CVE-2026-8723 qs.stringify crashes on null/undefined entries in comma-format arrays under encodeValuesOnly

What is CVE-2026-8723?

CVE-2026-8723 is a medium-severity vulnerability in ljharb qs, published on 16 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

### Summary

`qs.stringify` throws `TypeError` when called with `arrayFormat: 'comma'` and `encodeValuesOnly: true` on an array containing `null` or `undefined`. The throw is synchronous and not handled by any of qs's null-related options (`skipNulls`, `strictNullHandling`).

### Details

In the comma + `encodeValuesOnly` branch, `lib/stringify.js:145` mapped the array through the raw encoder before joining:

```js

obj = utils.maybeMap(obj, encoder);

```

`utils.encode` (`lib/utils.js:195`) reads `str.length` with no null guard, so a `null` or `undefined` element throws `TypeError`. `skipNulls` and `strictNullHandling` are both checked in the per-element loop below this line and never get a chance to run.

Same class of bug as the filter-array path fixed in 0c180a4. The vulnerable shape of the comma + `encodeValuesOnly` branch was introduced in 4c4b23d ("encode comma values more consistently", PR #463, 2023-01-19), first released in v6.11.1.

#### PoC

```js

const qs = require('qs');

qs.stringify({ a: [null, 'b'] }, { arrayFormat: 'comma', encodeValuesOnly: true });

qs.stringify({ a: [undefined, 'b'] }, { arrayFormat: 'comma', encodeValuesOnly: true });

qs.stringify({ a: [null] }, { arrayFormat: 'comma', encodeValuesOnly: true });

// TypeError: Cannot read properties of null (reading 'length')

// at encode (lib/utils.js:195:13)

// at Object.maybeMap (lib/utils.js:322:37)

// at stringify (lib/stringify.js:145:25)

```

#### Fix

`lib/stringify.js:145`, applied in 21f80b3 on `main` and released as v6.15.2:

```diff

- obj = utils.maybeMap(obj, encoder);

+ obj = utils.maybeMap(obj, function (v) {

+ return v == null ? v : encoder(v);

+ });

```

`null` and `undefined` now pass through `maybeMap` unchanged and reach the `join(',')` step as-is. For `{ a: [null, 'b'] }` this produces `a=,b`, matching the non-`encodeValuesOnly` comma path (which already joins before encoding and produces `a=%2Cb` for the same input). Single-element `[null]` arrays still collapse via the existing `obj.join(',') || null` and remain subject to `skipNulls` / `strictNullHandling` in the main loop.

### Affected versions

`>=6.11.1 <6.15.2` — fixed in v6.15.2.

The vulnerable code shape was introduced in 4c4b23d and first shipped in v6.11.1. Earlier versions — including all of 6.7.x, 6.8.x, 6.9.x, 6.10.x, and 6.11.0 — implemented the comma + `encodeValuesOnly` path differently (joining before encoding) and are not affected. Empirically verified across released versions.

### Impact

Application code that calls `qs.stringify` with both `arrayFormat: 'comma'` and `encodeValuesOnly: true` (both non-default) on input that may contain a `null` or `undefined` array element will throw synchronously instead of producing a query string. In a typical Node.js HTTP framework (Express, Fastify, Koa, hapi) the sync throw is caught by the framework's error boundary and the affected request returns a 500; the worker process does not exit and subsequent requests are unaffected. The "kills the worker process" framing applies only to call sites outside a request-handler error boundary (background jobs, startup paths, stream pipelines) or to deployments with framework error handling explicitly disabled.

The vulnerable input is a `null` or `undefined` entry inside an array; this is reachable from JSON request bodies or from application code constructing arrays from user input, but not from standard HTML form submissions (which produce strings or omitted fields, not literal `null`).

Weakness: CWE-476

Which versions are affected by CVE-2026-8723?

2 open source packages ship an affected version, including qs.

How do I fix CVE-2026-8723?

Upgrade qs to 6.15.2 or later.

Is CVE-2026-8723 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-8723 exists.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as AZL-87105, AZL-87108, CLEANSTART-2026-BN31244, CLEANSTART-2026-LT80709, CLEANSTART-2026-PI13683, CLSA-2026-1788262760 and 38 more.