[GHSA-6rw7-vpxm-498p] qs's arrayLimit bypass in its bracket notation allows DoS via memory exhaustion

advisories/github-reviewed/2025/12/GHSA-6rw7-vpxm-498p/GHSA-6rw7-vpxm-498p.json

@@ -1,18 +1,14 @@
 {
   "schema_version": "1.4.0",
   "id": "GHSA-6rw7-vpxm-498p",
-  "modified": "2025-12-30T21:02:54Z",
+  "modified": "2025-12-30T21:02:55Z",
   "published": "2025-12-30T21:02:54Z",
   "aliases": [
     "CVE-2025-15284"
   ],
   "summary": "qs's arrayLimit bypass in its bracket notation allows DoS via memory exhaustion",
   "details": "### Summary\n\nThe `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.\n\n### Details\n\nThe `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`).\n\n**Vulnerable code** (`lib/parse.js:159-162`):\n```javascript\nif (root === '[]' && options.parseArrays) {\n    obj = utils.combine([], leaf);  // No arrayLimit check\n}\n```\n\n**Working code** (`lib/parse.js:175`):\n```javascript\nelse if (index <= options.arrayLimit) {  // Limit checked here\n    obj = [];\n    obj[index] = leaf;\n}\n```\n\nThe 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.\n\n### PoC\n\n**Test 1 - Basic bypass:**\n```bash\nnpm install qs\n```\n\n```javascript\nconst qs = require('qs');\nconst result = qs.parse('a[]=1&a[]=2&a[]=3&a[]=4&a[]=5&a[]=6', { arrayLimit: 5 });\nconsole.log(result.a.length);  // Output: 6 (should be max 5)\n```\n\n**Test 2 - DoS demonstration:**\n```javascript\nconst qs = require('qs');\nconst attack = 'a[]=' + Array(10000).fill('x').join('&a[]=');\nconst result = qs.parse(attack, { arrayLimit: 100 });\nconsole.log(result.a.length);  // Output: 10000 (should be max 100)\n```\n\n**Configuration:**\n- `arrayLimit: 5` (test 1) or `arrayLimit: 100` (test 2)\n- Use bracket notation: `a[]=value` (not indexed `a[0]=value`)\n\n### Impact\n\nDenial of Service via memory exhaustion. Affects applications using `qs.parse()` with user-controlled input and `arrayLimit` for protection.\n\n**Attack scenario:**\n1. Attacker sends HTTP request: `GET /api/search?filters[]=x&filters[]=x&...&filters[]=x` (100,000+ times)\n2. Application parses with `qs.parse(query, { arrayLimit: 100 })`\n3. qs ignores limit, parses all 100,000 elements into array\n4. Server memory exhausted → application crashes or becomes unresponsive\n5. Service unavailable for all users\n\n**Real-world impact:**\n- Single malicious request can crash server\n- No authentication required\n- Easy to automate and scale\n- Affects any endpoint parsing query strings with bracket notation\n\n### Suggested Fix\n\nAdd `arrayLimit` validation to the bracket notation handler. The code already calculates `currentArrayLength` at line 147-151, but it's not used in the bracket notation handler at line 159.\n\n**Current code** (`lib/parse.js:159-162`):\n```javascript\nif (root === '[]' && options.parseArrays) {\n    obj = options.allowEmptyArrays && (leaf === '' || (options.strictNullHandling && leaf === null))\n        ? []\n        : utils.combine([], leaf);  // No arrayLimit check\n}\n```\n\n**Fixed code**:\n```javascript\nif (root === '[]' && options.parseArrays) {\n    // Use currentArrayLength already calculated at line 147-151\n    if (options.throwOnLimitExceeded && currentArrayLength >= options.arrayLimit) {\n        throw new RangeError('Array limit exceeded. Only ' + options.arrayLimit + ' element' + (options.arrayLimit === 1 ? '' : 's') + ' allowed in an array.');\n    }\n    \n    // If limit exceeded and not throwing, convert to object (consistent with indexed notation behavior)\n    if (currentArrayLength >= options.arrayLimit) {\n        obj = options.plainObjects ? { __proto__: null } : {};\n        obj[currentArrayLength] = leaf;\n    } else {\n        obj = options.allowEmptyArrays && (leaf === '' || (options.strictNullHandling && leaf === null))\n            ? []\n            : utils.combine([], leaf);\n    }\n}\n```\n\nThis makes bracket notation behaviour consistent with indexed notation, enforcing `arrayLimit` and converting to object when limit is exceeded (per README documentation).",
   "severity": [
-    {
-      "type": "CVSS_V3",
-      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
-    },
     {
       "type": "CVSS_V4",
       "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"