@xmldom/xmldom EUVD-2026-28289

Summary

The package serializes DocumentType node fields (internalSubset, publicId, systemId) verbatim without any escaping or validation. When these fields are set programmatically to attacker-controlled strings, XMLSerializer.serializeToString can produce output where the DOCTYPE declaration is terminated early and arbitrary markup appears outside it.

---

Details

DOMImplementation.createDocumentType(qualifiedName, publicId, systemId, internalSubset) validates only qualifiedName against the XML QName production. The remaining three arguments are stored as-is with no validation.

The XMLSerializer emits DocumentType nodes as:

<!DOCTYPE name[ PUBLIC pubid][ SYSTEM sysid][ [internalSubset]]>

All fields are pushed into the output buffer verbatim - no escaping, no quoting added.

internalSubset injection: The serializer wraps internalSubset with [ and ]. A value containing ]> closes the internal subset and the DOCTYPE declaration at the injection point. Any content after ]> in internalSubset appears outside the DOCTYPE in the serialized output as raw XML markup. Reported by @TharVid (GHSA-f6ww-3ggp-fr8h). Affected: @xmldom/xmldom ≥ 0.9.0 via createDocumentType API; 0.8.x only via direct property write.

publicId injection: The serializer emits publicId verbatim after PUBLIC with no quoting added. A value containing an injected system identifier (e.g., "pubid" SYSTEM "evil") breaks the intended quoting context, injecting a fake SYSTEM entry into the serialized DOCTYPE declaration. Identified during internal security research. Affected: both branches, all versions back to 0.1.0.

systemId injection: The serializer emits systemId verbatim. A value containing > terminates the DOCTYPE declaration early; content after > appears as raw XML markup outside the DOCTYPE context. Identified during internal security research. Affected: both branches, all versions back to 0.1.0.

The parse path is safe: the SAX parser enforces the PubidLiteral and SystemLiteral grammar productions, which exclude the relevant characters, and the internal subset parser only accepts a subset it can structurally validate. The vulnerability is reachable only through programmatic createDocumentType calls with attacker-controlled arguments.

---

Affected code

lib/dom.js - createDocumentType (lines 898-910):

createDocumentType: function (qualifiedName, publicId, systemId, internalSubset) {
    validateQualifiedName(qualifiedName);          // only qualifiedName is validated
    var node = new DocumentType(PDC);
    node.name = qualifiedName;
    node.nodeName = qualifiedName;
    node.publicId = publicId || '';               // stored verbatim
    node.systemId = systemId || '';               // stored verbatim
    node.internalSubset = internalSubset || '';   // stored verbatim
    node.childNodes = new NodeList();
    return node;
},

lib/dom.js - serializer DOCTYPE case (lines 2948-2964):

case DOCUMENT_TYPE_NODE:
    var pubid = node.publicId;
    var sysid = node.systemId;
    buf.push(g.DOCTYPE_DECL_START, ' ', node.name);
    if (pubid) {
        buf.push(' ', g.PUBLIC, ' ', pubid);
        if (sysid && sysid !== '.') {
            buf.push(' ', sysid);
        }
    } else if (sysid && sysid !== '.') {
        buf.push(' ', g.SYSTEM, ' ', sysid);
    }
    if (node.internalSubset) {
        buf.push(' [', node.internalSubset, ']');  // internalSubset emitted verbatim
    }
    buf.push('>');
    return;

---

PoC

internalSubset injection

const { DOMImplementation, XMLSerializer } = require('@xmldom/xmldom');

const impl = new DOMImplementation();
const doctype = impl.createDocumentType(
    'root',
    '',
    '',
    ']><injected/><![CDATA['
);
const doc = impl.createDocument(null, 'root', doctype);
const xml = new XMLSerializer().serializeToString(doc);
console.log(xml);
// <!DOCTYPE root []><injected/><![CDATA[]><root/>
//                   ^^^^^^^^^^  injected element outside DOCTYPE

publicId quoting context break

const { DOMImplementation, XMLSerializer } = require('@xmldom/xmldom');

const impl = new DOMImplementation();
const doctype = impl.createDocumentType(
    'root',
    '"injected PUBLIC_ID" SYSTEM "evil"',
    '',
    ''
);
const doc = impl.createDocument(null, 'root', doctype);
console.log(new XMLSerializer().serializeToString(doc));
// <!DOCTYPE root PUBLIC "injected PUBLIC_ID" SYSTEM "evil"><root/>
// quoting context broken - SYSTEM entry injected

systemId injection

const { DOMImplementation, XMLSerializer } = require('@xmldom/xmldom');

const impl = new DOMImplementation();
const doctype = impl.createDocumentType(
    'root',
    '',
    '"sysid"><injected attr="pwn"/>',
    ''
);
const doc = impl.createDocument(null, 'root', doctype);
console.log(new XMLSerializer().serializeToString(doc));
// <!DOCTYPE root SYSTEM "sysid"><injected attr="pwn"/>><root/>
// > in sysid closes DOCTYPE early; <injected/> appears as sibling element

---

Impact

An application that programmatically constructs DocumentType nodes from user-controlled data and then serializes the document can emit a DOCTYPE declaration where the internal subset is closed early or where injected SYSTEM entities or other declarations appear in the serialized output.

Downstream XML parsers that re-parse the serialized output and expand entities from the injected DOCTYPE declarations may be susceptible to XXE-class attacks if they enable entity expansion.

---

Fix Applied

> ⚠ Opt-in required. Protection is not automatic. Existing serialization calls remain > vulnerable unless { requireWellFormed: true } is explicitly passed. Applications that pass > untrusted data to createDocumentType() or write untrusted values directly to a > DocumentType node's publicId, systemId, or internalSubset properties should audit > all serializeToString() call sites and add the option.

XMLSerializer.serializeToString() now accepts an options object as a second argument. When { requireWellFormed: true } is passed, the serializer validates the DocumentType node's publicId, systemId, and internalSubset fields before emitting the DOCTYPE declaration and throws InvalidStateError if any field contains an injection sequence:

• publicId: throws if non-empty and does not match the XML PubidLiteral production (XML 1.0 [12])
• systemId: throws if non-empty and does not match the XML SystemLiteral production (XML 1.0 [11])
• internalSubset: throws if it contains ]> (which closes the internal subset and DOCTYPE declaration early)

All three checks apply regardless of how the invalid value entered the node - whether via createDocumentType arguments or a subsequent direct property write.

PoC - fixed path

const { DOMImplementation, XMLSerializer } = require('@xmldom/xmldom');
const impl = new DOMImplementation();

// internalSubset injection
const dt1 = impl.createDocumentType('root', '', '', ']><injected/><![CDATA[');
const doc1 = impl.createDocument(null, 'root', dt1);

// Default (unchanged): verbatim - injection present
console.log(new XMLSerializer().serializeToString(doc1));
// <!DOCTYPE root []><injected/><![CDATA[]><root/>

// Opt-in guard: throws InvalidStateError
try {
  new XMLSerializer().serializeToString(doc1, { requireWellFormed: true });
} catch (e) {
  console.log(e.name, e.message);
  // InvalidStateError: DocumentType internalSubset contains "]>"
}

The guard also covers post-creation property writes:

const dt2 = impl.createDocumentType('root', '', '');
dt2.systemId = '"sysid"><injected attr="pwn"/>';
const doc2 = impl.createDocument(null, 'root', dt2);
new XMLSerializer().serializeToString(doc2, { requireWellFormed: true });
// InvalidStateError: DocumentType systemId is not a valid SystemLiteral

Why the default stays verbatim

The W3C DOM Parsing and Serialization spec §3.2.1.3 defines a require well-formed flag whose default value is false. With the flag unset, the spec permits verbatim serialization of DOCTYPE fields. Unconditionally throwing would be a behavioral breaking change with no spec justification. The opt-in requireWellFormed: true flag allows applications that require injection safety to enable strict mode without breaking existing deployments.

Residual limitation

createDocumentType(qualifiedName, publicId, systemId[, internalSubset]) does not validate publicId, systemId, or internalSubset at creation time. This creation-time validation is a breaking change and is deferred to a future breaking release.

When the default serialization path is used (without requireWellFormed: true), all three fields are still emitted verbatim. Applications that do not pass requireWellFormed: true remain exposed.