What is the CVSS score of CVE-2026-33349?

CVE-2026-33349 has a CVSS 3.1 base score of 5.9 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H. EPSS exploitation probability: 0.0%.

Which versions of Red Hat are affected by CVE-2026-33349?

CVE-2026-33349 presents moderate security risk rated CVSS 5.9. Exploitation could compromise the security of affected deployments.. A patch is available.

How to fix or mitigate CVE-2026-33349?

Within 30 days: Identify affected systems and apply vendor patches as part of regular patch cycle. Monitor vendor channels for patch availability.

Red Hat CVE-2026-33349

MEDIUM

Improper Validation of Specified Quantity in Input (CWE-1284)

2026-03-19 https://github.com/NaturalIntelligence/fast-xml-parser

GHSA-jp2q-39xq-3w4g

Denial Of Service Red Hat

5.9

CVSS 3.1 · GitHub Advisory

Severity by source

GitHub Advisory PRIMARY

5.9 MEDIUM

AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Red Hat

5.9 MEDIUM

qualitative

Primary rating from GitHub Advisory.

CVSS VectorGitHub Advisory

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Lifecycle Timeline

Patch released

Mar 31, 2026 - 21:13 nvd

Patch available

Analysis Generated

Mar 19, 2026 - 20:00 vuln.today

CVE Published

Mar 19, 2026 - 19:13 nvd

MEDIUM 5.9

DescriptionGitHub Advisory

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.

AnalysisAI

The fast-xml-parser library contains a logic error in DocTypeReader.js where JavaScript's falsy evaluation of the number 0 causes entity size and count limits to be completely bypassed when explicitly configured to 0. An attacker who can supply crafted XML input to an application using fast-xml-parser with these limits set to 0 can trigger unbounded entity expansion, exhausting server memory and causing denial of service. A proof-of-concept exists demonstrating the vulnerability, and the CVSS score of 5.9 reflects medium severity with high attack complexity, though the real-world impact is significant for applications that explicitly configure these restrictive limits.

Technical ContextAI

fast-xml-parser (pkg:npm/fast-xml-parser) is a widely-used Node.js library for parsing and building XML documents. The vulnerability stems from a type-checking logic error in the DocTypeReader.js module, specifically in the readDocType and readEntityExp functions. While the OptionsBuilder.js correctly preserves a developer-supplied value of 0 using nullish coalescing (??), the DocTypeReader.js uses truthy evaluation (e.g., if (this.options.maxEntityCount &&...)) to enforce entity limits. Since 0 is falsy in JavaScript, when a developer explicitly sets maxEntityCount or maxEntitySize to 0 intending to prohibit all entities, the guard conditions short-circuit and the limits are never enforced. This is classified under CWE-1284 (Improper Validation of Specified Quantity in Input), as the validation logic fails to correctly handle edge case values that are valid but falsy in the host language. The vulnerability allows XML with DOCTYPE declarations containing unbounded entity definitions to bypass security controls designed to prevent entity expansion attacks.

RemediationAI

Upgrade fast-xml-parser to a patched version as specified in the official GitHub Advisory GHSA-jp2q-39xq-3w4g (https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-jp2q-39xq-3w4g). The recommended fix involves replacing truthy checks with explicit type checks: change if (this.options.maxEntityCount &&...) to if (typeof this.options.maxEntityCount === 'number' &&...) in DocTypeReader.js lines 30-32 and similarly for maxEntitySize at lines 128-130. As a workaround for applications unable to immediately patch, disable entity processing entirely by setting processEntities to false rather than configuring limits to 0, or use the enabled: false option. Do not rely on setting these limits to 0 as a security control until the patch is applied. Review all applications using fast-xml-parser to identify those with custom entity processing configurations and prioritize patching for instances parsing untrusted XML input.