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


- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 27 Oct 2026
- **Updated:** 27 Oct 2026
- **Also known as:** CLSA-2026-1793180904, GHSA-w4cr-vjpx-p82q

## What is CVE-2026-110910?

CVE-2026-110910 is a medium-severity vulnerability in ljharb qs, published on 27 Oct 2026. 

### Official description

### 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.

Weakness: CWE-116 (Improper Encoding or Escaping of Output)

## Which versions are affected by CVE-2026-110910?


## How do I fix CVE-2026-110910?

Upgrade to a fixed version.

## Is CVE-2026-110910 being exploited?

There is no known exploitation of CVE-2026-110910 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-110910 and fix it if it is affected.

What it is: CVE-2026-110910 is a medium-severity vulnerability in ljharb qs, published on 27 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-110910 in the wild and no public exploit code.

Affected product: ljharb qs. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-110910/ (Markdown: https://insights.sbomify.com/CVE-2026-110910/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-110910
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-110910
- OSV: https://osv.dev/vulnerability/CVE-2026-110910

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-110910/
Summaries are generated and may contain mistakes; the official description above is the record.