A DTD, or Document Type Definition, is an XML feature that describes the structure of a document. It can define elements, attributes, and entities, which lets parsers validate or expand XML content in a predictable way.
In security work, DTDs matter because entity expansion can become dangerous when a parser allows external entity resolution. An attacker can place a malicious entity in XML and cause the server to fetch local files or remote resources, leading to XML External Entity (XXE) attacks. In practice, this can expose configuration data, credentials, or system details from privileged services such as provisioning interfaces. Defenders reduce the risk by disabling DTD processing where it is not needed, blocking external entities, and using hardened XML parsers. If XML must be accepted from untrusted sources, safe parser settings are essential; otherwise, a harmless-looking document type declaration can become a data disclosure path.