A flaw in Node.js TLS hostname verification can cause a server certificate to be validated against a different IP address than the one the client connected to, when the host is a non-canonical IPv4 literal with a leading zero, such as `0127…

A flaw in Node.js TLS hostname verification can cause a server certificate to be validated against a different IP address than the one the client connected to, when the host is a non-canonical IPv4 literal with a leading zero, such as `0127.0.0.1`. `tls.checkServerIdentity()` normalizes the host following the WHATWG URL rules, which read a leading-zero octet as octal (`0127.0.0.1` becomes `87.0.0.1`), while the connection is made through the operating system resolver, which on some platforms such as macOS reads it as decimal (`127.0.0.1`). A server at the connected address can then be authenticated with a certificate issued for another IP address. This vulnerability affects Node.js **26.x**, **24.x**, and **22.x**.

Source published

23 Oct 2026 16:15:05 UTC

Source modified

23 Oct 2026 16:15:05 UTC

Observed here

23 Oct 2026 17:48:20 UTC

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CVSS 3.0 · 7.4 CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N