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.