xmldom: XML injection via unsafe CDATA serialization allows attacker-controlled markup insertion

Summary

@xmldom/xmldom allows attacker-controlled strings containing the CDATA terminator ]]> to be inserted into a CDATASection node. During serialization, XMLSerializer emitted the CDATA content verbatim without rejecting or safely splitting the terminator. As a result, data intended to remain text-only became active XML markup in the serialized output, enabling XML structure injection and downstream business-logic manipulation.

The sequence ]]> is not allowed inside CDATA content and must be rejected or safely handled during serialization. (MDN Web Docs)

Attack surface

Document.createCDATASection(data) is the most direct entry point, but it is not the only one. The WHATWG DOM spec intentionally does not validate ]]> in mutation methods — only createCDATASection carries that guard. The following paths therefore also allow ]]> to enter a CDATASection node and reach the serializer:

• CharacterData.appendData()
• CharacterData.replaceData()
• CharacterData.insertData()
• Direct assignment to .data
• Direct assignment to .textContent

(Note: assigning to .nodeValue does not update .data in this implementation — the serializer reads .data directly — so .nodeValue is not an exploitable path.)

Parse path

Parsing XML that contains a CDATA section is not affected. The SAX parser's non-greedy CDSect regex stops at the first ]]>, so parsed CDATA data never contains the terminator.

Impact

If an application uses xmldom to generate "trusted" XML documents that embed untrusted user input inside CDATA (a common pattern in exports, feeds, SOAP/XML integrations, etc.), an attacker can inject additional XML elements/attributes into the generated document.

This can lead to:

• Integrity violation of generated XML documents.
• Business-logic injection in downstream consumers (e.g., injecting <approved>true</approved>, <role>admin</role>, workflow flags, or other security-relevant elements).
• Unexpected privilege/workflow decisions if downstream logic assumes injected nodes cannot appear.

This issue does not require malformed parsers or browser behavior; it is caused by serialization producing attacker-influenced XML markup.

Root Cause (with file + line numbers)

File: lib/dom.js

1. No validation in createCDATASection

createCDATASection: function (data) accepts any string and appends it directly.

• Lines 2216–2221 (0.9.8)

2. Unsafe CDATA serialization

Serializer prints CDATA sections as:

<![CDATA[ + node.data + ]]>

without handling ]]> in the data.

• Lines 2919–2920 (0.9.8)

Because CDATA content is emitted verbatim, an embedded ]]> closes the CDATA section early and the remainder of the attacker-controlled payload is interpreted as markup in the serialized XML.

Proof of Concept — Fix A: createCDATASection now throws

On patched versions, passing ]]> directly to createCDATASection throws InvalidCharacterError instead of silently accepting the payload:

const { DOMImplementation } = require('./lib');

const doc = new DOMImplementation().createDocument(null, 'root', null);
try {
  doc.createCDATASection('SAFE]]><injected attr="pwn"/>');
  console.log('VULNERABLE — no error thrown');
} catch (e) {
  console.log('FIXED — threw:', e.name); // InvalidCharacterError
}

Expected output on patched versions:

FIXED — threw: InvalidCharacterError

Proof of Concept — Fix B: mutation vector now safe

On patched versions, injecting ]]> via a mutation method (appendData, replaceData, .data =, .textContent =) no longer produces injectable output. The serializer splits the terminator so the result round-trips as safe text:

const { DOMImplementation, XMLSerializer } = require('./lib');
const { DOMParser } = require('./lib');

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

// Start with safe data, then mutate to include the terminator
const cdata = doc.createCDATASection('safe');
doc.documentElement.appendChild(cdata);
cdata.appendData(']]><injected attr="pwn"/><more>TEXT</more><![CDATA[');

const out = new XMLSerializer().serializeToString(doc);
console.log('Serialized:', out);

const reparsed = new DOMParser().parseFromString(out, 'text/xml');
const injected = reparsed.getElementsByTagName('injected').length > 0;
console.log('Injected element found in reparsed doc:', injected);
// VULNERABLE: true  |  FIXED: false

Expected output on patched versions:

Serialized: <root><![CDATA[safe]]]]><![CDATA[><injected attr="pwn"/><more>TEXT</more><![CDATA[]]></root>
Injected element found in reparsed doc: false

Fix Applied

Both mitigations were implemented:

Option A — Strict/spec-aligned: reject ]]> in createCDATASection()

Document.createCDATASection(data) now throws InvalidCharacterError (per the WHATWG DOM spec) when data contains ]]>. This closes the direct entry point.

Code that previously passed a string containing ]]> to createCDATASection and relied on the silent/unsafe behaviour will now receive InvalidCharacterError. Use a mutation method such as appendData if you intentionally need ]]> in a CDATASection node's data (the serializer split in Option B will keep the output safe).

Option B — Defensive serialization: split the terminator during serialization

XMLSerializer now replaces every occurrence of ]]> in CDATA section data with the split sequence ]]]]><![CDATA[> before emitting. This closes all mutation-vector paths that Option A alone cannot guard, and means the serialized output is always well-formed XML regardless of how ]]> entered the node.

Update — 2026-04-xx (0.9.10 / 0.8.13)

splitCDATASections is deprecated

The CDATA split behavior introduced as Option B of this fix (replacing ]]> with]]]]><![CDATA[> during serialization) is deprecated as of 0.9.10 / 0.8.13.

This release introduces a requireWellFormed option on XMLSerializer.serializeToString(). When { requireWellFormed: true } is passed as the second argument, the serializer throws InvalidStateError if CDATA section data contains ]]> — this is the spec-aligned behavior (W3C DOM Parsing and Serialization, require well-formed flag) and the recommended migration path going forward. The split behavior is now controlled by an explicit splitCDATASections option (default true, preserving the current behavior). The three serialization behaviors are: | requireWellFormed | splitCDATASections | Behavior ||---|---|---|| false (default) | true (default) | Split ]]> → ]]]]><![CDATA[> (current behavior, deprecated) || true | — (ignored) | Throw InvalidStateError — spec-aligned, recommended |\ false | false | Emit verbatim — same as pre-0.9.9 behavior |

requireWellFormed: true takes precedence: the split path is unreachable when it is set.

Migration

Replace any reliance on the default split behavior with an explicit opt-in:

// After (explicit guard, spec-aligned): const xml = new XMLSerializer().serializeToString(doc, { requireWellFormed: true }); // Throws InvalidStateError if any CDATASection contains ']]>' ```

### Removal timeline
Both the `splitCDATASections` option and the underlying `]]>` → `]]]]><![CDATA[>` split mechanics will be removed in the next breaking (`0.10.0`) release. After removal, the only behaviors will be verbatim (default) and `requireWellFormed: true` (throws).

Removal is tracked in [xmldom/xmldom#999](https://github.com/xmldom/xmldom/issues/999).