Introduction to the Issue

Imagine deploying your CloudFormation stack and suddenly notice unexpected CNAME records being added to your AWS Hosted Zones. These CNAMEs aren’t part of your CloudFormation templates, and their presence raises eyebrows. Are these records a security risk, or is there a more benign explanation? This blog post delves into this perplexing issue, shedding light on why these CNAMEs appear and what you can do about them.

Background on AWS Certificate Manager (ACM)

AWS Certificate Manager (ACM) is a service that simplifies the process of provisioning, managing, and deploying SSL/TLS certificates for use with AWS services. These certificates are crucial for securing websites, ensuring that data transmitted between users and servers is encrypted. ACM automates many manual steps in certificate management, such as renewal and deployment, making it an essential tool for maintaining a secure cloud environment.

When ACM issues a certificate for a domain, validation is required to confirm domain ownership. This is often done using DNS validation, where a CNAME record is created in the domain’s hosted zone. Once ACM validates the domain, the CNAME record typically remains in place for as long as the certificate is active.

Observations and Troubleshooting Steps

The sudden appearance of CNAME records in your AWS Hosted Zones can be alarming, especially when they seem to have been added without your explicit input. To troubleshoot this issue, it’s essential first to identify the source of these records. Here are some steps you can take:

  1. Examine CloudFormation Events: Review the events generated during your CloudFormation stack deployment. This can provide clues about any resources created or modified that might be linked to the new CNAMEs.
  2. Check ACM Certificate Requests: Look at the ACM console to see if any certificates were issued when the CNAME records appeared. This can help you determine if ACM is behind the new entries.
  3. Analyze Stack Templates: Scrutinize your CloudFormation templates for any resources that might trigger certificate requests or DNS record modifications.
  4. Review Route 53 Hosted Zone Records: Compare the newly added CNAME records with the existing ones to understand their purpose and linkage to ACM-managed certificates.

The Puzzle of Automatically Inserted CNAMEs

The automatic insertion of CNAME records during CloudFormation deployments is closely tied to ACM’s DNS validation process. When a certificate is requested via ACM, the service may add a CNAME record to the relevant hosted zone for validation. However, these records are typically added by ACM and not directly by CloudFormation. The timing and nature of these additions can seem mysterious, especially if you must be aware of the automatic DNS validation process.

Hypotheses and Considerations

Several hypotheses could explain the unexpected CNAME additions:

  1. ACM Certificate Request: The most likely scenario is that a certificate request was made via ACM, which automatically added the CNAME records for domain validation.
  2. Pre-existing Certificates: Another possibility is that an existing ACM certificate is being reused in your stack, prompting ACM to revalidate the domain, resulting in new CNAME records.
  3. Third-Party Integrations: Using third-party services or automation tools that interact with ACM could trigger certificate requests and corresponding CNAME additions.

Call for Investigation

Given the potential implications of unauthorized or unexpected DNS changes, it’s crucial to investigate the root cause of these CNAME additions. Collaborate with your security team to ensure that no unauthorized ACM certificates are being issued, and review your CloudFormation templates and related resources thoroughly.

For those in the community who have encountered similar issues, sharing your experiences and solutions can help others navigate this complex situation.

About Teri Radichel

Teri Radichel is a renowned cybersecurity expert and author with extensive experience in cloud security, particularly in AWS environments. Her work often highlights the importance of understanding the nuances of cloud security and the potential risks associated with misconfigurations or unexpected changes in cloud environments.

Conclusion and Next Steps

The unexpected addition of CNAME records in AWS Hosted Zones during CloudFormation deployments can be unsettling, but the mystery can be unraveled systematically. Understanding ACM’s role in certificate management and DNS validation can help you better anticipate and manage these changes.

Next steps include:

  1. Regularly Monitor Hosted Zones: Keep an eye on your DNS records to detect unexpected changes early.
  2. Audit ACM Certificates: Periodically audit ACM certificates in your account to ensure no unauthorized certificates are being issued.
  3. Document Your Findings: Share your experiences and solutions with the broader community to help others avoid similar issues.

References

Route 53 template snippets

AWS::Route53::HostedZone