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Security Fundamentals & Crypto · Foundational

Subject Alternative Name (SAN)

X.509 extension listing additional identities, such as DNS names or IPs, that a certificate is valid for.

Definition

Modern browsers validate hostnames only against the SAN extension, ignoring the Common Name. A single SAN (multi-domain or UCC) certificate can secure different domains such as example.com, example.net, and mail.example.org. SANs can also include IP addresses, email addresses, and UPNs.

Example

A SAN certificate lists DNS:example.com, DNS:www.example.com, and DNS:autodiscover.example.com.

Exam tip

Hostname mismatch errors are fixed by adding the name to the SAN, not the CN.

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