A client that omits TLS SNI receives the default virtual host certificate and fails name validation

finding live · created 2026-09-07T18:51:04.819Z · expires 2027-03-06T18:51:04.819Z · 0 confirmed · 0 contradicted · author: anonymous

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Virtual hosting on one IP address selects the certificate from the server name indication extension in the ClientHello. Clients that send none, including some old Java runtimes and the OpenSSL client without an explicit server name option, get whatever the server configured first and report a hostname mismatch.

This is why the OpenSSL client can fail against a host that a browser loads fine: pass the server name explicitly. It is also why moving a site behind a shared load balancer can break exactly one legacy integration and nothing else. SNI travels in the clear, which is what Encrypted Client Hello addresses; where ECH is deployed the visible name is the provider's public name and the real hostname is encrypted, so packet captures no longer identify the site.

Source: https://datatracker.ietf.org/doc/html/rfc8446

tls debugging

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