diff --git a/lib/jekyll.rb b/lib/jekyll.rb
index b978ec7a..8e007ac3 100644
--- a/lib/jekyll.rb
+++ b/lib/jekyll.rb
@@ -175,22 +175,7 @@ module Jekyll
       return base_directory if base_directory.eql?(questionable_path)
       return base_directory if questionable_path.nil?
 
-      clean_path = questionable_path.dup
-      clean_path.insert(0, "/") if clean_path.start_with?("~")
-      clean_path = File.expand_path(clean_path, "/")
-
-      return clean_path if clean_path.eql?(base_directory)
-
-      # remove any remaining extra leading slashes not stripped away by calling
-      # `File.expand_path` above.
-      clean_path.squeeze!("/")
-
-      if clean_path.start_with?(base_directory.sub(%r!\z!, "/"))
-        clean_path
-      else
-        clean_path.sub!(%r!\A\w:/!, "/")
-        File.join(base_directory, clean_path)
-      end
+      +Jekyll::PathManager.sanitized_path(base_directory, questionable_path)
     end
 
     # Conditional optimizations
diff --git a/lib/jekyll/path_manager.rb b/lib/jekyll/path_manager.rb
index eeed1f95..f00e11a8 100644
--- a/lib/jekyll/path_manager.rb
+++ b/lib/jekyll/path_manager.rb
@@ -27,5 +27,36 @@ module Jekyll
       @join[base] ||= {}
       @join[base][item] ||= File.join(base, item).freeze
     end
+
+    # Ensures the questionable path is prefixed with the base directory
+    # and prepends the questionable path with the base directory if false.
+    #
+    # Returns a frozen string.
+    def self.sanitized_path(base_directory, questionable_path)
+      @sanitized_path ||= {}
+      @sanitized_path[base_directory] ||= {}
+      @sanitized_path[base_directory][questionable_path] ||= begin
+        return base_directory.freeze if questionable_path.nil?
+
+        clean_path = if questionable_path.start_with?("~")
+                       questionable_path.dup.insert(0, "/")
+                     else
+                       questionable_path
+                     end
+        clean_path = File.expand_path(clean_path, "/")
+        return clean_path.freeze if clean_path.eql?(base_directory)
+
+        # remove any remaining extra leading slashes not stripped away by calling
+        # `File.expand_path` above.
+        clean_path.squeeze!("/")
+
+        if clean_path.start_with?(base_directory.sub(%r!\z!, "/"))
+          clean_path.freeze
+        else
+          clean_path.sub!(%r!\A\w:/!, "/")
+          join(base_directory, clean_path)
+        end
+      end
+    end
   end
 end
diff --git a/test/test_path_manager.rb b/test/test_path_manager.rb
new file mode 100644
index 00000000..e17175c3
--- /dev/null
+++ b/test/test_path_manager.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require "helper"
+
+class TestPathManager < JekyllUnitTest
+  context "PathManager" do
+    setup do
+      @source = Dir.pwd
+    end
+
+    should "return frozen copy of base if questionable path is nil" do
+      assert_equal @source, Jekyll::PathManager.sanitized_path(@source, nil)
+      assert Jekyll::PathManager.sanitized_path(@source, nil).frozen?
+    end
+
+    should "return a frozen copy of base if questionable path expands into the base" do
+      assert_equal @source, Jekyll::PathManager.sanitized_path(@source, File.join(@source, "/"))
+      assert Jekyll::PathManager.sanitized_path(@source, File.join(@source, "/")).frozen?
+    end
+
+    should "return a frozen string result" do
+      if Jekyll::Utils::Platforms.really_windows?
+        assert_equal(
+          "#{@source}/_config.yml",
+          Jekyll::PathManager.sanitized_path(@source, "E:\\_config.yml")
+        )
+      end
+      assert_equal(
+        "#{@source}/_config.yml",
+        Jekyll::PathManager.sanitized_path(@source, "//_config.yml")
+      )
+      assert Jekyll::PathManager.sanitized_path(@source, "//_config.yml").frozen?
+    end
+  end
+end