---
id: CVE-2026-110816
title: >-
  ### Summary




  `qs.stringify` with `arrayFormat: 'comma'` throws `TypeError: Cannot convert
  object to primitive value` when an array element cannot be coerced to a
  string, such as an object with an own non-callable `toString` (`{ "toStri…
summary: >-
  ### Summary




  `qs.stringify` with `arrayFormat: 'comma'` throws `TypeError: Cannot convert
  object to primitive value` when an array element cannot be coerced to a
  string, such as an object with an own non-callable `toString` (`{ "toStri…
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
cwe:
  - CWE-248
  - CWE-704
published: '2026-10-14'
updated: '2026-10-14'
sourceUpdated: '2026-10-14T22:17:23.230'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-110816'
references:
  - url: >-
      https://github.com/ljharb/qs/commit/a952e7519338ef0cfb25694a2988848f93333727
    label: 7ffcee3d-2c14-4c3e-b844-86c6a321a158
  - url: 'https://github.com/ljharb/qs/security/advisories/GHSA-6pq8-jhm2-2vcw'
    label: 7ffcee3d-2c14-4c3e-b844-86c6a321a158
tags:
  - nvd
  - cve.org
ingestedAt: '2026-10-14T23:00:02.042Z'
vendor: ljharb
product: qs
affected:
  - qs >= 6.7.0 < 6.16.1
---

## Overview

### Summary



`qs.stringify` with `arrayFormat: 'comma'` throws `TypeError: Cannot convert object to primitive value` when an array element cannot be coerced to a string, such as an object with an own non-callable `toString` (`{ "toString": "x" }`) or a null-prototype object. The comma format joins array elements with `Array.prototype.join` and, under `encodeValuesOnly`, first passes each element through `String()` in `utils.encode`; neither guards against an element whose string coercion fails.



### Details



In `lib/stringify.js`, the comma branch collapses the array with `obj.join(',')`. For an element whose own `toString` is not callable, `ToPrimitive` skips it, falls through to `Object.prototype.valueOf` (which returns the object itself), and throws. Under `encodeValuesOnly: true`, the same element first reaches `String(str)` in `utils.encode` (`lib/utils.js`) and throws there. Nested arrays (`{ "a": [[{ "toString": "x" }]] }`) throw the same way, through `Array.prototype.toString`. The `indices`, `brackets`, and `repeat` formats walk into object elements instead of coercing them, and are not affected.



Such elements can come from untrusted input. `JSON.parse` produces `{ "a": [{ "toString": "x" }] }` with no qs option involved. `qs.parse("a[0][toString]=x", { allowPrototypes: true })` keeps the `toString` key, and `qs.parse("a[][b]=c", { plainObjects: true })` produces a null-prototype element; the default parse options drop `toString` and produce ordinary objects, which do not throw. Express 4's default `query parser`, Express 5's `extended` query parser, and body-parser's `urlencoded({ extended: true })` all call `qs.parse` with `allowPrototypes: true`.



#### PoC



```js



var qs = require("qs");



qs.stringify(JSON.parse("{\"a\":[{\"toString\":\"x\"}]}"), { arrayFormat: "comma" });



qs.stringify(qs.parse("a[0][toString]=x", { allowPrototypes: true }), { arrayFormat: "comma" });



qs.stringify(qs.parse("a[][b]=c", { plainObjects: true }), { arrayFormat: "comma", encodeValuesOnly: true });



// TypeError: Cannot convert object to primitive value



```



#### Fix



Applied in a952e75 on `main` and released as v6.16.1: an element whose string coercion would fail is coerced to its primitive `valueOf`, if any, or else to `Object.prototype.toString`, so it renders as `[object Object]`, the same as any other object element already does under the comma format. Nested arrays are walked the way `join` walks them. Every input that did not previously throw serializes exactly as before, and a `toString` or `valueOf` that itself throws still propagates.



### Affected versions



`>=6.7.0 <6.16.1`, fixed in v6.16.1.



The `join` path was introduced with the comma format in b84acba and first shipped in v6.7.0; v6.6.x and earlier ignore `arrayFormat: 'comma'` and are not affected. The `encodeValuesOnly` `String()` site was introduced in 4c4b23d (v6.11.1); before that, the same input throws at `join`. Every release from v6.7.0 through v6.16.0 is affected.



### Impact



An unauthenticated request can make any code path that re-serializes attacker-influenced data with `qs.stringify` and `arrayFormat: 'comma'` (for example, rebuilding a query string from `req.query` for a redirect or an upstream request, or serializing a parsed JSON body) throw synchronously. In a typical Node.js HTTP framework the throw is caught by the framework error boundary and the affected request returns a 500; the process survives and other requests are unaffected. Where the call runs outside an error boundary, such as an `async` Express 4 handler (where the throw becomes an unhandled promise rejection) or a bare `http` request handler, the process exits, so the impact in that case depends on the application error handling rather than on qs. The default `arrayFormat` (`indices`) is not affected.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.