{"cve":"CVE-2026-82417","nvd":{"published":"2026-08-30T00:16:34.657","modified":"2026-09-03T18:00:36.970","status":"Awaiting Analysis","description":"### Summary\n\n\n\n`qs.stringify` throws a `TypeError` when it serializes an object whose own `constructor` property has a truthy, non-callable `isBuffer` member. `utils.isBuffer` duck-types buffers by calling `obj.constructor.isBuffer(obj)` after checking only that the property is truthy, so a value such as `{ constructor: { isBuffer: \"x\" } }` makes the call throw `TypeError: obj.constructor.isBuffer is not a function`.\n\n\n\n### Details\n\n\n\n`lib/stringify.js:127` calls `utils.isBuffer` on every non-primitive value it serializes. `utils.isBuffer` (`lib/utils.js:332`) reads `obj.constructor.isBuffer` and invokes it without verifying that it is a function. `constructor` and `isBuffer` are ordinary property names, so any object carrying them as own properties reaches the unchecked call.\n\n\n\nSuch an object can be built from untrusted input. `qs.parse(\"x[constructor][isBuffer]=y\", { plainObjects: true })` or `{ allowPrototypes: true }` keeps the `constructor` key as an own property (the default parse options drop it), and `JSON.parse(\"{\\\"a\\\":{\\\"constructor\\\":{\\\"isBuffer\\\":\\\"x\\\"}}}\")` produces the same shape with no qs option involved. Express 4 with its default `query parser` setting and body-parser with `extended: true` both call `qs.parse` with `allowPrototypes: true`, so on those stacks `req.query` and `req.body` can carry the shape directly.\n\n\n\n#### PoC\n\n\n\n```js\n\n\n\nvar qs = require(\"qs\");\n\n\n\nqs.stringify(qs.parse(\"x[constructor][isBuffer]=y\", { plainObjects: true }));\n\n\n\nqs.stringify(JSON.parse(\"{\\\"a\\\":{\\\"constructor\\\":{\\\"isBuffer\\\":\\\"x\\\"}}}\"));\n\n\n\n// TypeError: obj.constructor.isBuffer is not a function\n\n\n\n//     at Object.isBuffer (lib/utils.js:332:78)\n\n\n\n//     at stringify (lib/stringify.js:127:45)\n\n\n\n```\n\n\n\n#### Fix\n\n\n\n`lib/utils.js`, applied in e83d321 on `main` and released as v6.16.0:\n\n\n\n```diff\n\n\n\n- return !!(obj.constructor && obj.constructor.isBuffer && obj.constructor.isBuffer(obj));\n\n\n\n+ return !!(obj.constructor && typeof obj.constructor.isBuffer === \"function\" && obj.constructor.isBuffer(obj));\n\n\n\n```\n\n\n\nReal `Buffer`, `safer-buffer`, and browserify `buffer` polyfill instances serialize exactly as before; only the throw is removed.\n\n\n\n### Affected versions\n\n\n\n`>=2.2.5 <6.16.0`, fixed in v6.16.0.\n\n\n\nThe unguarded duck-type was introduced in 3768a75 and first shipped in v2.2.5 (September 2014). v2.2.4 and earlier used `Buffer.isBuffer` and are not affected. Every release from v2.2.5 through v6.15.3 contains the unguarded call.\n\n\n\n### Impact\n\n\n\nAn unauthenticated request can make any code path that re-serializes attacker-influenced data with `qs.stringify` (for example, rebuilding a query string from `req.query` for a redirect or an upstream request, or serializing a parsed JSON body) throw synchronously. In a typical Node.js HTTP framework the throw is caught by the framework error boundary and the affected request returns a 500; the process survives and other requests are unaffected. Where the call runs outside an error boundary, such as an `async` Express 4 handler (where the throw becomes an unhandled promise rejection) or a background job, the process exits, so the impact in that case depends on the application error handling rather than on qs.","cvss_v4":{"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N","score":6.3},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","score":5.3},"cwes":["CWE-248","CWE-703"],"references":["https://github.com/ljharb/qs/commit/e83d321ffafb38cf210683ac31714fce6ce1c6c6","https://github.com/ljharb/qs/security/advisories/GHSA-4mjr-xmp4-gh2g","https://github.com/ljharb/qs/security/advisories/GHSA-4mjr-xmp4-gh2g"],"products":[],"source":"nvd","synced_at":"2026-10-19T22:50:50.842544443+00:00"},"epss":{"cve":"CVE-2026-82417","score":0.00396,"percentile":0.33816,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-4mjr-xmp4-gh2g","url":"https://osv.dev/vulnerability/GHSA-4mjr-xmp4-gh2g","summary":"qs: Denial of Service via Attacker Controlled isBuffer","modified":"2026-09-10T03:51:14.991984096Z","packages":[{"ecosystem":"npm","name":"qs","purl":"pkg:npm/qs","ranges":[">=2.2.5 <6.16.0"],"fixed":["6.16.0"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}