{"id":"CVE-2026-110910","url":"https://www.cyber-defence.io/tools/cve/CVE-2026-110910","generatedAt":"2026-10-27T22:46:40.290Z","title":"### Summary `qs.stringify` with `allowEmptyArrays: true` writes the key of an empty array without encoding it, even though keys are encoded by default.","description":"### 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- `>=6.12.0 <6.12.6`, fixed in v6.12.6\n\n\n\n- `>=6.13.0 <6.13.4`, fixed in v6.13.4\n\n\n\n- `>=6.14.0 <6.14.3`, fixed in v6.14.3\n\n\n\n- `>=6.15.0 <6.15.4`, fixed in v6.15.4\n\n\n\n- `>=6.16.0 <6.16.2`, 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.","published":"2026-10-27T07:16:32.000Z","lastModified":"2026-10-27T20:17:12.000Z","status":"Awaiting Analysis","sourceIdentifier":"7ffcee3d-2c14-4c3e-b844-86c6a321a158","cvss":{"version":"4.0","score":6.3,"severity":"MEDIUM","vector":"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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/M"},"cwe":["CWE-116"],"affected":[],"epss":null,"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"low","text":"Lower severity and no EPSS score yet. Track it; it rarely justifies an emergency change on its own."},"changes":[{"kind":"new","label":"New CVE","at":"2026-10-27T07:16:32.000Z","detail":{"score":"6.3","severity":"MEDIUM","publishedAt":"2026-10-27T07:16:32.263Z"},"summary":"New CVE, medium severity (CVSS 6.3)."}],"references":[{"url":"https://github.com/ljharb/qs/commit/033526fc82f2e80a3d896df2bce9137e34cb5dc5","source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158"},{"url":"https://github.com/ljharb/qs/commit/234d5051f5071efaa025a7ca9e4d34a56a460a1b","source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158"},{"url":"https://github.com/ljharb/qs/commit/5d3b1b9ec6afd0cfc760c8e12862873789a32fc8","source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158"},{"url":"https://github.com/ljharb/qs/commit/6b8bf1a53d7b0c5e7e434eaae8e2c0cffd5c2492","source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158"},{"url":"https://github.com/ljharb/qs/commit/ad445aa23e4c3a97394c3b9a5f9d7a7159eb1f17","source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158"},{"url":"https://github.com/ljharb/qs/commit/f9dd4830cb922363822cdfdca2cceb3ed9792224","source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158"},{"url":"https://github.com/ljharb/qs/issues/626","source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158"},{"url":"https://github.com/ljharb/qs/pull/561","source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158"},{"url":"https://github.com/ljharb/qs/security/advisories/GHSA-w4cr-vjpx-p82q","source":"7ffcee3d-2c14-4c3e-b844-86c6a321a158"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-110910","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}