CVE-2026-82417

EUVD-2026-68140 · Published Aug 30, 2026

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.

Weaknesses

CWE-703CWE-248

CVSS scores

v4.06.3CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
v3.15.3CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

MITRE ATT&CK

Inferred

Where this vulnerability fits in an attack

ReconImpact

Initial Access

T1190Exploit Public-Facing Application

References

Potentially impacted assets

See if this affects your attack surface

Latest trending attack

Criticalyesterday

Google Brotli decoder - Remote Code Execution via compressed requests (Exploitation Check)

This control is the second step of the Patrowl response to CVE-2026-111497 (Node2Shell). It complements the first control of 22 October 2026, which focused on the Node.js runtime, and does not replace it. Why a second control CVE-2026-111497 is an out-of-bounds write in the Google Brotli decoder, the library that decompresses Brotli ("br") data. It was first published as a Node.js vulnerability, because Node.js bundles this decoder. Later publications showed that the flaw is in the library itself. Every product that embeds an affected version of the decoder is concerned, whatever its language or vendor. A public proof of concept demonstrates code execution. Any server that decompresses request bodies sent with "Content-Encoding: br" exposes the decoder to unauthenticated attackers. Known cases include Express 5 and NestJS 11 on Node.js, Koa and Fastify setups, Bun, Envoy and Istio with request decompression enabled, and any application or reverse proxy that decodes Brotli request bodies with an affected library. The flaw also works in the other direction. A malicious server can compromise an unpatched client that decodes a Brotli response, for example Node.js fetch(), Python clients (requests, httpx, aiohttp, Scrapy) with the Brotli or brotlicffi package, or libcurl builds with Brotli support. What this control checks Patrowl sends small, valid Brotli-compressed requests to each asset and observes whether the server decodes them. The check does not depend on the technology: it detects the behaviour, whatever the stack behind it. These requests are what any legitimate client could send. They never reach the vulnerable code path and cannot affect the availability of the service. How to read the result Potentially impacted means the asset decodes Brotli request bodies: the attack surface is exposed. It does not confirm that the asset is vulnerable. The version of the Brotli decoder in use is not visible from the outside. This check covers applications that parse request bodies on their root path. A Not impacted result does not rule out exposure on other routes, nor on the client side. Checking the versions on the servers remains the only exhaustive verification. Affected and fixed versions Google Brotli 1.0.3 to 1.2.0 are affected, fixed in 1.0.10, 1.1.1 and 1.2.1. Fixed releases of the main products embedding it: Node.js 22.23.4, 24.22.1 and 26.12.1 (end-of-life lines 20.x and older and 25.x receive no fix), Bun 1.4.4, Envoy 1.37.8, 1.38.6, 1.39.3 and 1.40.0, Istio 1.29.9, 1.30.6 and 1.31.2, Python Brotli 1.2.1 and brotlicffi 1.2.1.0. Products that link against the system library depend on the Brotli package of the Linux distribution. Recommendations Update every component that embeds the Brotli decoder (only real fix). Start with the assets flagged by this control. Restart the processes after the update. Rebuild containers, single-executable applications and appliances that bundle an affected runtime or library. Reduce the exposure until the update is deployed. Reject requests with "Content-Encoding: br" at the reverse proxy, load balancer or WAF, case-insensitively and including lists such as "gzip, br". On Envoy and Istio, disable request decompression for Brotli. Review outbound calls. List the features that fetch content from servers you do not control, and patch the services that host them first.