xmldom: DocType `name` Injection Bypasses requireWellFormed

Summary

The @xmldom/xmldom serializer emits DocumentType.name verbatim into the <!DOCTYPE …> declaration with no well-formedness guard. GHSA-f6ww-3ggp-fr8h (CVE-2026-41674) hardened the serializer's requireWellFormed path for a DocumentType's sibling fields — publicId, systemId, and internalSubset — but it did not add any check for name. A > (or whitespace) in the name terminates the doctype declaration early, letting the remaining characters become sibling markup in the serialized output.

Because requireWellFormed: true — the recommended mitigation for the prior xmldom injection CVEs — performs no validation on the DocType name, this is a bypass of that control, in the same family as the open element-name (GHSA-w2rr-34g9-rvrj) and attribute-name (GHSA-4w3w-2rp5-g8jm) name-injection advisories.

Details

The serializer's DOCUMENT_TYPE_NODE case runs the requireWellFormed block only against publicId, systemId, and internalSubset, then pushes n.name directly into the buffer between the <!DOCTYPE prefix and the closing >:

• 0.9.x (v0.9.10, bb7a085): serializer DocType case, lib/dom.js#L3256-L3283 — the requireWellFormed block (#L3259-L3269) validates publicId/systemId/internalSubset but not name, which is emitted verbatim at #L3270.
• 0.8.x (v0.8.13, e5c1480): serializer DocType case, lib/dom.js#L1914-L1946 — same structure; name is emitted verbatim at #L1928.
• unscoped xmldom (v0.6.0, c80a161): lib/dom.js#L1105 emits node.name verbatim; this line predates requireWellFormed, so there is no well-formedness path at all.

Enabling write paths

DocumentType.name is a plain, writable own-property, so the enabling vector differs by line:

• 0.9.x — createDocumentType() validates the name via validateQualifiedName (lib/dom.js#L925-L936, validation at #L926), so the deliverable vector is a direct property write (dt.name = 'html><script>…') to the unguarded own-property.
• 0.8.x — createDocumentType() does not validate the name (lib/dom.js#L456-L464), so the malicious name is reachable directly through createDocumentType() as well as via direct property write.
• unscoped xmldom (<= 0.6.0) — createDocumentType() does not validate the name (lib/dom.js#L286), same as 0.8.x.

This is the same structural root cause the sibling name-injection advisories share: the serializer's requireWellFormed path validates content delimiters but no name field, and every name-like field is a plain writable property, so mutation / direct property-write bypasses any creation-time check.

Root Cause

1. The serializer's requireWellFormed DocType block checks publicId, systemId, and internalSubset (the fields hardened by GHSA-f6ww-3ggp-fr8h) but has no check for name.
2. DocumentType.name is a plain writable own-property; on 0.8.x and the unscoped package createDocumentType() does not validate it either.
3. The serializer emits name directly between the doctype delimiters: <!DOCTYPE ${name}…>.

Proof of Concept

Run against @xmldom/xmldom v0.9.10 (commit bb7a085):

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

const impl = new DOMImplementation();
const serializer = new XMLSerializer();

// 0.9.x createDocumentType validates the name, so overwrite it via direct property write
const dt = impl.createDocumentType('html', '', '');
dt.name = 'html><script xmlns="http://www.w3.org/1999/xhtml">alert(1)</script';
const doc = impl.createDocument(null, 'r', dt);

const output = serializer.serializeToString(doc, { requireWellFormed: true });
console.log(output);
// Output: <!DOCTYPE html><script xmlns="http://www.w3.org/1999/xhtml">alert(1)</script><r/>
//
// requireWellFormed: true did NOT prevent the injection (no exception thrown).
// The injected <script> is well-formed XHTML that a browser would execute.

Confirmed runtime behavior:

• 0.9.x — createDocumentType() rejects the malicious name at creation (InvalidCharacterError); a direct write to dt.name bypasses that, and serializeToString(…, { requireWellFormed: true }) emits the breakout with no exception.
• Re-parse confirmation — re-parsing the output shows the injected <script> is a real second top-level element originating entirely from the DocType name: the parser rejects it with HierarchyRequestError: Only one element can be added and only after doctype. A comment-injection variant (dt.name = 'html><!--INJECTED--', output <!DOCTYPE html><!--INJECTED--><r/>) re-parses cleanly and the injected comment node is enumerable, confirming the injected node is structurally live.
• 0.8.x (v0.8.13, e5c1480) — createDocumentType('html><script>…', '', '') accepts the malicious name directly (no creation-time validation), and serializeToString(doc, null, null, { requireWellFormed: true }) produces <!DOCTYPE html><script>alert(1)</script><r/> with no exception.

A browser reproduction does not apply: browsers keep DocumentType.name readonly, so the direct-write vector cannot be reproduced in a browser DOM. The injection is specific to xmldom exposing name as writable and serializing it without a guard.

Impact

Applications that build a DocumentType node with an attacker-influenced name — via direct property write on any affected line, or via createDocumentType() on 0.8.x and the unscoped package — and serialize the document are vulnerable to XML/markup injection:

• XML structure injection — breaking out of the <!DOCTYPE …> declaration to inject arbitrary sibling elements, comments, or additional markup into the output.
• XSS via XHTML — if the serialized output is served as XHTML or processed by a browser-based XML parser, an injected <script> element (in the XHTML namespace) executes.
• requireWellFormed bypass — applications that adopted requireWellFormed: true as a mitigation for the prior injection CVEs (including the sibling DocType fields fixed by GHSA-f6ww-3ggp-fr8h) remain vulnerable through the DocType name.

Fix Applied

Under requireWellFormed, the serializer validates the DocType name as a well-formed XML Name and throws InvalidStateError when it is not — matching the sibling publicId/systemId/internalSubset checks. Non-breaking and opt-in; ships on both maintained versions. No creation-time change is made: 0.9.x already validates the name at createDocumentType, and the 0.8.x/unscoped creation gap cannot be closed without a breaking change, so it is left unfixed. See the XML Name production.

⚠ Opt-in required. Protection is not automatic. Existing serialization calls remain vulnerable unless { requireWellFormed: true } is explicitly passed. Applications that serialize untrusted DOM content should audit all serializeToString() call sites and add it.

Proof of Concept - fixed path

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

const impl = new DOMImplementation();
const serializer = new XMLSerializer();

const dt = impl.createDocumentType('html', '', '');
dt.name = 'html><script xmlns="http://www.w3.org/1999/xhtml">alert(1)</script';
const doc = impl.createDocument(null, 'r', dt);

// Default path: the ill-formed name is still emitted verbatim (injection present).
console.log(serializer.serializeToString(doc));
// <!DOCTYPE html><script xmlns="http://www.w3.org/1999/xhtml">alert(1)</script><r/>

// Opt-in path: serialization throws instead of emitting the breakout.
serializer.serializeToString(doc, { requireWellFormed: true });
// throws InvalidStateError

Why the default stays verbatim

W3C DOM Parsing's require-well-formed flag defaults to false, and the browser XMLSerializer emits the name verbatim in that default mode. Unconditionally throwing would be an unjustified breaking change against that specified default, so the guard is opt-in behind { requireWellFormed: true }.

Residual limitation

The default serialization path still emits the ill-formed DocType name verbatim; protection applies only when requireWellFormed: true is passed. No creation-time validation is added for the DocType name: 0.9.x already validates at createDocumentType, and the 0.8.x/unscoped creation gap is left unfixed — it cannot be closed without a breaking change.