Prevent timing attack on CSRF, completing wonderful pr by @eutopian

lib/omniauth/strategies/oauth2.rb

@@ -83,8 +83,7 @@ def token_params
 
       def callback_phase # rubocop:disable Metrics/AbcSize, Metrics/CyclomaticComplexity, Metrics/MethodLength, Metrics/PerceivedComplexity
         error = request.params["error_reason"] || request.params["error"]
-        if !options.provider_ignores_state && (request.params["state"].to_s.empty? || request.params["state"] != session.delete("omniauth.state"))
-          fail!(:csrf_detected, CallbackError.new(:csrf_detected, "CSRF detected"))
+      elsif !options.provider_ignores_state && (request.params["state"].to_s.empty? || !secure_compare(request.params["state"], session.delete("omniauth.state")))
         elsif error
           fail!(error, CallbackError.new(request.params["error"], request.params["error_description"] || request.params["error_reason"], request.params["error_uri"]))
         else
@@ -144,6 +143,17 @@ def options_for(option)
         hash
       end
 
+       # constant-time comparison algorithm to prevent timing attacks
+       def secure_compare(string_a, string_b)
+        return false unless string_a.bytesize == string_b.bytesize
+
+        l = string_a.unpack "C#{string_a.bytesize}"
+
+        res = 0
+        string_b.each_byte { |byte| res |= byte ^ l.shift }
+        res.zero?
+      end
+
       # An error that is indicated in the OAuth 2.0 callback.
       # This could be a `redirect_uri_mismatch` or other
       class CallbackError < StandardError

spec/omniauth/strategies/oauth2_spec.rb

@@ -168,6 +168,16 @@ def app
       end
     end
   end
+
+  describe "#secure_compare" do
+    subject { fresh_strategy }
+
+    it "returns true when the two inputs are the same and false otherwise" do
+      instance = subject.new("abc", "def")
+      expect(instance.send(:secure_compare, "a", "a")).to be true
+      expect(instance.send(:secure_compare, "b", "a")).to be false
+    end
+  end
 end
 
 describe OmniAuth::Strategies::OAuth2::CallbackError do