XML External Entities (XXE)

Attackers can make malicious use of external entity references in XML documents, due to vulnerabilities in old XML parsers. These can be used to gain access to internal files, scan ports, and execute code remotely.

An XML External Entity attack is a type of attack against an application that parses XML input. This attack occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This attack may lead to the disclosure of confidential data, denial of service, server side request forgery, port scanning from the perspective of the machine where the parser is located, and other system impacts.

Source : XML_External_Entity_{(XXE)}_Processing

Readable Material : XML_External_Entity_Prevention_Cheat_Sheet