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Response snapshot Oct 27, 2026, 17:26 UTC

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VulnerabilityCVE-2026-110816

qs : qs.stringify with arrayFormat 'comma' throws an uncaught TypeError on array elements carrying a non-callable own toString (DoS)

`qs.stringify(obj, { arrayFormat: 'comma' })` throws an uncaught `TypeError: Cannot convert object to primitive value` when an array in `obj` contains an element that carries an own, non-callable `toString` property (for example `{ a: [ { toString: 'x' } ] }`).

Published Updated

At a glance

Exploited
Not known
Severity
6.3 Medium (CVSS 4.0)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: qs: 6.16.1. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-6pq8-jhm2-2vcw (opens a new tab)

What happened

## Summary

`qs.stringify(obj, { arrayFormat: 'comma' })` throws an uncaught `TypeError: Cannot convert object to primitive value` when an array in `obj` contains an element that carries an own, non-callable `toString` property (for example `{ a: [ { toString: 'x' } ] }`). A remote, unauthenticated attacker who can influence an object that the application later re-serializes with the comma array format triggers the throw.

In a service that lacks a synchronous error boundary around the serialization call, the uncaught exception crashes the worker process, producing a denial of service.

## Root Cause

Continue reading

When `arrayFormat: 'comma'` is selected, the comma branch collapses the whole array into a single value via `Array.prototype.join`, or, under `encodeValuesOnly: true`, maps each element through the value encoder before joining. Both paths perform an implicit string coercion (`ToPrimitive` with hint `"string"`) on each element.

For an object whose own `toString` is a non-callable value, coercion skips the shadowed non-callable `toString`, falls through to `Object.prototype.valueOf` (which returns the object rather than a primitive), and throws.

The comma branch runs this coercion before the per-element null and skip handling in the main loop, so nothing downstream can neutralize the hostile element, and the throw is never caught inside the library.

```text lib/stringify.js (comma-array branch) line 152-154: obj = utils.maybeMap(obj, function (v) { return v == null ? v : encoder(v); }); // encodeValuesOnly path line 156: objKeys = [{ value: obj.length > 0 ? obj.join(',') || null : void undefined }]; // default comma path lib/utils.js line 292: string = String(str); // value encoder coercion reached via the encodeValuesOnly path ```

## Impact

Prerequisites: the application calls `qs.stringify` with `arrayFormat: 'comma'` (a documented, supported option) on an object whose shape can be influenced by attacker input. Attacker-controllable sources include `qs.

parse(input, { allowPrototypes: true })`, which retains a `toString` segment (Express 4's default query parser, Express 5's `extended` query parser, and body-parser's `extended` urlencoded parser all set it), `qs.parse(input, { plainObjects: true })`, whose null-prototype elements throw even without a `toString` key (eg, `a[][b]=c`), and `JSON.parse` of a request body, which needs no qs parse option at all.

No authentication or user interaction is required. Demonstrated capability: a single request forces an uncaught synchronous `TypeError` in the serialization call. In a naive single-process Node service with no synchronous error boundary, this crashes the process and takes the service offline (availability impact).

Under a framework such as Express that traps synchronous handler throws, the plausible outcome is a per-request 500 rather than a full outage; that lower-impact case is noted but not the worst credible supported deployment. There is no confidentiality or integrity impact, no memory unsafety, and no prototype pollution.

## Proof of Concept

`Install qs at the affected version (for example 6.16.0) and run the following script with Node.js. No network setup is required; it demonstrates the throw in isolation.`

```js var qs = require('qs');

// Source A: qs.parse with allowPrototypes retains the hostile toString segment qs.stringify(qs.parse('a[0][toString]=x', { allowPrototypes: true }), { arrayFormat: 'comma' });

// Source B: a JSON request body, no qs parse option needed qs.stringify(JSON.parse('{"a":[{"toString":"x"}]}'), { arrayFormat: 'comma' });

// Also affected: the encodeValuesOnly sub-path qs.stringify(JSON.parse('{"a":[{"toString":"x"}]}'), { arrayFormat: 'comma', encodeValuesOnly: true }); ```

```text A default-join (allowPrototypes) => THREW: TypeError: Cannot convert object to primitive value B default-join (JSON.parse) => THREW: TypeError: Cannot convert object to primitive value C encodeValuesOnly path => THREW: TypeError: Cannot convert object to primitive value N2 callable toString => NO THROW; output="a=z" N3 isBuffer vector (CVE-2026-82417) => NO THROW; output="a%5Bconstructor%5D%5BisBuffer%5D=x" qs version: 6.16.0 ```

## Attack Chain

1. **Exposure:** A remote, unauthenticated request reaches an endpoint whose handler eventually serializes attacker-influenced data with `qs.stringify(obj, { arrayFormat: 'comma' })`. 2. **Control:** The attacker supplies data that becomes `{ a: [ { toString: } ] }`, either via `a[0][toString]=x` parsed with `allowPrototypes`, or via a JSON body `{"a":[{"toString":"x"}]}`. 3.

**Path:** In `stringify`, the primitive/buffer guard at `lib/stringify.js:134` is false for an array, so control reaches the comma branch at `lib/stringify.js:149`. 4. **Guard:** On the `encodeValuesOnly` path, the closure `v == null ? v : encoder(v)` at `lib/stringify.js:152-154` passes only `null`/`undefined` through; a non-null object still reaches the encoder.

The default path skips that closure entirely and reaches `obj.join(',')` at `lib/stringify.js:156` with no guard at all. 5. **Primitive:** The element is coerced to a string (`Array.prototype.join`, or `String(str)` at `lib/utils.js:292`). `ToPrimitive` skips the non-callable own `toString`, `valueOf` returns the object, and a `TypeError` is thrown. 6.

**Result:** The uncaught synchronous exception propagates out of the serialization call, failing the request and, in a service without a synchronous error boundary, crashing the process (denial of service).

## Bypass Evidence

This is a different element type on the code path CVE-2026-8723 (GHSA-q8mj-m7cp-5q26) patched, plus the default comma path, which that fix never covered. CVE-2026-8723 addressed a `TypeError` crash on `null`/`undefined` entries in the comma plus `encodeValuesOnly` path. Its fix (commit 21f80b3) wraps the encoder in `function (v) { return v == null ? v : encoder(v); }`, so it only passes `null` and `undefined` through. A non-null element with a hostile own `toString` fa

Affected package ranges reported by GitHub: qs: >= 6.7.0, <= 6.16.0

Scores and how they were measured

CVSS base severity

6.3

Version 4.0As of

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-6pq8-jhm2-2vcw (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-6pq8-jhm2-2vcw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 27). qs : qs.stringify with arrayFormat 'comma' throws an uncaught TypeError on array elements carrying a non-callable own toString (DoS) (CVE-2026-110816). https://kevwire.com/record/CVE-2026-110816

MLA

"qs : qs.stringify with arrayFormat 'comma' throws an uncaught TypeError on array elements carrying a non-callable own toString (DoS) (CVE-2026-110816)." KEVWire, 27 Oct. 2026, kevwire.com/record/CVE-2026-110816.

Chicago

KEVWire. "qs : qs.stringify with arrayFormat 'comma' throws an uncaught TypeError on array elements carrying a non-callable own toString (DoS) (CVE-2026-110816)." Last modified October 27, 2026. https://kevwire.com/record/CVE-2026-110816.

Evidence summary (plain text)

qs : qs.stringify with arrayFormat 'comma' throws an uncaught TypeError on array elements carrying a non-callable own toString (DoS) (Oct 27, 2026).
`qs.stringify(obj, { arrayFormat: 'comma' })` throws an uncaught `TypeError: Cannot convert object to primitive value` when an array in `obj` contains an element that carries an own, non-callable `toString` property (for example `{ a: [ { toString: 'x' } ] }`). A remote, unauthenticated attacker who can influence an object that the application later re-serializes with the comma array format triggers the throw. In a service that lacks a synchronous error boundary around the serialization call, the uncaught exception crashes the worker process, producing a denial of service.

When `arrayFormat: 'comma'` is selected, the comma branch collapses the whole array into a single value via `Array.prototype.join`, or, under `encodeValuesOnly: true`, maps each element through the value encoder before joining. Both paths perform an implicit string coercion (`ToPrimitive` with hint `"string"`) on each element. For an object whose own `toString` is a non-callable value, coercion skips the shadowed non-callable `toString`, falls through to `Object.prototype.valueOf` (which returns the object rather than a primitive), and throws. The comma branch runs this coercion before the per-element null and skip handling in the main loop, so nothing downstream can neutralize the hostile element, and the throw is never caught inside the library.
CVSS 6.3 (version 4.0; evidence as of Oct 27, 2026; https://github.com/advisories/GHSA-6pq8-jhm2-2vcw).
Sources:
GitHub-reviewed advisory GHSA-6pq8-jhm2-2vcw: https://github.com/advisories/GHSA-6pq8-jhm2-2vcw
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-110816.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 17:25 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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