Methods
A
C
F
H
M
N
P
R
U
V
X
Included Modules
Constants
AUTHENTICITY_TOKEN_LENGTH = 32
 
NULL_ORIGIN_MESSAGE = <<-MSG.strip_heredoc The browser returned a 'null' origin for a request with origin-based forgery protection turned on. This usually means you have the 'no-referrer' Referrer-Policy header enabled, or that you the request came from a site that refused to give its origin. This makes it impossible for Rails to verify the source of the requests. Likely the best solution is to change your referrer policy to something less strict like same-origin or strict-same-origin. If you cannot change the referrer policy, you can disable origin checking with the Rails.application.config.action_controller.forgery_protection_origin_check setting. MSG
 
Instance Private methods
any_authenticity_token_valid?()

Checks if any of the authenticity tokens from the request are valid.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 294
def any_authenticity_token_valid? # :doc:
  request_authenticity_tokens.any? do |token|
    valid_authenticity_token?(session, token)
  end
end
compare_with_real_token(token, session)
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 371
def compare_with_real_token(token, session) # :doc:
  ActiveSupport::SecurityUtils.fixed_length_secure_compare(token, real_csrf_token(session))
end
form_authenticity_param()

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

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 409
def form_authenticity_param # :doc:
  params[request_forgery_protection_token]
end
handle_unverified_request()
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 241
def handle_unverified_request # :doc:
  forgery_protection_strategy.new(self).handle_unverified_request
end
mark_for_same_origin_verification!()

GET requests are checked for cross-origin JavaScript after rendering.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 266
def mark_for_same_origin_verification! # :doc:
  @marked_for_same_origin_verification = request.get?
end
marked_for_same_origin_verification?()

If the verify_authenticity_token before_action ran, verify that JavaScript responses are only served to same-origin GET requests.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 272
def marked_for_same_origin_verification? # :doc:
  @marked_for_same_origin_verification ||= false
end
masked_authenticity_token(session, form_options: {})

Creates a masked version of the authenticity token that varies on each request. The masking is used to mitigate SSL attacks like BREACH.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 313
def masked_authenticity_token(session, form_options: {}) # :doc:
  action, method = form_options.values_at(:action, :method)

  raw_token = if per_form_csrf_tokens && action && method
    action_path = normalize_action_path(action)
    per_form_csrf_token(session, action_path, method)
  else
    real_csrf_token(session)
  end

  one_time_pad = SecureRandom.random_bytes(AUTHENTICITY_TOKEN_LENGTH)
  encrypted_csrf_token = xor_byte_strings(one_time_pad, raw_token)
  masked_token = one_time_pad + encrypted_csrf_token
  Base64.strict_encode64(masked_token)
end
non_xhr_javascript_response?()

Check for cross-origin JavaScript responses.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 277
def non_xhr_javascript_response? # :doc:
  content_type =~ %r(\Atext/javascript) && !request.xhr?
end
normalize_action_path(action_path)
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 439
def normalize_action_path(action_path) # :doc:
  uri = URI.parse(action_path)
  uri.path.chomp("/")
end
per_form_csrf_token(session, action_path, method)
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 394
def per_form_csrf_token(session, action_path, method) # :doc:
  OpenSSL::HMAC.digest(
    OpenSSL::Digest::SHA256.new,
    real_csrf_token(session),
    [action_path, method.downcase].join("#")
  )
end
protect_against_forgery?()

Checks if the controller allows forgery protection.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 414
def protect_against_forgery? # :doc:
  allow_forgery_protection
end
real_csrf_token(session)
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 389
def real_csrf_token(session) # :doc:
  session[:_csrf_token] ||= SecureRandom.base64(AUTHENTICITY_TOKEN_LENGTH)
  Base64.strict_decode64(session[:_csrf_token])
end
request_authenticity_tokens()

Possible authenticity tokens sent in the request.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 301
def request_authenticity_tokens # :doc:
  [form_authenticity_param, request.x_csrf_token]
end
unmask_token(masked_token)
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 363
def unmask_token(masked_token) # :doc:
  # Split the token into the one-time pad and the encrypted
  # value and decrypt it.
  one_time_pad = masked_token[0...AUTHENTICITY_TOKEN_LENGTH]
  encrypted_csrf_token = masked_token[AUTHENTICITY_TOKEN_LENGTH..-1]
  xor_byte_strings(one_time_pad, encrypted_csrf_token)
end
valid_authenticity_token?(session, encoded_masked_token)

Checks the client's masked token to see if it matches the session token. Essentially the inverse of masked_authenticity_token.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 332
def valid_authenticity_token?(session, encoded_masked_token) # :doc:
  if encoded_masked_token.nil? || encoded_masked_token.empty? || !encoded_masked_token.is_a?(String)
    return false
  end

  begin
    masked_token = Base64.strict_decode64(encoded_masked_token)
  rescue ArgumentError # encoded_masked_token is invalid Base64
    return false
  end

  # See if it's actually a masked token or not. In order to
  # deploy this code, we should be able to handle any unmasked
  # tokens that we've issued without error.

  if masked_token.length == AUTHENTICITY_TOKEN_LENGTH
    # This is actually an unmasked token. This is expected if
    # you have just upgraded to masked tokens, but should stop
    # happening shortly after installing this gem.
    compare_with_real_token masked_token, session

  elsif masked_token.length == AUTHENTICITY_TOKEN_LENGTH * 2
    csrf_token = unmask_token(masked_token)

    compare_with_real_token(csrf_token, session) ||
      valid_per_form_csrf_token?(csrf_token, session)
  else
    false # Token is malformed.
  end
end
valid_per_form_csrf_token?(token, session)
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 375
def valid_per_form_csrf_token?(token, session) # :doc:
  if per_form_csrf_tokens
    correct_token = per_form_csrf_token(
      session,
      normalize_action_path(request.fullpath),
      request.request_method
    )

    ActiveSupport::SecurityUtils.fixed_length_secure_compare(token, correct_token)
  else
    false
  end
end
valid_request_origin?()

Checks if the request originated from the same origin by looking at the Origin header.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 429
def valid_request_origin? # :doc:
  if forgery_protection_origin_check
    # We accept blank origin headers because some user agents don't send it.
    raise InvalidAuthenticityToken, NULL_ORIGIN_MESSAGE if request.origin == "null"
    request.origin.nil? || request.origin == request.base_url
  else
    true
  end
end
verified_request?()

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

  • Is it a GET or HEAD request? GETs should be safe and idempotent

  • Does the form_authenticity_token match the given token value from the params?

  • Does the X-CSRF-Token header match the form_authenticity_token?

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 288
def verified_request? # :doc:
  !protect_against_forgery? || request.get? || request.head? ||
    (valid_request_origin? && any_authenticity_token_valid?)
end
verify_authenticity_token()

The actual before_action that is used to verify the CSRF token. Don't override this directly. Provide your own forgery protection strategy instead. If you override, you'll disable same-origin <script> verification.

Lean on the protect_from_forgery declaration to mark which actions are due for same-origin request verification. If protect_from_forgery is enabled on an action, this before_action flags its after_action to verify that JavaScript responses are for XHR requests, ensuring they follow the browser's same-origin policy.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 226
def verify_authenticity_token # :doc:
  mark_for_same_origin_verification!

  if !verified_request?
    if logger && log_warning_on_csrf_failure
      if valid_request_origin?
        logger.warn "Can't verify CSRF token authenticity."
      else
        logger.warn "HTTP Origin header (#{request.origin}) didn't match request.base_url (#{request.base_url})"
      end
    end
    handle_unverified_request
  end
end
verify_same_origin_request()

If verify_authenticity_token was run (indicating that we have forgery protection enabled for this request) then also verify that we aren't serving an unauthorized cross-origin response.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 256
def verify_same_origin_request # :doc:
  if marked_for_same_origin_verification? && non_xhr_javascript_response?
    if logger && log_warning_on_csrf_failure
      logger.warn CROSS_ORIGIN_JAVASCRIPT_WARNING
    end
    raise ActionController::InvalidCrossOriginRequest, CROSS_ORIGIN_JAVASCRIPT_WARNING
  end
end
xor_byte_strings(s1, s2)
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 402
def xor_byte_strings(s1, s2) # :doc:
  s2_bytes = s2.bytes
  s1.each_byte.with_index { |c1, i| s2_bytes[i] ^= c1 }
  s2_bytes.pack("C*")
end