# CVE-2026-82417

## Summary

- **CVE ID:** CVE-2026-82417
- **Severity:** MEDIUM
- **CVSS Score:** 6.3 (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)
- **CWE:** CWE-248, CWE-703
- **Published:** Aug 29, 2026
- **Last Modified:** Aug 31, 2026

## Description

### Summary



`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`.



### Details



`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.



Such 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.



#### PoC



```js



var qs = require("qs");



qs.stringify(qs.parse("x[constructor][isBuffer]=y", { plainObjects: true }));



qs.stringify(JSON.parse("{\"a\":{\"constructor\":{\"isBuffer\":\"x\"}}}"));



// TypeError: obj.constructor.isBuffer is not a function



//     at Object.isBuffer (lib/utils.js:332:78)



//     at stringify (lib/stringify.js:127:45)



```



#### Fix



`lib/utils.js`, applied in e83d321 on `main` and released as v6.16.0:



```diff



- return !!(obj.constructor && obj.constructor.isBuffer && obj.constructor.isBuffer(obj));



+ return !!(obj.constructor && typeof obj.constructor.isBuffer === "function" && obj.constructor.isBuffer(obj));



```



Real `Buffer`, `safer-buffer`, and browserify `buffer` polyfill instances serialize exactly as before; only the throw is removed.



### Affected versions



`>=2.2.5 <6.16.0`, fixed in v6.16.0.



The 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.



### Impact



An 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.

## Affected Products

- ljharb — qs (2.2.5)

## References

- [CNA](https://github.com/ljharb/qs/security/advisories/GHSA-4mjr-xmp4-gh2g)
- [CNA](https://github.com/ljharb/qs/commit/e83d321ffafb38cf210683ac31714fce6ce1c6c6)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.26%
- **EPSS Percentile:** 17.8

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._