xmldom: Processing Instruction Target Injection Bypasses requireWellFormed

Summary

Document.createProcessingInstruction() in @xmldom/xmldom performs no validation on the target parameter. The requireWellFormed: true serializer option validates only for : in the target and a case-insensitive xml prefix, but does not check for > characters. A > in the target breaks the processing instruction boundary (<?...?>), allowing injection of arbitrary content into the serialized XML output.

Details

Document.createProcessingInstruction(target, data) at lib/dom.js around line 2413 accepts any string as the target parameter and stores it on the PI node without validation.

During serialization, the requireWellFormed code path (around line 3286) performs two checks on PI targets:

1. Rejects targets containing : (namespace prefix check)
2. Rejects targets matching xml case-insensitively (reserved prefix)

However, it does NOT validate that the target conforms to the XML Name production, and critically does NOT check for > characters. Since processing instructions are serialized as <?target data?>, a > in the target prematurely closes the PI, causing the remaining content to be interpreted as document content by any downstream XML parser.

Root Cause

1. createProcessingInstruction() performs no validation on target
2. The serializer's requireWellFormed check is incomplete -- it only checks for : and xml, missing characters that break PI syntax (>, ?, whitespace)
3. The serializer emits the target verbatim: <?${target} ${data}?>

Proof of Concept

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

const impl = new DOMImplementation();
const serializer = new XMLSerializer();
const doc = impl.createDocument(null, 'root', null);

// PI target containing > breaks the PI boundary
const pi = doc.createProcessingInstruction('a>', 'data');
doc.documentElement.appendChild(pi);

const output = serializer.serializeToString(doc, { requireWellFormed: true });
console.log(output);
// Output: <root><?a> data?></root>
//
// The > in the target closes the PI prematurely.
// A downstream XML parser sees:
//   - Processing instruction: <?a?>  (target "a", no data)
//   - Text content: " data?>"
//
// requireWellFormed: true did NOT prevent the injection.

Injecting elements via PI target

const pi2 = doc.createProcessingInstruction(
  'a?><script xmlns="http://www.w3.org/1999/xhtml">alert(1)</script><?b',
  ''
);
doc.documentElement.appendChild(pi2);

const output2 = serializer.serializeToString(doc, { requireWellFormed: true });
console.log(output2);
// Output includes:
//   <?a?><script xmlns="http://www.w3.org/1999/xhtml">alert(1)</script><?b ?>
//
// The injected <script> element is valid XHTML that a browser would execute.

Impact

Applications that create processing instructions with user-controlled target strings and serialize the result are vulnerable to XML injection. This enables:

• XML structure injection: Breaking the PI boundary to inject arbitrary elements, text, or additional processing instructions into the output
• XSS via XHTML: If the serialized output is served as XHTML or processed by a browser-based XML parser, injected script elements will execute
• XXE chain: Injected DOCTYPE declarations or entity references could trigger XXE in downstream XML parsers that consume the output
• requireWellFormed bypass: The existing well-formedness checks are incomplete and provide a false sense of security

Fix Applied

Under requireWellFormed, the serializer validates a processing-instruction target as an XML NCName (a Name with no colon) and rejects a case-insensitive xml, throwing InvalidStateError when the target is ill-formed — so a >, ?, or whitespace in the target is now refused.
On 0.9.12 this replaces an earlier check that already rejected a colon or xml, so the no-colon rule is preserved.
0.8.15 had no processing-instruction target check at all, so the whole target validation is new there.
Non-breaking and opt-in. 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 doc = impl.createDocument(null, 'root', null);

// PI target containing > breaks the PI boundary
const pi = doc.createProcessingInstruction('a>', 'data');
doc.documentElement.appendChild(pi);

// Default path: emits the ill-formed target verbatim.
console.log(serializer.serializeToString(doc));
// Output: <root><?a> data?></root>

// Opt-in path: the target check now rejects the break-out character.
try {
  serializer.serializeToString(doc, { requireWellFormed: true });
} catch (e) {
  console.log(e.name); // InvalidStateError
}

Why the default stays verbatim

W3C DOM Parsing's require-well-formed flag defaults to false, and the browser XMLSerializer emits the target verbatim in that default mode. Unconditionally throwing on an ill-formed PI target would diverge from that platform behavior and would be an unjustified breaking change, so the stricter validation is gated behind { requireWellFormed: true }. (See the W3C XML Name production and XML Processing Instructions.)

Residual limitation

The default serialization path still emits the ill-formed target verbatim -- only the opt-in requireWellFormed path is protected. Creation-time validation of the target in createProcessingInstruction() is breaking and is deferred to the next breaking release, tracked at xmldom/xmldom#1073.