lib/DocumentToText.php in OpenCats before 0.9.4-3 has XXE that allows remote users to read files on the underlying operating system. The attacker must upload a file in the docx or odt format.
https://github.com/opencats/OpenCATS/pull/440
https://euvd.enisa.europa.eu/vulnerability/EUVD-2019-4859
http://packetstormsecurity.com/files/164253/OpenCats-0.9.4-2-XML-Injection.html