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:
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