From 69fc0e1b5e6a3227576d67587c386142ef65854e Mon Sep 17 00:00:00 2001 From: Bradley Buda Date: Tue, 19 Aug 2014 14:29:26 -0700 Subject: Auth token mask from breach-mitigation-rails gem This merges in the code from the breach-mitigation-rails gem that masks authenticity tokens on each request by XORing them with a random set of bytes. The masking is used to make it impossible for an attacker to steal a CSRF token from an SSL session by using techniques like the BREACH attack. The patch is pretty simple - I've copied over the [relevant code](https://github.com/meldium/breach-mitigation-rails/blob/master/lib/breach_mitigation/masking_secrets.rb) and updated the tests to pass, mostly by adjusting stubs and mocks. --- .../metal/request_forgery_protection.rb | 68 +++++++++++++++++++++- .../controller/request_forgery_protection_test.rb | 11 ++-- 2 files changed, 71 insertions(+), 8 deletions(-) diff --git a/actionpack/lib/action_controller/metal/request_forgery_protection.rb b/actionpack/lib/action_controller/metal/request_forgery_protection.rb index 0efa0fb259..15d030a0e5 100644 --- a/actionpack/lib/action_controller/metal/request_forgery_protection.rb +++ b/actionpack/lib/action_controller/metal/request_forgery_protection.rb @@ -240,6 +240,8 @@ module ActionController #:nodoc: content_type =~ %r(\Atext/javascript) && !request.xhr? end + AUTHENTICITY_TOKEN_LENGTH = 32 + # Returns true or false if a request is verified. Checks: # # * is it a GET or HEAD request? Gets should be safe and idempotent @@ -247,13 +249,73 @@ module ActionController #:nodoc: # * Does the X-CSRF-Token header match the form_authenticity_token def verified_request? !protect_against_forgery? || request.get? || request.head? || - form_authenticity_token == form_authenticity_param || - form_authenticity_token == request.headers['X-CSRF-Token'] + valid_authenticity_token?(session, form_authenticity_param) || + valid_authenticity_token?(session, request.headers['X-CSRF-Token']) end # Sets the token value for the current session. def form_authenticity_token - session[:_csrf_token] ||= SecureRandom.base64(32) + masked_authenticity_token(session) + end + + # Creates a masked version of the authenticity token that varies + # on each request. The masking is used to mitigate SSL attacks + # like BREACH. + def masked_authenticity_token(session) + one_time_pad = SecureRandom.random_bytes(AUTHENTICITY_TOKEN_LENGTH) + encrypted_csrf_token = xor_byte_strings(one_time_pad, real_csrf_token(session)) + masked_token = one_time_pad + encrypted_csrf_token + Base64.strict_encode64(masked_token) + end + + # Checks the client's masked token to see if it matches the + # session token. Essentially the inverse of + # +masked_authenticity_token+. + def valid_authenticity_token?(session, encoded_masked_token) + return false if encoded_masked_token.nil? || encoded_masked_token.empty? + + 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 + # 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] + csrf_token = xor_byte_strings(one_time_pad, encrypted_csrf_token) + + compare_with_real_token csrf_token, session + + else + false # Token is malformed + end + end + + def compare_with_real_token(token, session) + # Borrow a constant-time comparison from Rack + Rack::Utils.secure_compare(token, real_csrf_token(session)) + end + + def real_csrf_token(session) + session[:_csrf_token] ||= SecureRandom.base64(AUTHENTICITY_TOKEN_LENGTH) + Base64.strict_decode64(session[:_csrf_token]) + end + + def xor_byte_strings(s1, s2) + s1.bytes.zip(s2.bytes).map { |(c1,c2)| c1 ^ c2 }.pack('c*') end # The form's authenticity parameter. Override to provide your own. diff --git a/actionpack/test/controller/request_forgery_protection_test.rb b/actionpack/test/controller/request_forgery_protection_test.rb index 2a5aad9c0e..5e2510fb2e 100644 --- a/actionpack/test/controller/request_forgery_protection_test.rb +++ b/actionpack/test/controller/request_forgery_protection_test.rb @@ -125,8 +125,9 @@ end module RequestForgeryProtectionTests def setup @token = "cf50faa3fe97702ca1ae" - - SecureRandom.stubs(:base64).returns(@token) + @controller.stubs(:form_authenticity_token).returns(@token) + @controller.stubs(:valid_authenticity_token?).with{ |_, t| t == @token }.returns(true) + @controller.stubs(:valid_authenticity_token?).with{ |_, t| t != @token }.returns(false) @old_request_forgery_protection_token = ActionController::Base.request_forgery_protection_token ActionController::Base.request_forgery_protection_token = :custom_authenticity_token end @@ -386,7 +387,7 @@ class RequestForgeryProtectionControllerUsingResetSessionTest < ActionController end test 'should emit a csrf-param meta tag and a csrf-token meta tag' do - SecureRandom.stubs(:base64).returns(@token + '<=?') + @controller.stubs(:form_authenticity_token).returns(@token + '<=?') get :meta assert_select 'meta[name=?][content=?]', 'csrf-param', 'custom_authenticity_token' assert_select 'meta[name=?][content=?]', 'csrf-token', 'cf50faa3fe97702ca1ae<=?' @@ -466,7 +467,7 @@ class CustomAuthenticityParamControllerTest < ActionController::TestCase super @old_logger = ActionController::Base.logger @logger = ActiveSupport::LogSubscriber::TestHelper::MockLogger.new - @token = "foobar" + @token = Base64.strict_encode64(SecureRandom.random_bytes(32)) @old_request_forgery_protection_token = ActionController::Base.request_forgery_protection_token ActionController::Base.request_forgery_protection_token = @token end @@ -478,7 +479,7 @@ class CustomAuthenticityParamControllerTest < ActionController::TestCase def test_should_not_warn_if_form_authenticity_param_matches_form_authenticity_token ActionController::Base.logger = @logger - SecureRandom.stubs(:base64).returns(@token) + @controller.stubs(:valid_authenticity_token?).returns(:true) begin post :index, :custom_token_name => 'foobar' -- cgit v1.2.3