### Summary
`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.
### Details
In `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.
#### PoC
```js
var qs = require(\"qs\");
var query = qs.stringify({ q: \"hi\", \"x&apiKey=evil&y\": [] }, { allowEmptyArrays: true });
// \"q=hi&x&apiKey=evil&y[]\"
qs.parse(query);
// { q: \"hi\", x: \"\", apiKey: \"evil\", y: [\"\"] }
qs.stringify({ \"x#\": [], z: \"1\" }, { allowEmptyArrays: true });
// \"x#[]&z=1\": in a URL, everything after \"#\" becomes the fragment
```
#### Fix
Applied 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).
### Affected versions
- `>=6.12.0 <6.12.6`, fixed in v6.12.6
- `>=6.13.0 <6.13.4`, fixed in v6.13.4
- `>=6.14.0 <6.14.3`, fixed in v6.14.3
- `>=6.15.0 <6.15.4`, fixed in v6.15.4
- `>=6.16.0 <6.16.2`, fixed in v6.16.2
v6.17.0 and later are not affected.
`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.
### Impact
An 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.
"}]}],"references":[{"url":"https://github.com/ljharb/qs/security/advisories/GHSA-w4cr-vjpx-p82q","tags":["vendor-advisory"]},{"url":"https://github.com/ljharb/qs/commit/234d5051f5071efaa025a7ca9e4d34a56a460a1b","tags":["patch"]},{"url":"https://github.com/ljharb/qs/commit/6b8bf1a53d7b0c5e7e434eaae8e2c0cffd5c2492","tags":["patch"]},{"url":"https://github.com/ljharb/qs/commit/033526fc82f2e80a3d896df2bce9137e34cb5dc5","tags":["patch"]},{"url":"https://github.com/ljharb/qs/commit/f9dd4830cb922363822cdfdca2cceb3ed9792224","tags":["patch"]},{"url":"https://github.com/ljharb/qs/commit/ad445aa23e4c3a97394c3b9a5f9d7a7159eb1f17","tags":["patch"]},{"url":"https://github.com/ljharb/qs/commit/5d3b1b9ec6afd0cfc760c8e12862873789a32fc8","tags":["patch"]},{"url":"https://github.com/ljharb/qs/pull/561","tags":["patch"]},{"url":"https://github.com/ljharb/qs/issues/626","tags":["issue-tracking"]}],"metrics":[{"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}],"cvssV3_1":{"version":"3.1","attackVector":"NETWORK","attackComplexity":"HIGH","privilegesRequired":"NONE","userInteraction":"NONE","scope":"UNCHANGED","confidentialityImpact":"NONE","integrityImpact":"LOW","availabilityImpact":"NONE","baseSeverity":"LOW","baseScore":3.7,"vectorString":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N"}},{"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}],"cvssV4_0":{"attackVector":"NETWORK","attackComplexity":"LOW","attackRequirements":"PRESENT","privilegesRequired":"NONE","userInteraction":"NONE","vulnConfidentialityImpact":"NONE","subConfidentialityImpact":"NONE","vulnIntegrityImpact":"LOW","subIntegrityImpact":"NONE","vulnAvailabilityImpact":"NONE","subAvailabilityImpact":"NONE","exploitMaturity":"NOT_DEFINED","Safety":"NOT_DEFINED","Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","valueDensity":"NOT_DEFINED","vulnerabilityResponseEffort":"NOT_DEFINED","providerUrgency":"NOT_DEFINED","version":"4.0","baseSeverity":"MEDIUM","baseScore":6.3,"vectorString":"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"}}],"workarounds":[{"lang":"en","value":"Leave `allowEmptyArrays` off (the default), or check key names that come from untrusted input against an allowlist before passing the object to `qs.stringify`.","supportingMedia":[{"type":"text/html","base64":false,"value":"Leave `allowEmptyArrays` off (the default), or check key names that come from untrusted input against an allowlist before passing the object to `qs.stringify`.
"}]}],"solutions":[{"lang":"en","value":"Upgrade to qs 6.17.0 or later, or, on an older release line, to 6.12.6, 6.13.4, 6.14.3, 6.15.4, or 6.16.2.","supportingMedia":[{"type":"text/html","base64":false,"value":"Upgrade to qs 6.17.0 or later, or, on an older release line, to 6.12.6, 6.13.4, 6.14.3, 6.15.4, or 6.16.2.
"}]}],"credits":[{"lang":"en","value":"DucMinhNe","type":"finder"},{"lang":"en","value":"DucMinhNe","type":"remediation developer"},{"lang":"en","value":"ljharb","type":"remediation reviewer"}],"source":{"discovery":"EXTERNAL"}}}}