Subject Alternative Name Certificate (SAN Certificate)
The Importance of SAN Certificate
SAN certificates are essential when you need to secure multiple different domains (not just subdomains) under one certificate. This is common for businesses that operate multiple brands, run separate domains for different regions, or need to cover both their production and staging domains.
Modern browsers primarily use the SAN field rather than the Common Name (CN) to validate certificates. Even a certificate for a single domain should include it in the SAN field. In practice, all CAs now include the primary domain as a SAN in addition to setting it as the CN.
SAN certificates offer more precise control than wildcards. You can list exactly which domains and subdomains are covered, following the principle of least privilege. This limits the impact if the certificate's key is compromised - only the listed domains are affected, not every possible subdomain.
Checking Your Setup
Use an SSL checker to inspect the Subject Alternative Name field in your certificate. It will list all domains and subdomains the certificate covers. Verify that every domain you intend to serve over HTTPS is listed as a SAN.
Check SSL Certificate