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Why Browsers Reject Valid SSL Certs: The SAN Mismatch Explained

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A common TLS error — NET::ERR_CERT_COMMON_NAME_INVALID in Chrome or SSL_ERROR_BAD_CERT_DOMAIN in Firefox — occurs when a server presents a certificate that does not cover the hostname being requested, even if the certificate itself is valid and trusted. Since around 2017, browsers stopped using the certificate's Common Name (CN) field for hostname verification and now rely exclusively on the Subject Alternative Name (SAN) extension, which lists the exact hostnames a certificate is authorized for. This means a certificate can appear correct to a human reader yet still be rejected if the requested hostname is absent from its SAN list. The most frequent triggers include mismatches between apex and www domains, wildcard certificates failing to cover sub-subdomains or bare apex domains, and servers falling back to a default certificate that belongs to a different site. Resolving the error requires reissuing the certificate with all required hostnames explicitly listed in the SAN field.

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Why Browsers Reject Valid SSL Certs: The SAN Mismatch Explained · ShortSingh