### 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.
"}],"value":"### Summary\n\n\n\n`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.\n\n\n\n### Details\n\n\n\nIn `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.\n\n\n\nSuch 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`.\n\n\n\n#### PoC\n\n\n\n```js\n\n\n\nvar qs = require(\"qs\");\n\n\n\nqs.stringify(JSON.parse(\"{\\\"a\\\":[{\\\"toString\\\":\\\"x\\\"}]}\"), { arrayFormat: \"comma\" });\n\n\n\nqs.stringify(qs.parse(\"a[0][toString]=x\", { allowPrototypes: true }), { arrayFormat: \"comma\" });\n\n\n\nqs.stringify(qs.parse(\"a[][b]=c\", { plainObjects: true }), { arrayFormat: \"comma\", encodeValuesOnly: true });\n\n\n\n// TypeError: Cannot convert object to primitive value\n\n\n\n```\n\n\n\n#### Fix\n\n\n\nApplied 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.\n\n\n\n### Affected versions\n\n\n\n`>=6.7.0 <6.16.1`, fixed in v6.16.1.\n\n\n\nThe `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.\n\n\n\n### Impact\n\n\n\nAn 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."}],"metrics":[{"cvssV3_1":{"attackComplexity":"LOW","attackVector":"NETWORK","availabilityImpact":"LOW","baseScore":5.3,"baseSeverity":"MEDIUM","confidentialityImpact":"NONE","integrityImpact":"NONE","privilegesRequired":"NONE","scope":"UNCHANGED","userInteraction":"NONE","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","version":"3.1"},"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}]},{"cvssV4_0":{"Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","Safety":"NOT_DEFINED","attackComplexity":"LOW","attackRequirements":"PRESENT","attackVector":"NETWORK","baseScore":6.3,"baseSeverity":"MEDIUM","exploitMaturity":"NOT_DEFINED","privilegesRequired":"NONE","providerUrgency":"NOT_DEFINED","subAvailabilityImpact":"NONE","subConfidentialityImpact":"NONE","subIntegrityImpact":"NONE","userInteraction":"NONE","valueDensity":"NOT_DEFINED","vectorString":"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","version":"4.0","vulnAvailabilityImpact":"LOW","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","vulnerabilityResponseEffort":"NOT_DEFINED"},"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}]}],"problemTypes":[{"descriptions":[{"cweId":"CWE-248","description":"CWE-248 Uncaught Exception","lang":"en","type":"CWE"}]},{"descriptions":[{"cweId":"CWE-704","description":"CWE-704 Incorrect Type Conversion or Cast","lang":"en","type":"CWE"}]}],"providerMetadata":{"dateUpdated":"2026-10-14T22:05:24.015Z","orgId":"7ffcee3d-2c14-4c3e-b844-86c6a321a158","shortName":"harborist"},"references":[{"tags":["vendor-advisory"],"url":"https://github.com/ljharb/qs/security/advisories/GHSA-6pq8-jhm2-2vcw"},{"tags":["patch"],"url":"https://github.com/ljharb/qs/commit/a952e7519338ef0cfb25694a2988848f93333727"}],"solutions":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"Upgrade to qs 6.16.1 or later.
"}],"value":"Upgrade to qs 6.16.1 or later."}],"source":{"discovery":"EXTERNAL"},"title":"qs.stringify with arrayFormat comma throws TypeError on array elements that cannot be coerced to a string","workarounds":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"Use the default `arrayFormat` (`indices`), or pass a `filter` function to `qs.stringify` that replaces non-primitive array elements before the comma format joins them. Alternatively, wrap `qs.stringify` calls on externally influenced objects in try/catch, and avoid `allowPrototypes: true` / `plainObjects: true` when parsed untrusted input is fed back into `qs.stringify`.
"}],"value":"Use the default `arrayFormat` (`indices`), or pass a `filter` function to `qs.stringify` that replaces non-primitive array elements before the comma format joins them. Alternatively, wrap `qs.stringify` calls on externally influenced objects in try/catch, and avoid `allowPrototypes: true` / `plainObjects: true` when parsed untrusted input is fed back into `qs.stringify`."}]}},"cveMetadata":{"assignerOrgId":"7ffcee3d-2c14-4c3e-b844-86c6a321a158","assignerShortName":"harborist","cveId":"CVE-2026-110816","datePublished":"2026-10-14T22:05:24.015Z","dateReserved":"2026-10-14T21:40:55.805Z","dateUpdated":"2026-10-14T22:05:24.015Z","state":"PUBLISHED"},"dataType":"CVE_RECORD","dataVersion":"5.2"},"nvd":{"publishedDate":"2026-10-14 22:17:23","lastModifiedDate":"2026-10-14 22:17:23","problem_types":["CWE-248","CWE-704","CWE-248 CWE-248 Uncaught Exception","CWE-704 CWE-704 Incorrect Type Conversion or Cast"],"metrics":{"cvssMetricV40":[{"source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158","type":"Secondary","cvssData":{"version":"4.0","vectorString":"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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","baseScore":6.3,"baseSeverity":"MEDIUM","attackVector":"NETWORK","attackComplexity":"LOW","attackRequirements":"PRESENT","privilegesRequired":"NONE","userInteraction":"NONE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","vulnAvailabilityImpact":"LOW","subConfidentialityImpact":"NONE","subIntegrityImpact":"NONE","subAvailabilityImpact":"NONE","exploitMaturity":"NOT_DEFINED","confidentialityRequirement":"NOT_DEFINED","integrityRequirement":"NOT_DEFINED","availabilityRequirement":"NOT_DEFINED","modifiedAttackVector":"NOT_DEFINED","modifiedAttackComplexity":"NOT_DEFINED","modifiedAttackRequirements":"NOT_DEFINED","modifiedPrivilegesRequired":"NOT_DEFINED","modifiedUserInteraction":"NOT_DEFINED","modifiedVulnConfidentialityImpact":"NOT_DEFINED","modifiedVulnIntegrityImpact":"NOT_DEFINED","modifiedVulnAvailabilityImpact":"NOT_DEFINED","modifiedSubConfidentialityImpact":"NOT_DEFINED","modifiedSubIntegrityImpact":"NOT_DEFINED","modifiedSubAvailabilityImpact":"NOT_DEFINED","Safety":"NOT_DEFINED","Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","valueDensity":"NOT_DEFINED","vulnerabilityResponseEffort":"NOT_DEFINED","providerUrgency":"NOT_DEFINED"}}],"cvssMetricV31":[{"source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158","type":"Secondary","cvssData":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","baseScore":5.3,"baseSeverity":"MEDIUM","attackVector":"NETWORK","attackComplexity":"LOW","privilegesRequired":"NONE","userInteraction":"NONE","scope":"UNCHANGED","confidentialityImpact":"NONE","integrityImpact":"NONE","availabilityImpact":"LOW"},"exploitabilityScore":3.9,"impactScore":1.4}]},"configurations":[]},"legacy_mitre":{"record":{"CveYear":"2026","CveId":"110816","Ordinal":"1","Title":"qs.stringify with arrayFormat comma throws TypeError on array el","CVE":"CVE-2026-110816","Year":"2026"},"notes":[{"CveYear":"2026","CveId":"110816","Ordinal":"1","NoteData":"### Summary\n\n\n\n`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.\n\n\n\n### Details\n\n\n\nIn `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.\n\n\n\nSuch 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`.\n\n\n\n#### PoC\n\n\n\n```js\n\n\n\nvar qs = require(\"qs\");\n\n\n\nqs.stringify(JSON.parse(\"{\\\"a\\\":[{\\\"toString\\\":\\\"x\\\"}]}\"), { arrayFormat: \"comma\" });\n\n\n\nqs.stringify(qs.parse(\"a[0][toString]=x\", { allowPrototypes: true }), { arrayFormat: \"comma\" });\n\n\n\nqs.stringify(qs.parse(\"a[][b]=c\", { plainObjects: true }), { arrayFormat: \"comma\", encodeValuesOnly: true });\n\n\n\n// TypeError: Cannot convert object to primitive value\n\n\n\n```\n\n\n\n#### Fix\n\n\n\nApplied 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.\n\n\n\n### Affected versions\n\n\n\n`>=6.7.0 <6.16.1`, fixed in v6.16.1.\n\n\n\nThe `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.\n\n\n\n### Impact\n\n\n\nAn 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.","Type":"Description","Title":"qs.stringify with arrayFormat comma throws TypeError on array el"}]}}}