Summary "qs.stringify" with "arrayFormat: 'comma'" throws "TypeError: Cannot convert object to primitive value" when an array element cannot be coerced to a string, such as an object with an own non-callable "toString" ("{ "toString": "x" }") or a null-prototype object. The comma format joins array elements with "Array.prototype.join" and, under "encodeValuesOnly", first passes each element through "String()" in "utils.encode"; neither guards against an element whose string coercion fails. Details In "lib/stringify.js", the comma branch collapses the array with "obj.join(',')". For an element whose own "toString" is not callable, "ToPrimitive" skips it, falls through to "Object.prototype.valueOf" (which returns the object itself), and throws. Under "encodeValuesOnly: true", the same element first reaches "String(str)" in "utils.encode" ("lib/utils.js") and throws there. Nested arrays ("{ "a": [[{ "toString": "x" }]] }") throw the same way, through "Array.prototype.toString". The "indices", "brackets", and "repeat" formats walk into object elements instead of coercing them, and are not affected. Such elements can come from untrusted input. "JSON.parse" produces "{ "a": [{ "toString": "x" }] }" with no qs option involved. "qs.parse("a[0][toString]=x", { allowPrototypes: true })" keeps the "toString" key, and "qs.parse("a[][b]=c", { plainObjects: true })" produces a null-prototype element; the default parse options drop "toString" and produce ordinary objects, which do not throw. Express 4's default "query parser", Express 5's "extended" query parser, and body-parser's "urlencoded({ extended: true })" all call "qs.parse" with "allowPrototypes: true". PoC Fix Applied in a952e75 on "main" and released as v6.16.1: an element whose string coercion would fail is coerced to its primitive "valueOf", if any, or else to "Object.prototype.toString", so it renders as "[object Object]", the same as any other object element already does under the comma format. Nested arrays are walked the way "join" walks them. Every input that did not previously throw serializes exactly as before, and a "toString" or "valueOf" that itself throws still propagates. Affected versions ">=6.7.0 <6.16.1", fixed in v6.16.1. The "join" path was introduced with the comma format in b84acba and first shipped in v6.7.0; v6.6.x and earlier ignore "arrayFormat: 'comma'" and are not affected. The "encodeValuesOnly" "String()" site was introduced in 4c4b23d (v6.11.1); before that, the same input throws at "join". Every release from v6.7.0 through v6.16.0 is affected. Impact An unauthenticated request can make any code path that re-serializes attacker-influenced data with "qs.stringify" and "arrayFormat: 'comma'" (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 bare "http" request handler, the process exits, so the impact in that case depends on the application error handling rather than on qs. The default "arrayFormat" ("indices") is not affected.