Improper certificate validation for non-canonical IPv4 hostnames allows DNS SANs to authenticate IP endpoints

Summary

A flaw in Node.js TLS hostname verification allows a certificate with only a DNS SAN to authenticate an IP endpoint when the client connects using a non-canonical IPv4 spelling, such as 0177.0.0.1, 0x7f.0.0.1, 2130706433 or 127.1.

tls.checkServerIdentity() did not classify these hosts as IP addresses, but normalized them to their canonical form and matched the result against the certificate DNS names. A certificate containing DNS:127.0.0.1 and no IP Address:127.0.0.1 SAN was therefore accepted, while the same certificate is correctly rejected for 127.0.0.1. Exploitation requires a certificate with a numeric DNS SAN issued by a CA trusted by the client, and a platform resolver that accepts these spellings, such as glibc.

This vulnerability affects Node.js 26.x, 24.x, and 22.x.