Qs Project: >> Qs >> 6.10.0 Security Vulnerabilities
### 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.
CVSS Score
3.7
EPSS Score
0.0
Published
2026-02-12
Improper Input Validation vulnerability in qs (parse modules) allows HTTP DoS.This issue affects qs: < 6.14.1.
Summary
The arrayLimit option in qs did not enforce limits for bracket notation (a[]=1&a[]=2), only for indexed notation (a[0]=1). This is a consistency bug; arrayLimit should apply uniformly across all array notations.
Note: The default parameterLimit of 1000 effectively mitigates the DoS scenario originally described. With default options, bracket notation cannot produce arrays larger than parameterLimit regardless of arrayLimit, because each a[]=valueconsumes one parameter slot. The severity has been reduced accordingly.
Details
The arrayLimit option only checked limits for indexed notation (a[0]=1&a[1]=2) but did not enforce it for bracket notation (a[]=1&a[]=2).
Vulnerable code (lib/parse.js:159-162):
if (root === '[]' && options.parseArrays) {
obj = utils.combine([], leaf); // No arrayLimit check
}
Working code (lib/parse.js:175):
else if (index <= options.arrayLimit) { // Limit checked here
obj = [];
obj[index] = leaf;
}
The bracket notation handler at line 159 uses utils.combine([], leaf) without validating against options.arrayLimit, while indexed notation at line 175 checks index <= options.arrayLimit before creating arrays.
PoC
const qs = require('qs');
const result = qs.parse('a[]=1&a[]=2&a[]=3&a[]=4&a[]=5&a[]=6', { arrayLimit: 5 });
console.log(result.a.length); // Output: 6 (should be max 5)
Note on parameterLimit interaction: The original advisory's "DoS demonstration" claimed a length of 10,000, but parameterLimit (default: 1000) caps parsing to 1,000 parameters. With default options, the actual output is 1,000, not 10,000.
Impact
Consistency bug in arrayLimit enforcement. With default parameterLimit, the practical DoS risk is negligible since parameterLimit already caps the total number of parsed parameters (and thus array elements from bracket notation). The risk increases only when parameterLimit is explicitly set to a very high value.
CVSS Score
3.7
EPSS Score
0.001
Published
2025-12-29
qs before 6.10.3, as used in Express before 4.17.3 and other products, allows attackers to cause a Node process hang for an Express application because an __ proto__ key can be used. In many typical Express use cases, an unauthenticated remote attacker can place the attack payload in the query string of the URL that is used to visit the application, such as a[__proto__]=b&a[__proto__]&a[length]=100000000. The fix was backported to qs 6.9.7, 6.8.3, 6.7.3, 6.6.1, 6.5.3, 6.4.1, 6.3.3, and 6.2.4 (and therefore Express 4.17.3, which has "deps: qs@6.9.7" in its release description, is not vulnerable).
CVSS Score
7.5
EPSS Score
0.014
Published
2022-11-26