CVE-2026-82417

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

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:

- 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.

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

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:

- 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.

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…

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

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:

- 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.

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

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:

- 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.

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.