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