### 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.
`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.
Metrics
Affected Vendors & Products
No data.
No data.
OpenCVE Enrichment is a feature of OpenCVE that uses AI to automatically link vendors and products to CVEs. Learn more on GitHub.
| Vendors | Products |
|---|---|
|
Ljharb
Subscribe
|
Qs
Subscribe
|
Advisories
No advisories yet.
Fixes
Solution
Upgrade to qs 6.17.0 or later.
Workaround
Leave `allowEmptyArrays` off (the default), or check key names that come from untrusted input against an allowlist before passing the object to `qs.stringify`.
References
History
Tue, 27 Oct 2026 09:00:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| First Time appeared |
Ljharb
Ljharb qs |
|
| Vendors & Products |
Ljharb
Ljharb qs |
Tue, 27 Oct 2026 07:15:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| 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 `#` ∴ 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. | |
| Title | qs.stringify with allowEmptyArrays does not encode the keys of empty arrays, allowing query string parameter injection | |
| Weaknesses | CWE-116 | |
| References |
| |
| Metrics |
cvssV3_1
|
Projects
Sign in to view the affected projects.
MITRE
Status: PUBLISHED
Assigner: harborist
Published:
Updated: 2026-10-27T06:43:13.416Z
Reserved: 2026-10-15T03:01:58.840Z
Link: CVE-2026-110910
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-10-27T07:16:32.263
Modified: 2026-10-27T07:16:32.263
Link: CVE-2026-110910
Redhat
No data.
OpenCVE Enrichment
Updated: 2026-10-27T09:15:18Z
Weaknesses