WAR: Unused Customer Master Keys

The security and cost-effectiveness of your cloud storage on AWS depend on managing your encryption keys responsibly. Identifying and addressing unused Customer Master Keys (CMKs) aligns with two key principles of the AWS Well-Architected Framework:

• Security Pillar (SE): Specifically, SE.3: Key Management ensures proper key handling practices to protect the confidentiality and integrity of your data.
• Cost Optimization Pillar (CO): CO.2: Rightsizing focuses on using resources efficiently to avoid unnecessary spending for underutilized resources.

We will explore what constitutes an unused CMK, the security implications of keeping them around, and strategies for optimizing your CMK usage.

What are Unused Customer Master Keys (CMKs)?

CMKs are the foundation of AWS Key Management Service (KMS). They are security keys that you create and manage to encrypt and decrypt your data at rest and in transit within AWS services. A CMK is considered unused if it meets one of the following criteria:

• No Encrypted Data References: The CMK has no data keys or other resources encrypted under its control. This means it's not actively being used for any encryption operations.
• Obsolete Keys: The CMK might have been used previously but is no longer referenced for any data encryption tasks due to application updates, data migrations, or other changes.

Why are Unused Customer Master Keys a Concern?

Even unused CMKs can pose security risks and impact your costs:

• Security Risks: Unused CMKs, if not managed properly, can be potential targets for attackers. An attacker who gains access to an unused CMK might be able to decrypt previously encrypted data if they can also compromise the data encryption key.
• Unnecessary Costs: You incur charges for each CMK in your account, regardless of its usage. Unused CMKs can lead to wasted spending on resources that are not contributing to data security.

Identifying Unused Customer Master Keys:

Here are some methods to identify unused CMKs in your AWS environment:

• AWS Key Management Service Console: Use the KMS console to list all your CMKs. Look for keys with no data keys or encrypted resources associated with them.
• AWS CloudTrail Logs: Enable CloudTrail logging for KMS events and analyze the logs to identify CMKs that haven't been used for encryption or decryption operations for an extended period.
• AWS Config Rules: Configure AWS Config to monitor CMK usage and trigger alerts for unused keys.

Strategies for Dealing with Unused Customer Master Keys:

Once you've identified unused CMKs, here are some options to optimize your security posture and costs:

• Schedule Deletion for Unused CMKs: AWS allows scheduling the deletion of unused CMKs. This ensures complete removal of the key and eliminates associated charges after a waiting period.
• Rotate Keys for Previously Used CMKs (if applicable): If an unused CMK was previously used for encryption, consider rotating the key to enhance overall data security. This involves creating a new CMK and re-encrypting the data using the new key while scheduling the old CMK for deletion.

Automating Unused CMK Management:

• AWS Lambda Functions: Develop Lambda functions that can be triggered by CloudTrail or Config events to automate actions like scheduling deletion of unused CMKs.

Benefits of Addressing Unused Customer Master Keys:

By proactively identifying and addressing unused CMKs, you can achieve the following benefits:

• Enhanced Security: Reduce the attack surface by minimizing the number of CMKs in your environment and following best practices for key rotation.
• Optimized Costs: Eliminate unnecessary charges associated with unused encryption keys.
• Improved Key Management: Maintain a clean and organized set of CMKs, simplifying key management and reducing the risk of human error.

Alignment with the Well-Architected Framework:

Following these strategies aligns with the Security and Cost Optimization pillars of the Well-Architected Framework. By effectively managing your CMKs, you can ensure the confidentiality and integrity of your data while maintaining a cost-efficient and secure cloud storage environment.