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.