{"id":"UBUNTU-CVE-2026-110910","details":"### 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 - `\u003e=6.12.0 \u003c6.12.6`, fixed in v6.12.6 - `\u003e=6.13.0 \u003c6.13.4`, fixed in v6.13.4 - `\u003e=6.14.0 \u003c6.14.3`, fixed in v6.14.3 - `\u003e=6.15.0 \u003c6.15.4`, fixed in v6.15.4 - `\u003e=6.16.0 \u003c6.16.2`, 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.","modified":"2026-11-04T03:30:19.741723127Z","published":"2026-11-03T00:00:00Z","upstream":["CVE-2026-110910"],"references":[{"type":"REPORT","url":"https://ubuntu.com/security/CVE-2026-110910"},{"type":"REPORT","url":"https://www.cve.org/CVERecord?id=CVE-2026-110910"},{"type":"REPORT","url":"https://github.com/ljharb/qs/security/advisories/GHSA-w4cr-vjpx-p82q"},{"type":"REPORT","url":"https://github.com/ljharb/qs/issues/626"},{"type":"REPORT","url":"https://github.com/ljharb/qs/commit/033526fc82f2e80a3d896df2bce9137e34cb5dc5"},{"type":"REPORT","url":"https://github.com/ljharb/qs/commit/234d5051f5071efaa025a7ca9e4d34a56a460a1b"},{"type":"REPORT","url":"https://github.com/ljharb/qs/commit/5d3b1b9ec6afd0cfc760c8e12862873789a32fc8"},{"type":"REPORT","url":"https://github.com/ljharb/qs/commit/6b8bf1a53d7b0c5e7e434eaae8e2c0cffd5c2492"},{"type":"REPORT","url":"https://github.com/ljharb/qs/commit/ad445aa23e4c3a97394c3b9a5f9d7a7159eb1f17"},{"type":"REPORT","url":"https://github.com/ljharb/qs/commit/f9dd4830cb922363822cdfdca2cceb3ed9792224"},{"type":"REPORT","url":"https://github.com/ljharb/qs/pull/561"}],"affected":[{"package":{"name":"node-qs","ecosystem":"Ubuntu:Pro:14.04:LTS","purl":"pkg:deb/ubuntu/node-qs?arch=source&distro=esm-infra-legacy%2Ftrusty"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["0.4.2-1","0.6.5-1","0.6.5-1ubuntu0.1~esm1"],"ecosystem_specific":{"binaries":[{"binary_version":"0.6.5-1ubuntu0.1~esm1","binary_name":"node-qs"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-110910.json"}},{"package":{"name":"node-qs","ecosystem":"Ubuntu:16.04:LTS","purl":"pkg:deb/ubuntu/node-qs?arch=source&distro=xenial"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.2.4-1"],"ecosystem_specific":{"binaries":[{"binary_name":"node-qs","binary_version":"2.2.4-1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-110910.json"}},{"package":{"name":"node-qs","ecosystem":"Ubuntu:18.04:LTS","purl":"pkg:deb/ubuntu/node-qs?arch=source&distro=bionic"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.2.4-1","2.2.4-1ubuntu1"],"ecosystem_specific":{"binaries":[{"binary_name":"node-qs","binary_version":"2.2.4-1ubuntu1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-110910.json"}},{"package":{"name":"node-qs","ecosystem":"Ubuntu:Pro:20.04:LTS","purl":"pkg:deb/ubuntu/node-qs?arch=source&distro=esm-apps%2Ffocal"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["6.5.2-1","6.9.0+ds-1","6.9.1+ds-1","6.9.1+ds-1ubuntu0.1~esm1"],"ecosystem_specific":{"binaries":[{"binary_name":"node-qs","binary_version":"6.9.1+ds-1ubuntu0.1~esm1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-110910.json"}},{"package":{"name":"node-qs","ecosystem":"Ubuntu:22.04:LTS","purl":"pkg:deb/ubuntu/node-qs?arch=source&distro=jammy"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["6.9.4+ds-1","6.10.1+ds-1","6.10.2+ds+~6.9.7-1","6.10.3+ds+~6.9.7-1"],"ecosystem_specific":{"binaries":[{"binary_version":"6.10.3+ds+~6.9.7-1","binary_name":"node-qs"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-110910.json"}},{"package":{"name":"node-qs","ecosystem":"Ubuntu:24.04:LTS","purl":"pkg:deb/ubuntu/node-qs?arch=source&distro=noble"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["6.11.0+ds+~6.9.7-3","6.11.0+ds+~6.9.7-4"],"ecosystem_specific":{"binaries":[{"binary_name":"node-qs","binary_version":"6.11.0+ds+~6.9.7-4"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-110910.json"}},{"package":{"name":"node-qs","ecosystem":"Ubuntu:26.10","purl":"pkg:deb/ubuntu/node-qs?arch=source&distro=stonking"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["6.14.1+ds+~6.14.0-1","6.15.2+ds+~6.15.1-1"],"ecosystem_specific":{"binaries":[{"binary_name":"node-qs","binary_version":"6.15.2+ds+~6.15.1-1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-110910.json"}},{"package":{"name":"node-qs","ecosystem":"Ubuntu:26.04:LTS","purl":"pkg:deb/ubuntu/node-qs?arch=source&distro=resolute"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["6.13.0+ds+~6.9.16-1","6.14.1+ds+~6.14.0-1"],"ecosystem_specific":{"binaries":[{"binary_version":"6.14.1+ds+~6.14.0-1","binary_name":"node-qs"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-110910.json"}}],"schema_version":"1.9.2","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N"},{"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"},{"type":"Ubuntu","score":"medium"}]}