The tests used to test maven-checkstyle are using old DTDs and unsecure "http" locations. This is related to CVE-2019-9658. The security issue is in the parser that is used to parse this file. It leaves any code that parses this file vulnerable to XXE via a MITM.
All doctypes used in maven-checkstyle repo (configurations, suppressions, import controls, etc...) should be updated to use the latest URL and public DOCTYPEs.
<!DOCTYPE module PUBLIC
"-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"