### Summary `qs.stringify` throws a `TypeError` when it serializes an object whose own `constructor` property has a truthy, non-callable

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

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 4var qs = require("qs");
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 8qs.stringify(qs.parse("x[constructor][isBuffer]=y", { plainObjects: true }));
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12qs.stringify(JSON.parse("{\"a\":{\"constructor\":{\"isBuffer\":\"x\"}}}"));
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16// TypeError: obj.constructor.isBuffer is not a function
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20//     at Object.isBuffer (lib/utils.js:332:78)
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24//     at stringify (lib/stringify.js:127:45)
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Fix

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

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 4- return !!(obj.constructor && obj.constructor.isBuffer && obj.constructor.isBuffer(obj));
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 8+ return !!(obj.constructor && typeof obj.constructor.isBuffer === "function" && obj.constructor.isBuffer(obj));
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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.