---
id: "CVE-2026-110910"
kind: "vulnerability"
aliases: ["CVE-2026-110910"]
title: "qs: empty-array keys skip encoding, letting callers inject query parameters"
severity: "medium"
cvss: 6.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["javascript", "node.js", "package", "qs", "self-hosted"]
not_affected_if: ["allowemptyarrays-off", "default-config", "qs-before-6.12.0", "trusted-keys-only"]
published_at: "2026-10-27T07:16:32+00:00"
first_seen_at: "2026-10-27T07:30:44+00:00"
updated_at: "2026-10-27T07:31:57+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-110910", "first_seen_at": "2026-10-27T07:30:44+00:00"}]
---

# qs: empty-array keys skip encoding, letting callers inject query parameters

The qs library's stringify function, when allowEmptyArrays is true, writes the key of an empty array without URL-encoding it. A key containing &, =, or # therefore turns into query-string syntax, so attacker-chosen key names can add parameters or truncate the query.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `npm ls qs` to see which qs version is installed and where it is pulled in.
- Search the code for `allowEmptyArrays` passed to `qs.stringify`, especially where object keys come from user input.
- Check whether that code forwards the serialized query string to an upstream service.

## Fix

- Upgrade qs to `6.17.0` or later.
- Until upgraded, stop passing user-controlled keys through `qs.stringify` with `allowEmptyArrays: true`, or turn that option off.

## Timeline

- 2026-10-27T07:30:44+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-110910

## From the sources

### 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 `#` ∴ 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: 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 <6.17.0`, fixed in v6.17.0.



`allowEmptyArrays` was introduced in f22b2bc and first shipped in v6.12.0, with this early return. Every release from v6.12.0 through v6.16.1 is affected. 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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_