CVE-2026-21637: Incomplete fix for CVE-2026-21637: loadSNI() in _tls_wrap.js lacks try/catch leading to Remote DoS

A flaw in Node.js TLS error handling leaves SNICallback invocations unprotected against synchronous exceptions, while the equivalent ALPN and PSK callbacks were already addressed in CVE-2026-21637. This represents an incomplete fix of that prior vulnerability.

When an SNICallback throws synchronously on unexpected input the exception bypasses TLS error handlers and propagates as an uncaught exception, crashing the Node.js process.

This vulnerability affects all Node.js versions that received the CVE-2026-21637 fix, including 20.x, 22.x, 24.x, and 25.x, on any TLS server where SNICallback may throw on unexpected servername input.

— Node.Js

A flaw in Node.js TLS error handling leaves SNICallback invocations unprotected against synchronous exceptions, while the equivalent ALPN and PSK callbacks were already addressed in CVE-2026-21637. This represents an incomplete fix of that prior vulnerability.

When an SNICallback throws synchronously on unexpected input the exception bypasses TLS error handlers and propagates as an uncaught exception, crashing the Node.js process.

— Node.Js

CVE-2026-21637 is regarding a vulnerability in Node.js TLS error handling allows remote attackers to crash or exhaust resources of a TLS server when pskCallback or ALPNCallback are in use. Synchronous exceptions thrown during these callbacks bypass standard TLS error handling paths (tlsClientError and error), causing either immediate process termination or silent file descriptor leaks that eventually lead to denial of service. Because these callbacks process attacker-controlled input during the TLS handshake, a remote client can repeatedly trigger the issue. HackerOne created this CVE on their behalf. The documented Visual Studio updates incorporate updates in Node.js which address this vulnerability. Please see CVE-2026-21637 for more information.

— Microsoft

HackerOne: CVE-2026-21637 TLS PSK/ALPN Callback Exceptions Bypass Error Handlers

— Microsoft