xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. Prior to @xmldom/xmldom versions 0.8.15 and 0.9.12, and in xmldom version 0.6.0 and earlier, the DOCUMENT_TYPE_NODE branch in lib/dom.js validates publicId, systemId, and internalSubset under requireWellFormed: true but emits DocumentType.name verbatim. A name containing > or whitespace can terminate the <!DOCTYPE ...> declaration and inject sibling markup; the value can be supplied through createDocumentType() on the 0.8.x and unscoped lines or through a direct DocumentType.name property write on every affected line. The default path and legacy creation-time behavior remain permissive, while the vulnerable strict path fails to enforce an XML Name. This issue is fixed in @xmldom/xmldom versions 0.8.15 and 0.9.12; no fixed version is available for xmldom.