XML external entity injection abuses the parts of the XML specification that most applications never need: the Document Type Definition (DTD) and its entities. When a parser accepts attacker-controlled XML with entity resolution left enabled, a declaration such as <!ENTITY xxe SYSTEM "file:///etc/passwd"> makes the parser fetch an external resource and splice its contents into the document wherever &xxe; appears. The input stays valid XML, so it passes through SOAP endpoints, REST bodies, SVG and DOCX uploads, SAML, and any other XML-backed interface.

Three outcomes follow from that single primitive. Disclosure reads local files and returns them in the response or through a side channel. SSRF and out-of-band retrieval point the entity at internal hosts, cloud metadata, or an attacker server, reaching resources the parser's network can see. Amplification nests entities so a tiny document expands into gigabytes, exhausting memory for denial of service.

This subtree covers in-band file disclosure, out-of-band exfiltration, SSRF, entity-expansion attacks, and the blind variants that leak data through parser errors when nothing is reflected.