{"id":"CVE-2026-110910","summary":"qs.stringify with allowEmptyArrays does not encode the keys of empty arrays, allowing query string parameter injection","details":"### Summary\n\n\n\n`qs.stringify` with `allowEmptyArrays: true` writes the key of an empty array without encoding it, even though keys are encoded by default. A key that contains `&` (or the configured `delimiter`), `=`, or `#` therefore becomes query string syntax instead of data, so a caller-supplied key name can add parameters to the serialized query string, or cut it short.\n\n\n\n### Details\n\n\n\nIn `lib/stringify.js`, the `allowEmptyArrays` branch returns `adjustedPrefix + '[]'` early, before the code that passes every other key through `encoder` and `formatter`. So `{ \"x&k=v\": [\"1\"] }` serializes as `x%26k%3Dv%5B0%5D=1`, but `{ \"x&k=v\": [] }` serializes as `x&k=v[]`. The early return applies at any depth (`{ \"a\": { \"b&k=v\": [] } }` gives `a[b&k=v][]`), under every `arrayFormat`, with `allowDots`, and with a custom `encoder`, which is never called for that key. With `encode: false` or `encodeValuesOnly: true`, keys are intentionally left unencoded on every path; those configurations are not affected.\n\n\n\n#### PoC\n\n\n\n```js\n\n\n\nvar qs = require(\"qs\");\n\n\n\nvar query = qs.stringify({ q: \"hi\", \"x&apiKey=evil&y\": [] }, { allowEmptyArrays: true });\n\n\n\n// \"q=hi&x&apiKey=evil&y[]\"\n\n\n\nqs.parse(query);\n\n\n\n// { q: \"hi\", x: \"\", apiKey: \"evil\", y: [\"\"] }\n\n\n\nqs.stringify({ \"x#\": [], z: \"1\" }, { allowEmptyArrays: true });\n\n\n\n// \"x#[]&z=1\": in a URL, everything after \"#\" becomes the fragment\n\n\n\n```\n\n\n\n#### Fix\n\n\n\nApplied in 234d505 on `main` and released as v6.17.0, and backported to v6.16.2, v6.15.4, v6.14.3, v6.13.4, and v6.12.6: the key of an empty array goes through `encoder` and `formatter` exactly as the key of any other value does, so `{ \"x&apiKey=evil&y\": [] }` now serializes as `x%26apiKey%3Devil%26y[]`. Keys that need no encoding serialize as before, with two exceptions that match every other path: the brackets of a nested key are now encoded (`a[b][]` becomes `a%5Bb%5D[]`, which `qs.parse` reads identically), and with `encodeDotInKeys`, a dot in the key becomes `%252E` instead of `%2E` (which `decodeDotInKeys` previously misread as nesting).\n\n\n\n### Affected versions\n\n\n\n- `\u003e=6.12.0 \u003c6.12.6`, fixed in v6.12.6\n\n\n\n- `\u003e=6.13.0 \u003c6.13.4`, fixed in v6.13.4\n\n\n\n- `\u003e=6.14.0 \u003c6.14.3`, fixed in v6.14.3\n\n\n\n- `\u003e=6.15.0 \u003c6.15.4`, fixed in v6.15.4\n\n\n\n- `\u003e=6.16.0 \u003c6.16.2`, fixed in v6.16.2\n\n\n\nv6.17.0 and later are not affected.\n\n\n\n`allowEmptyArrays` was introduced in f22b2bc and first shipped in v6.12.0, with this early return. Earlier versions have no `allowEmptyArrays` option and are not affected.\n\n\n\n### Impact\n\n\n\nAn application that passes an object whose key names come from untrusted input to `qs.stringify` with `allowEmptyArrays: true` (for example, to forward a parsed JSON body or user-selected fields to an upstream service) can have parameters injected into the resulting query string. The injected parameters are not keys of the object, so deleting or overwriting a key on the object (such as setting a trusted `apiKey` after copying the user's fields) does not remove them; whether the attacker's value or the application's wins depends on how the receiving side handles a duplicate parameter. A `#` in such a key ends the query string early when the result is placed in a URL. `allowEmptyArrays` is off by default, and only keys whose value (after any `filter`) is an empty array are affected.","aliases":["GHSA-w4cr-vjpx-p82q"],"modified":"2026-11-11T10:47:29.707354507Z","published":"2026-10-27T06:42:42.365Z","related":["openSUSE-SU-2026:12429-1"],"database_specific":{"cna_assigner":"harborist","cwe_ids":["CWE-116"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/110xxx/CVE-2026-110910.json"},"references":[{"type":"WEB","url":"https://npmjs.com/qs"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/110xxx/CVE-2026-110910.json"},{"type":"ADVISORY","url":"https://github.com/ljharb/qs/security/advisories/GHSA-w4cr-vjpx-p82q"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-110910"},{"type":"REPORT","url":"https://github.com/ljharb/qs/issues/626"},{"type":"FIX","url":"https://github.com/ljharb/qs/commit/033526fc82f2e80a3d896df2bce9137e34cb5dc5"},{"type":"FIX","url":"https://github.com/ljharb/qs/commit/234d5051f5071efaa025a7ca9e4d34a56a460a1b"},{"type":"FIX","url":"https://github.com/ljharb/qs/commit/5d3b1b9ec6afd0cfc760c8e12862873789a32fc8"},{"type":"FIX","url":"https://github.com/ljharb/qs/commit/6b8bf1a53d7b0c5e7e434eaae8e2c0cffd5c2492"},{"type":"FIX","url":"https://github.com/ljharb/qs/commit/ad445aa23e4c3a97394c3b9a5f9d7a7159eb1f17"},{"type":"FIX","url":"https://github.com/ljharb/qs/commit/f9dd4830cb922363822cdfdca2cceb3ed9792224"},{"type":"FIX","url":"https://github.com/ljharb/qs/pull/561"},{"type":"PACKAGE","url":"https://github.com/ljharb/qs"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/ljharb/qs","events":[{"introduced":"062334aa584809829822c077f5f9f41c1e253dab"},{"fixed":"3a29fca09e65c7b3fe9cf61060138a0a2e4025fe"},{"introduced":"5cf516c0dd557d85d5f18d4a916c96cd9cfc2305"},{"fixed":"268ad70b08925d18190c061f3dba25c3bd51bacb"},{"introduced":"32dcc637f90be175c0d4617b429263115a87e3a8"},{"fixed":"7877f73f5efee1f8aec49154f360411c276de2ad"},{"introduced":"d9b4c66303375493c68c42d68e363e50b1753771"},{"fixed":"20d6a8acaaff5dccf71a412680675fbdc6a1a786"},{"introduced":"bb9379e01fad04c601478acd6152143cb20c984b"},{"fixed":"136912a619c2d81c090ca196ea02b6c193580d1d"},{"fixed":"234d5051f5071efaa025a7ca9e4d34a56a460a1b"},{"fixed":"6b8bf1a53d7b0c5e7e434eaae8e2c0cffd5c2492"},{"fixed":"033526fc82f2e80a3d896df2bce9137e34cb5dc5"},{"fixed":"f9dd4830cb922363822cdfdca2cceb3ed9792224"},{"fixed":"ad445aa23e4c3a97394c3b9a5f9d7a7159eb1f17"},{"fixed":"5d3b1b9ec6afd0cfc760c8e12862873789a32fc8"}],"database_specific":{"extracted_events":[{"strategy":"StandardRange","range":[{"introduced":"6.12.0"},{"fixed":"6.12.6"}],"source":"AFFECTED_FIELD"},{"source":"AFFECTED_FIELD","strategy":"StandardRange","range":[{"introduced":"6.13.0"},{"fixed":"6.13.4"}]},{"range":[{"introduced":"6.14.0"},{"fixed":"6.14.3"}],"source":"AFFECTED_FIELD","strategy":"StandardRange"},{"range":[{"introduced":"6.15.0"},{"fixed":"6.15.4"}],"source":"AFFECTED_FIELD","strategy":"StandardRange"},{"strategy":"StandardRange","range":[{"introduced":"6.16.0"},{"fixed":"6.16.2"}],"source":"AFFECTED_FIELD"},{"range":[{"fixed":"234d5051f5071efaa025a7ca9e4d34a56a460a1b"}],"source":"REFERENCES_COMMIT"},{"range":[{"fixed":"6b8bf1a53d7b0c5e7e434eaae8e2c0cffd5c2492"}],"source":"REFERENCES_COMMIT"},{"range":[{"fixed":"033526fc82f2e80a3d896df2bce9137e34cb5dc5"}],"source":"REFERENCES_COMMIT"},{"range":[{"fixed":"f9dd4830cb922363822cdfdca2cceb3ed9792224"}],"source":"REFERENCES_COMMIT"},{"source":"REFERENCES_COMMIT","range":[{"fixed":"ad445aa23e4c3a97394c3b9a5f9d7a7159eb1f17"}]},{"range":[{"fixed":"5d3b1b9ec6afd0cfc760c8e12862873789a32fc8"}],"source":"REFERENCES_COMMIT"}]}}],"versions":["v6.15.3","v6.16.1","v6.14.2","v6.13.3","v6.12.5","v6.16.0","v6.15.2","v6.15.1","v6.15.0","v6.12.4","v6.13.2","v6.12.3","v6.13.1","v6.14.1","v6.14.0","v6.13.0","v6.12.2","v6.12.1","v6.12.0"],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-110910.json"}}],"schema_version":"1.9.2","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N"}]}