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 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.