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What Are the Security Risks of Wildcard SSL Certificates?

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The choice of using a wildcard certificate often involves carefully assessing the certificate’s benefits and comparing them to the potential wildcard certificate risks and vulnerabilities they introduce. Every website owner must strike a balance between the utility they seek and the security they need.

A wildcard SSL/TLS certificate offers some big advantages to website owners. Namely:

  • Securing your domain and virtually unlimited first-level subdomains under a single certificate.
  • Saving time by simplifying your certificate management responsibilities.
  • Not shouldering additional costs for securing those subdomains. 
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But that kind of power also comes with limitations, which translate into wildcard certificate security risks and vulnerabilities. We’re here to explore those concerns and help you make an informed decision about how best to secure your site.

3 Wildcard Certificates Security Risks & Vulnerabilities You Should Know

A wildcard SSL/TLS certificate operates on the principle of domain validation for a primary domain and all its subdomains using an asterisk (*) as a placeholder for the subdomain in the Common Name (CN) field. This would look like * For example, a single wildcard SSL/TLS certificate covers the main domain, as well as an unlimited number of first-level subdomains (,, etc.).

Wildcard Certificates Security Risks & Vulnerabilities
Image caption: A snapshot of a wildcard SSL certificate as seen in a web browser’s certificate viewer. It displays the use of an asterisk (*) in the common name (CN) field to denote secure coverage for all subdomains under the main domain

To secure subdomains at multiple levels, you would require a multi-domain SSL certificate. This is also known as a SAN certificate (the SAN stands for Subject Alternative Name), which we’ll speak to more a little later.

When embracing the benefits of using wildcard SSL certificates, companies must skillfully sidestep three key security pitfalls to keep their domains safe.

Wildcard Security Risk #1: Using a Single Key for All Subdomains

Using a wildcard SSL certificate means you have a single certificate and keypair to worry about. However, this approach creates a single point of failure and increases the risk of compromise if you fail to manage those assets properly.

The consequences could be severe should this key fall into the wrong hands, whether through mishandling, theft, or another form of security lapse. Attackers could potentially impersonate any of the subdomains covered by the wildcard certificate. This opens the door to data breaches and enables these bad actors to conduct phishing campaigns with greater credibility, leveraging your domain’s trusted status.

Wildcard Security Risk #2: Facing ALPACA and Attack Surface Expansion

In some cases, the benefits of wildcard SSL certificates in simplifying subdomain security are countered by the risks they can introduce. Firstly, they expand your site’s attack surface — a security breach of one subdomain can compromise the safety of all subdomains under the certificate’s umbrella. Each subdomain could be running different services and applications, and not all may have the same level of security measures in place. This makes the less secure ones potential gateways to the others.

Moreover, the same wildcard SSL/TLS certificates are prone to use in ALPACA — Application Layer Protocols Allowing Cross-Protocol Attacks. These protocol confusion attacks target servers with uniform certificates, enabling threat actors to breach one service to compromise others within the same domain. The exploitation of these vulnerabilities could lead to critical security incidents, including session cookie theft or execution of arbitrary code, thereby threatening the integrity and confidentiality of the entire network’s communication ecosystem.

Wildcard Security Risk #3: Managing Renewals and Revocations

Wildcard certificates offer a simplified renewal process for securing multiple subdomains. However, this comes with distinct disadvantages. If a certificate is due for renewal or is compromised and gets revoked, the update process can be complex when the certificate is deployed across multiple servers that are in different locations. This singular management point can also reduce oversight, as it may allow for the creation of unauthorized subdomains, potentially exposing the network to unforeseen risks.

Additionally, in the event of revocation, operational challenges may arise when transitioning each subdomain to a new certificate. This changeover period, however brief, amplifies the risk of downtime and security vulnerabilities. Therefore, this consolidation of certificate management necessitates a robust and vigilant administrative protocol to counteract the inherent risks.

Evaluating the Trade-Offs of Wildcard Certificate Risks and Benefits

On one hand, wildcard certificates help to bridge the gap between convenience and caution. However, they can also be a double-edged sword, offering streamlined management of subdomains while raising the stakes for security. Step into the details of this trade-off below.

Pros of Using a Wildcard CertificateCons of Using a Wildcard Certificate
Subdomain SecurityOne certificate secures an unlimited number of first-level subdomains. This helps ensure that public key cryptography is available to secure connections for users on all of your first-level subdomains.A single compromised key endangers all of your first-level subdomains, and wildcard certificates can’t be issued with extended validation (EV) due to CA/Browser Forum validation guidelines.
Certificate ManagementIt simplifies management with a single certificate for all subdomains and allows for easily adding new subdomains to your domain at any time.Managing certificates for second-level subdomains requires an additional wildcard certificate, and a compromise can necessitate revocation and re-issuance for all subdomains.
Cost EfficiencyPurchasing a single certificate is more cost-effective than buying separate certificates for each subdomain.Security breaches can incur significant indirect costs, magnifying the impact due to the single certificate covering multiple subdomains.
Operational EfficiencyYou can streamline administrative tasks for certificate renewals and revocations with a single certificate.If a subdomain is compromised, it can affect all others, requiring careful key management across multiple servers.
Attack SurfaceFacilitates quicker disaster recovery processes due to centralized certificate management.If not configured properly, a wildcard certificate expands your attack surface, posing risks to all subdomains and web apps secured by the wildcard certificate.
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8 Best Practices for Reducing Wildcard SSL Certificate Risks

To continue using wildcard certificates while maintaining security, organizations may implement certain practices. These could include:

  1. Implement strong certificate and key management best practices: Ensure secure certificate lifecycle management and safeguard keys using stringent protocols and secure systems. This includes the use of on-prem or cloud HSMs, access controls, and periodic inventories and audits.
  2. Limit the scope of each certificate in terms of its application: Confine wildcard certificates to their intended roles and domains, clearly defining their reach to minimize risks and prevent overextension.
  3. Implement strong access management to secure your certificates and keys: Restrict access to certificates and keys to essential personnel with operational roles, reinforcing secure handling and reducing insider threat risk.
  4. Keep your subdomain inventory up to date: Keep a detailed inventory of all active subdomains to help manage the scope of the wildcard certificate and identify unauthorized usage.
  5. Limit subdomain exposure to online threats: Restricting the use of wildcard certificates to environments where subdomains are not exposed to the internet or are within a controlled infrastructure can limit exposure.
  6. Conduct proactive security surveillance: Performing regular security audits and monitoring for suspicious activity can aid in the early detection of a security breach.
  7. Plan for incident response in advance: Develop an incident response plan that includes procedures for dealing with the potential compromise or outage of any digital certificate, including wildcards. This plan should be part of a broader approach to mitigating risks and promptly recovering from any security incidents.
  8. Leverage renewal automation feature: Implementing certificate management tools can automate the renewal and revocation processes, reducing the operational risks associated with human error. 

Incorporating these practices can help mitigate the wildcard certificate disadvantages while leveraging their operational benefits.

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Maximize Subdomain Security: Use a SAN Certificate

To mitigate the risks associated with wildcard SSL certificates while maintaining their benefits, organizations can opt for the use of SAN (Subject Alternative Name) certificates instead. SAN certificates, also known as multi-domain certificates, allow for precise specification of each covered domain or subdomain rather than an open-ended wildcard approach. This means that the scope of the certificate is explicitly defined and restricted to the list of URLs provided in the SAN field of the certificate. Consequently, organizations can manage their websites better by using SAN certificates. With one certificate, they can cover many subdomains without the risks that come with a wildcard certificate. It’s important to set up the SAN entries the right way so as to cover only legitimate and necessary subdomains. Doing so optimizes efficiency without compromising security.

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