What Are Wildcard Certificates?

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by josheph bell

March 25, 2025

Learn how wildcard certificates work, their advantages, and the security risks associated with their use.

Introduction

A wildcard certificate is an SSL/TLS certificate that can secure an unlimited number of subdomains under a single main domain. It enables encrypted communication between web servers and users by verifying a domain’s identity and securing data transmission.

Instead of issuing separate SSL certificates for each subdomain, a wildcard certificate allows all subdomains under a main domain to be secured with a single certificate, simplifying SSL/TLS certificate management.

Example:

A wildcard certificate for *.example.com secures:

  • www.example.com
  • shop.example.com
  • mail.example.com

But not:

  • example.com (The root domain often requires a separate certificate)
  • sub.sub.example.com (Lower-level subdomains are not covered)

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How Wildcard Certificates Work

1. Use of the Wildcard Character ("*") in the Certificate

  • The * character acts as a placeholder for any subdomain at the same hierarchy level.
  • Any subdomain matching this pattern can be secured without needing additional certificates.

2. Issuance by a Certificate Authority (CA)

  • Wildcard certificates are issued by trusted Certificate Authorities (CAs) such as Let’s Encrypt, DigiCert, or GlobalSign.
  • The CA verifies the main domain’s ownership to ensure that only authorized users can obtain the certificate.

3. Encryption and Security

  • Wildcard certificates use TLS encryption to secure data transmissions between servers and clients.
  • They can be used for web servers, email servers, APIs, and other services.

Advantages of Wildcard Certificates

1. Cost Efficiency

  • Instead of purchasing separate certificates for each subdomain, a single wildcard certificate is sufficient, reducing costs.
  • This is particularly beneficial for businesses managing multiple subdomains within a single domain.

2. Simplified Management

  • Administrators do not need to track, renew, and configure multiple certificates.
  • Wildcard certificates reduce administrative overhead, especially in large IT infrastructures.

3. Flexibility for New Subdomains

  • New subdomains can be added at any time without requiring a new certificate.
  • This is particularly useful for companies that frequently deploy new services or web applications under a main domain.

Security Risks and Challenges

1. Risk of Private Key Compromise

  • Since a wildcard certificate is used across multiple subdomains and servers, a compromised private key poses a significant security risk.
  • If an attacker gains access to the private key, all subdomains become vulnerable, greatly expanding the attack surface.

2. Difficulties in Access Control

  • A wildcard certificate can be used by multiple systems and teams.
  • Without strict security policies, there is a risk that unauthorized personnel could gain access to the certificate.

3. Limited Control Over Subdomains

  • If multiple departments or teams within a company manage their own subdomains, centrally managing a single wildcard certificate can be challenging.
  • A compromised server with access to the certificate could endanger all subdomains.

4. No Protection for Multiple Subdomain Levels

  • Wildcard certificates only secure one level of subdomains.
  • Lower-level subdomains, such as api.internal.example.com, are not covered by *.example.com.
  • In such cases, separate certificates or SAN (Subject Alternative Name) certificates are required.

Best Practices for Secure Use of Wildcard Certificates

1. Secure Handling of Private Keys

  • The private key of a wildcard certificate should never be stored unprotected across multiple servers.
  • Using a Hardware Security Module (HSM) can help securely store private keys.

2. Certificate Visibility and Monitoring

  • Companies should track wildcard certificates using Certificate Transparency Logs and security scans.
  • Regular audits ensure that the certificate is not being misused.

3. Restricted Administrator Access

  • Not every team or department should have unrestricted access to the certificate.
  • Implementing Role-Based Access Control (RBAC) helps minimize the risk of unauthorized access.

4. Automated Certificate Renewal and Rotation

  • Wildcard certificates should be renewed regularly to minimize security risks.
  • Automation tools such as Let’s Encrypt with ACME clients help update and deploy certificates automatically.

Alternatives to Wildcard Certificates

While wildcard certificates offer many benefits, alternative solutions may be better suited depending on the use case:

1. SAN Certificates (Subject Alternative Name)

  • Allows securing multiple specific domains and subdomains in a single certificate.
  • Provides more precise control, as only defined subdomains are covered.

2. Individual Certificates for Each Subdomain

  • Offers the highest level of security, as each service has its own certificate.
  • Reduces the risk of a single point of failure if one certificate is compromised.

3. Short-Lived Certificates

  • Some organizations use certificates with shorter lifespans (e.g., 90 days) to reduce the risk of compromised keys.
  • Automated renewal tools help efficiently manage this strategy.

Are Wildcard Certificates the Best Choice for Your Business?

Wildcard certificates provide a cost-effective, flexible, and time-saving solution for securing multiple subdomains. However, they introduce significant security risks if the private key is not adequately protected.

Businesses should carefully assess whether a wildcard solution or an alternative, such as SAN certificates or individual certificates, is the more secure option for their IT infrastructure. With strong access controls, secure key management, and automated renewal processes, wildcard certificates can be used effectively and securely.

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