CVE-2026-2391: Qs Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-w7fw-mjwx-w883",
  "modified" : "2026-02-12T20:07:59Z",
  "published" : "2026-02-12T17:04:39Z",
  "aliases" : [ "CVE-2026-2391" ],
  "summary" : "qs's arrayLimit bypass in comma parsing allows denial of service",
  "details" : "### Summary\nThe `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).\n\n### Details\nWhen 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.\n\n**Vulnerable code** (lib/parse.js: lines ~40-50):\n```js\nif (val && typeof val === 'string' && options.comma && val.indexOf(',') > -1) {\n    return val.split(',');\n}\n\nif (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\nreturn val;\n```\nThe `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).\n\n### PoC\n**Test 1 - Basic bypass:**\n```\nnpm install qs\n```\n\n```js\nconst qs = require('qs');\n\nconst payload = 'a=' + ','.repeat(25);  // 26 elements after split (bypasses arrayLimit: 5)\nconst options = { comma: true, arrayLimit: 5, throwOnLimitExceeded: true };\n\ntry {\n  const result = qs.parse(payload, options);\n  console.log(result.a.length);  // Outputs: 26 (bypass successful)\n} catch (e) {\n  console.log('Limit enforced:', e.message);  // Not thrown\n}\n```\n**Configuration:**\n- `comma: true`\n- `arrayLimit: 5`\n- `throwOnLimitExceeded: true`\n\nExpected: Throws \"Array limit exceeded\" error.\nActual: Parses successfully, creating an array of length 26.\n\n\n### Impact\nDenial of Service (DoS) via memory exhaustion.\n\n### Suggested Fix\nMove the `arrayLimit` check before the comma split in `parseArrayValue`, and enforce it on the resulting array length. Use `currentArrayLength` (already calculated upstream) for consistency with bracket notation fixes.\n\n**Current code** (lib/parse.js: lines ~40-50):\n```js\nif (val && typeof val === 'string' && options.comma && val.indexOf(',') > -1) {\n    return val.split(',');\n}\n\nif (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\nreturn val;\n```\n\n**Fixed code:**\n```js\nif (val && typeof val === 'string' && options.comma && val.indexOf(',') > -1) {\n    const splitArray = val.split(',');\n    if (splitArray.length > options.arrayLimit - currentArrayLength) {  // Check against remaining limit\n        if (options.throwOnLimitExceeded) {\n            throw new RangeError('Array limit exceeded. Only ' + options.arrayLimit + ' element' + (options.arrayLimit === 1 ? '' : 's') + ' allowed in an array.');\n        } else {\n            // Optionally convert to object or truncate, per README\n            return splitArray.slice(0, options.arrayLimit - currentArrayLength);\n        }\n    }\n    return splitArray;\n}\n\nif (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\nreturn val;\n```\nThis aligns behavior with indexed and bracket notations, reuses `currentArrayLength`, and respects `throwOnLimitExceeded`. Update README to note the consistent enforcement.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "qs"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "6.7.0"
      }, {
        "fixed" : "6.14.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 6.14.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/ljharb/qs/security/advisories/GHSA-w7fw-mjwx-w883"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-2391"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ljharb/qs/commit/f6a7abff1f13d644db9b05fe4f2c98ada6bf8482"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ljharb/qs"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-12T17:04:39Z",
    "nvd_published_at" : "2026-02-12T05:17:11Z"
  }
}