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
var qs = require("qs");qs.stringify(JSON.parse("{\"a\":[{\"toString\":\"x\"}]}"), { arrayFormat: "comma" });
qs.stringify(qs.parse("a[0][toString]=x", { allowPrototypes: true }), { arrayFormat: "comma" });
qs.stringify(qs.parse("a[][b]=c", { plainObjects: true }), { arrayFormat: "comma", encodeValuesOnly: true });
// TypeError: Cannot convert object to primitive value
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.