diff --git a/lib/omniauth/strategies/oauth2.rb b/lib/omniauth/strategies/oauth2.rb
index 28b3ae1..ff6b4d5 100644
--- a/lib/omniauth/strategies/oauth2.rb
+++ b/lib/omniauth/strategies/oauth2.rb
@@ -67,7 +67,7 @@ def callback_phase # rubocop:disable AbcSize, CyclomaticComplexity, MethodLength
         error = request.params["error_reason"] || request.params["error"]
         if error
           fail!(error, CallbackError.new(request.params["error"], request.params["error_description"] || request.params["error_reason"], request.params["error_uri"]))
-        elsif !options.provider_ignores_state && (request.params["state"].to_s.empty? || request.params["state"] != session.delete("omniauth.state"))
+        elsif !options.provider_ignores_state && (request.params["state"].to_s.empty? || !secure_compare(request.params["state"], session.delete("omniauth.state")))
           fail!(:csrf_detected, CallbackError.new(:csrf_detected, "CSRF detected"))
         else
           self.access_token = build_access_token
@@ -105,6 +105,17 @@ 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
diff --git a/spec/omniauth/strategies/oauth2_spec.rb b/spec/omniauth/strategies/oauth2_spec.rb
index 2008cc9..33d69b6 100644
--- a/spec/omniauth/strategies/oauth2_spec.rb
+++ b/spec/omniauth/strategies/oauth2_spec.rb
@@ -87,6 +87,16 @@ def app
       instance.callback_phase
     end
   end
+
+  describe "#secure_params" 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