Merge pull request #209 from koic/avoid_redos_in_ruby_3_1_and_earlier

Fix ReDoS vulnerability in `StringUtils#underscore`

lib/mcp/string_utils.rb

@@ -16,8 +16,8 @@ module MCP
 
     def underscore(camel_cased_word)
       camel_cased_word
-        .gsub(/([A-Z]+)([A-Z][a-z])/, '\1_\2')
+        .gsub(/(?<=[A-Z])(?=[A-Z][a-z])/, "_")
-        .gsub(/([a-z\d])([A-Z])/, '\1_\2')
+        .gsub(/(?<=[a-z\d])(?=[A-Z])/, "_")
         .tr("-", "_")
         .downcase
     end

test/mcp/string_utils_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "test_helper"
+require "timeout"
 
 module MCP
   class StringUtilsTest < Minitest::Test
@@ -18,5 +19,18 @@ module MCP
       assert_equal("test", StringUtils.handle_from_class_name("Module::Submodule::Test"))
       assert_equal("test_class", StringUtils.handle_from_class_name("Module::Submodule::TestClass"))
     end
+
+    def test_handle_from_class_name_does_not_cause_redos
+      # A long string of uppercase letters followed by a non-lowercase character
+      # would trigger catastrophic backtracking with the vulnerable regex patterns.
+      malicious_input = "A" * 50_000 + "!"
+
+      result = nil
+      Timeout.timeout(1) do
+        result = StringUtils.handle_from_class_name(malicious_input)
+      end
+
+      assert_equal("a" * 50_000 + "!", result)
+    end
   end
 end