What is a Wildcard SSL Certificate?

What Is a Wildcard SSL Certificate and How Does It Work?

A wildcard SSL certificate is a type of digital certificate used to secure a domain and all of its first-level subdomains with a single certificate. This is particularly useful for businesses or organizations that manage multiple subdomains under the same parent domain.

How It Works:

When a wildcard SSL certificate is issued, it uses an asterisk (*) in the domain name field—for example, *.example.com. The asterisk acts as a placeholder, allowing the certificate to secure:

• www.example.com

• mail.example.com

• blog.example.com

• store.example.com

However, it does not cover second-level subdomains like shop.mail.example.com.

Wildcard certificates offer both cost-efficiency and administrative simplicity. Instead of purchasing and managing separate certificates for each subdomain, you can deploy one certificate across your infrastructure. This reduces overhead and makes certificate renewals easier to manage.

Wildcard certificates are typically available in both Domain Validated (DV) and Organization Validated (OV) forms. While they do not secure the root domain alone (i.e., example.com without the subdomain), most certificate authorities offer options that include both example.com and *.example.com in the SAN (Subject Alternative Name) field.

Are Wildcard Certificates Secure, and What Are Their Limitations?

Wildcard certificates are considered secure in terms of the encryption they provide. Like any SSL/TLS certificate, they enable HTTPS connections and encrypt data in transit, protecting it from interception or tampering.

But here’s the catch:

Wildcard certificates come with a key management caveat. Since one certificate (and more critically, one private key) is used across multiple subdomains, a single point of compromise can put your entire domain structure at risk.

If one subdomain’s server is breached and the private key is stolen, then all other subdomains using that wildcard certificate are potentially compromised, too. This is especially risky in environments where different teams or departments manage different subdomains.

Limitations:

• Only covers one level of subdomains.

• Not ideal for environments requiring strict separation between services.

• Limited compatibility with some legacy systems or platforms.

• Certificate Transparency (CT) logging may disclose internal subdomains unintentionally.

Best Practices:

• Use Hardware Security Modules (HSMs) or secure key storage.

• Consider certificate segmentation—use different certificates for higher-risk services.

• Monitor your certificate’s use and rotate keys periodically.

So, yes, wildcard certificates are secure—but only if you treat the private key like it holds the keys to your digital kingdom (because it does).

How Do Wildcard Certificates Differ From SAN Certificates?

Wildcard and SAN (Subject Alternative Name) certificates are both used to secure multiple domains or subdomains, but they operate in fundamentally different ways.

Wildcard Certificates:

• Secure all first-level subdomains under a single domain.

• Example: *.example.com secures mail.example.com, login.example.com, etc.

• Simple, scalable, and great for dynamic environments where subdomains are added frequently.

SAN Certificates:

• Secure specific domain names, which may include different domains or subdomains.

• Example: A SAN cert could secure:

  • example.com

  • www.example.net

  • mail.example.org

• Offers more control and visibility, since each domain must be explicitly listed.

Key Differences:

Choose wildcard if you want simplicity and speed. Choose SAN if you need precision and tighter access control across varied domains or services.

Can Wildcard Certificates Be Used for Multiple Levels of Subdomains?

This is a common question—and one that trips up many people new to certificate management. The answer is no—standard wildcard certificates do not secure multi-level subdomains.

For example:

A wildcard for *.example.com will secure:

• mail.example.com

• www.example.com

But not:

• admin.mail.example.com

• support.dev.example.com

To cover multiple levels of subdomains, you’d need to:

• Purchase a separate wildcard for each subdomain level (e.g., *.mail.example.com)

• Use a SAN certificate that lists each needed subdomain explicitly

• Consider a multi-wildcard or wildcard SAN certificate (offered by select CAs), though these can be more expensive and complex to manage

Why the restriction?

It’s a security and specificity issue. Allowing one wildcard to secure an entire domain tree would create massive surface area for potential abuse. That’s why Certificate Authorities—and the browsers that validate certificates—limit the scope to one subdomain level.

So if your web architecture is built on deeper domain trees, plan your certificate strategy accordingly. Wildcards are flexible, but not omnipotent.