Fix possible XSS vector in JS escape helper

This commit escapes dollar signs and backticks to prevent JS XSS issues
when using the `j` or `javascript_escape` helper

CVE-2020-5267

Author: tenderlove

actionview/lib/action_view/helpers/javascript_helper.rb

@@ -12,7 +12,9 @@ module JavaScriptHelper
         "\n"    => '\n',
         "\r"    => '\n',
         '"'     => '\\"',
-        "'"     => "\\'"
+        "'"     => "\\'",
+        "`"     => "\\`",
+        "$"     => "\\$"
       }
 
       JS_ESCAPE_MAP[(+"\342\200\250").force_encoding(Encoding::UTF_8).encode!] = "
"
@@ -29,7 +31,7 @@ def escape_javascript(javascript)
         if javascript.empty?
           result = ""
         else
-          result = javascript.gsub(/(\\|<\/|\r\n|\342\200\250|\342\200\251|[\n\r"'])/u) { |match| JS_ESCAPE_MAP[match] }
+          result = javascript.gsub(/(\\|<\/|\r\n|\342\200\250|\342\200\251|[\n\r"']|[`]|[$])/u) { |match| JS_ESCAPE_MAP[match] }
         end
         javascript.html_safe? ? result.html_safe : result
       end

actionview/test/template/javascript_helper_test.rb

@@ -36,6 +36,14 @@ def test_escape_javascript
     assert_equal %(dont <\\/close> tags), j(%(dont </close> tags))
   end
 
+  def test_escape_backtick
+    assert_equal "\\`", escape_javascript("`")
+  end
+
+  def test_escape_dollar_sign
+    assert_equal "\\$", escape_javascript("$")
+  end
+
   def test_escape_javascript_with_safebuffer
     given = %('quoted' "double-quoted" new-line:\n </closed>)
     expect = %(\\'quoted\\' \\"double-quoted\\" new-line:\\n <\\/closed>)