An XML External Entity (XXE) vulnerability exists in the Ambari/Oozie project, allowing an attacker to inject malicious XML entities. This vulnerability occurs due to insecure parsing of XML input using the "DocumentBuilderFactory" class without disabling external entity resolution. An attacker can exploit this vulnerability to read arbitrary files on the server or perform server-side request forgery (SSRF) attacks. The issue has been fixed in both Ambari 2.7.9 and the trunk branch.