---
id: CVE-2026-110910
title: >-
  qs.stringify with allowEmptyArrays does not encode the keys of empty arrays,
  allowing query string parameter injection
summary: >-
  ### Summary




  `qs.stringify` with `allowEmptyArrays: true` writes the key of an empty array
  without encoding it, even though keys are encoded by default. A key that
  contains `&` (or the configured `delimiter`), `=`, or `#` therefore bec…
severity: low
cvss: 3.7
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N'
cvssSource: cna
cwe:
  - CWE-116
vendor: ljharb
product: qs
affected:
  - qs >= 6.12.0 < 6.12.6
  - qs >= 6.13.0 < 6.13.4
  - qs >= 6.14.0 < 6.14.3
  - qs >= 6.15.0 < 6.15.4
  - qs >= 6.16.0 < 6.16.2
published: '2026-10-27'
updated: '2026-10-27'
sourceUpdated: '2026-10-27T19:32:36.456Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-110910'
references:
  - url: 'https://github.com/ljharb/qs/security/advisories/GHSA-w4cr-vjpx-p82q'
  - url: >-
      https://github.com/ljharb/qs/commit/234d5051f5071efaa025a7ca9e4d34a56a460a1b
  - url: >-
      https://github.com/ljharb/qs/commit/6b8bf1a53d7b0c5e7e434eaae8e2c0cffd5c2492
  - url: >-
      https://github.com/ljharb/qs/commit/033526fc82f2e80a3d896df2bce9137e34cb5dc5
  - url: >-
      https://github.com/ljharb/qs/commit/f9dd4830cb922363822cdfdca2cceb3ed9792224
  - url: >-
      https://github.com/ljharb/qs/commit/ad445aa23e4c3a97394c3b9a5f9d7a7159eb1f17
  - url: >-
      https://github.com/ljharb/qs/commit/5d3b1b9ec6afd0cfc760c8e12862873789a32fc8
  - url: 'https://github.com/ljharb/qs/pull/561'
  - url: 'https://github.com/ljharb/qs/issues/626'
tags:
  - cve.org
epss: 0.00229
epssPercentile: 0.13601
ingestedAt: '2026-10-27T16:11:59.237Z'
---

## Overview

### Summary



`qs.stringify` with `allowEmptyArrays: true` writes the key of an empty array without encoding it, even though keys are encoded by default. A key that contains `&` (or the configured `delimiter`), `=`, or `#` therefore becomes query string syntax instead of data, so a caller-supplied key name can add parameters to the serialized query string, or cut it short.



### Details



In `lib/stringify.js`, the `allowEmptyArrays` branch returns `adjustedPrefix + '[]'` early, before the code that passes every other key through `encoder` and `formatter`. So `{ "x&k=v": ["1"] }` serializes as `x%26k%3Dv%5B0%5D=1`, but `{ "x&k=v": [] }` serializes as `x&k=v[]`. The early return applies at any depth (`{ "a": { "b&k=v": [] } }` gives `a[b&k=v][]`), under every `arrayFormat`, with `allowDots`, and with a custom `encoder`, which is never called for that key. With `encode: false` or `encodeValuesOnly: true`, keys are intentionally left unencoded on every path; those configurations are not affected.



#### PoC



```js



var qs = require("qs");



var query = qs.stringify({ q: "hi", "x&apiKey=evil&y": [] }, { allowEmptyArrays: true });



// "q=hi&x&apiKey=evil&y[]"



qs.parse(query);



// { q: "hi", x: "", apiKey: "evil", y: [""] }



qs.stringify({ "x#": [], z: "1" }, { allowEmptyArrays: true });



// "x#[]&z=1": in a URL, everything after "#" becomes the fragment



```



#### Fix



Applied in 234d505 on `main` and released as v6.17.0, and backported to v6.16.2, v6.15.4, v6.14.3, v6.13.4, and v6.12.6: the key of an empty array goes through `encoder` and `formatter` exactly as the key of any other value does, so `{ "x&apiKey=evil&y": [] }` now serializes as `x%26apiKey%3Devil%26y[]`. Keys that need no encoding serialize as before, with two exceptions that match every other path: the brackets of a nested key are now encoded (`a[b][]` becomes `a%5Bb%5D[]`, which `qs.parse` reads identically), and with `encodeDotInKeys`, a dot in the key becomes `%252E` instead of `%2E` (which `decodeDotInKeys` previously misread as nesting).



### Affected versions



- `>=6.12.0 <6.12.6`, fixed in v6.12.6



- `>=6.13.0 <6.13.4`, fixed in v6.13.4



- `>=6.14.0 <6.14.3`, fixed in v6.14.3



- `>=6.15.0 <6.15.4`, fixed in v6.15.4



- `>=6.16.0 <6.16.2`, fixed in v6.16.2



v6.17.0 and later are not affected.



`allowEmptyArrays` was introduced in f22b2bc and first shipped in v6.12.0, with this early return. Earlier versions have no `allowEmptyArrays` option and are not affected.



### Impact



An application that passes an object whose key names come from untrusted input to `qs.stringify` with `allowEmptyArrays: true` (for example, to forward a parsed JSON body or user-selected fields to an upstream service) can have parameters injected into the resulting query string. The injected parameters are not keys of the object, so deleting or overwriting a key on the object (such as setting a trusted `apiKey` after copying the user's fields) does not remove them; whether the attacker's value or the application's wins depends on how the receiving side handles a duplicate parameter. A `#` in such a key ends the query string early when the result is placed in a URL. `allowEmptyArrays` is off by default, and only keys whose value (after any `filter`) is an empty array are affected.

## Affected

- `qs >= 6.12.0 < 6.12.6`
- `qs >= 6.13.0 < 6.13.4`
- `qs >= 6.14.0 < 6.14.3`
- `qs >= 6.15.0 < 6.15.4`
- `qs >= 6.16.0 < 6.16.2`

## Remediation

Upgrade to qs 6.17.0 or later, or, on an older release line, to 6.12.6, 6.13.4, 6.14.3, 6.15.4, or 6.16.2.

### Workarounds

Leave `allowEmptyArrays` off (the default), or check key names that come from untrusted input against an allowlist before passing the object to `qs.stringify`.