REXML is an XML toolkit for Ruby. The REXML gem before 3.2.6 has a denial of service vulnerability when it parses an XML that has many `<`s in an attribute value. Those who need to parse untrusted XMLs may be impacted to this vulnerability. The REXML gem 3.2.7 or later include the patch to fix this vulnerability. As a workaround, don't parse untrusted XMLs.