{"id":"CVE-2026-110816","summary":"qs.stringify with arrayFormat comma throws TypeError on array elements that cannot be coerced to a string","details":"### 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`\u003e=6.7.0 \u003c6.16.1`, 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.","aliases":["GHSA-6pq8-jhm2-2vcw"],"modified":"2026-10-15T10:47:27.705586014Z","published":"2026-10-14T22:05:24.015Z","database_specific":{"cna_assigner":"harborist","cwe_ids":["CWE-248","CWE-704"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/110xxx/CVE-2026-110816.json","unresolved_ranges":[{"source":"AFFECTED_FIELD","extracted_events":[{"introduced":"6.7.0"},{"fixed":"6.16.1"}]}]},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/110xxx/CVE-2026-110816.json"},{"type":"PACKAGE","url":"https://github.com/ljharb/qs"},{"type":"FIX","url":"https://github.com/ljharb/qs/commit/a952e7519338ef0cfb25694a2988848f93333727"},{"type":"ADVISORY","url":"https://github.com/ljharb/qs/security/advisories/GHSA-6pq8-jhm2-2vcw"},{"type":"WEB","url":"https://npmjs.com/qs"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-110816"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/ljharb/qs","events":[{"introduced":"125e103b61f2bef245970f5a2a8dceffe5aab59a"},{"fixed":"bb9379e01fad04c601478acd6152143cb20c984b"}],"database_specific":{"extracted_events":[{"introduced":"v6.7.0"},{"fixed":"v6.16.0"}],"source":"DESCRIPTION"}}],"versions":["v6.15.3","v6.15.2","v6.15.1","v6.15.0","v6.14.2","v6.10.5","v6.11.2","v6.12.3","v6.13.1","v6.14.1","v6.14.0","v6.13.0","v6.12.2","v6.12.1","v6.12.0","v6.11.1","v6.11.0","v6.10.4","v6.10.3","v6.10.2","v6.10.1","v6.10.0","v6.9.6","v6.9.5","v6.9.4","v6.9.3","v6.9.2","v6.9.1","v6.9.0","v6.8.0","v6.7.0"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-110816.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}]}