HIGH 7.5 Microsoft PoC

CVE-2026-21710

A flaw in Node.js HTTP request handling causes an uncaught `TypeError` when a request is received with a header named `__proto__` and the application accesses `req.headersDistinct`. When this occurs, `dest["__proto__"]` resolves to `Object.prototype` rather than `undefined`, causing `.push()` to be called on a non-array. This exception is thrown synchronously inside a property getter and cannot be intercepted by `error` event listeners, meaning it cannot be handled without wrapping every `req.headersDistinct` access in a `try/catch`. * This vulnerability affects all Node.js HTTP servers on **20.x, 22.x, 24.x, and v25.x**

Microsoft Security Update 2026-Mar: A flaw in Node.js HTTP request handling causes an uncaught `TypeError` when a request is received with a header named `__proto__` and the application accesses `req.headersDistinct`. When this occurs, `dest["__proto__"]` resolves to `Object.prototype` rather than `undefined`, causing `.push()` to be called on a non-array. This exception is thrown synchronously inside a property getter and cannot be intercepted by `error` event listeners, meaning it cannot be handled without wrapping every `req.headersDistinct` access in a `try/catch`. * This vulnerability affects all Node.js HTTP servers on **20.x, 22.x, 24.x, and v25.x**

Affected Products

References

Published: 2026-03-10 · Source: Microsoft · Feed updated: 2026-09-25
This high severity vulnerability with a CVSS score of 7.5 was published on 2026-03-10 via Microsoft. 🚨 A public proof-of-concept exploit is available on GitHub. EPSS score: 25.0% (top 2% of all CVEs by exploitation probability). Affected: cbl2 nodejs18 18.20.3-12, cbl2 nodejs18 18.20.3-12 on CBL Mariner 2.0, azl3 nodejs 20.14.0-14 on Azure Linux 3.0 and 4 more.
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