XXE - XML External Entities vulnerability, it is a type of injection attack that occurs when an XML parser has the external entities DTD allowed for usage and does not properly parse the input given to it. As a result, an attacker can potentially inject malicious code to read files from the local system, perform other remote execution, and potentially expand to a DOS attack by using XML external entities expansion styled attack. To prevent XXE, if possible, try to disable external entities all together.