<h3>Summary</h3>
The arrayLimit option in qs does not enforce limits for comma-separated values when comma: true is enabled, allowing attackers to cause denial-of-service via memory exhaustion. This is a bypass of the array limit enforcement, similar to the bracket notation bypass addressed in GHSA-6rw7-vpxm-498p (CVE-2025-15284).
<h3>Details</h3>
When the comma option is set to true (not the default, but configurable in applications), qs allows parsing comma-separated strings as arrays (e.g., ?param=a,b,c becomes ['a', 'b', 'c']). However, the limit check for arrayLimit (default: 20) and the optional throwOnLimitExceeded occur after the comma-handling logic in parseArrayValue, enabling a bypass. This permits creation of arbitrarily large arrays from a single parameter, leading to excessive memory allocation.
Vulnerable code (lib/parse.js: lines ~40-50):
```js
if (val && typeof val === 'string' && options.comma && val.indexOf(',') > -1) {
return val.split(',');
}
if (options.throwOnLimitExceeded && currentArrayLength >= options.arrayLimit) {
throw new RangeError('Array limit exceeded. Only ' + options.arrayLimit + ' element' + (options.arrayLimit === 1 ? '' : 's') + ' allowed in an array.');
}
return val;
``
Thesplit(',')returns the array immediately, skipping the subsequent limit check. Downstream merging viautils.combinedoes not prevent allocation, even if it marks overflows for sparse arrays.This discrepancy allows attackers to send a single parameter with millions of commas (e.g.,?param=,,,,,,,,...), allocating massive arrays in memory without triggering limits. It bypasses the intent ofarrayLimit, which is enforced correctly for indexed (a[0]=) and bracket (a[]=`) notations (the latter fixed in v6.14.1 per GHSA-6rw7-vpxm-498p).
<h3>PoC</h3>
Test 1 - Basic bypass:
npm install qs
```js
const qs = require('qs');
const payload = 'a=' + ','.repeat(25); // 26 elements after split (bypasses arrayLimit: 5)
const options = { comma: true, arrayLimit: 5, throwOnLimitExceeded: true };
try {
const result = qs.parse(payload, options);
console.log(result.a.length); // Outputs: 26 (bypass successful)
} catch (e) {
console.log('Limit enforced:', e.message); // Not thrown
}
``
**Configuration:**
-comma: true-arrayLimit: 5-throwOnLimitExceeded: true`
Expected: Throws "Array limit exceeded" error.
Actual: Parses successfully, creating an array of length 26.
<h3>Impact</h3>
Denial of Service (DoS) via memory exhaustion.