CVE-2026-2391

### Summary

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).

### Details

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;

```

The `split(',')` returns the array immediately, skipping the subsequent limit check. Downstream merging via `utils.combine` does 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 of `arrayLimit`, which is enforced correctly for indexed (`a[0]=`) and bracket (`a[]=`) notations (the latter fixed in v6.14.1 per GHSA-6rw7-vpxm-498p).

### PoC

**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.

### Impact

Denial of Service (DoS) via memory exhaustion.