Understanding Cross Account KMS: Secure Key Sharing Across AWS Accounts

Cross account kms refers to the practice of allowing a customer’s customer master key (CMK) in one AWS account to be used by resources in another account. This pattern is common in organizations that separate production, development, and auditing functions across multiple accounts but still need centralized encryption management. When done correctly, cross account kms can improve security, reduce key sprawl, and simplify governance. When misunderstood, it can expose data or violate compliance requirements. This article walks through what cross account kms is, how it works, and how to implement it responsibly.

What is cross account kms?

In the AWS Key Management Service (KMS) ecosystem, a CMK controls cryptographic operations such as Encrypt, Decrypt, ReEncrypt, GenerateDataKey, and more. Cross account kms means granting another AWS account permission to use a CMK in your account. This permission can be granted through the key policy, IAM policies, or via grants. The goal is to enable cryptographic operations across account boundaries while maintaining strict access control and traceability. In practice, you might use cross account kms to let a data producer in one account encrypt data that a data consumer in another account can decrypt, or to allow shared services to access encrypted resources without moving keys between accounts.

Key concepts you should know

Before configuring cross account kms, it helps to understand a few core ideas:

• CMK policy vs. IAM policy: The CMK policy is the primary access control for the key. To enable cross account usage, you typically add statements that allow the other account’s principals to perform cryptographic operations. IAM user or role policies in the other account can provide additional, more granular permissions, but the key policy is the gatekeeper for the key itself.
• Grants: Grants are a temporary or scoped mechanism to delegate permissions to a specific principal for a CMK. A grant can be used for cross account kms scenarios when you want to give a particular role in the other account a narrowly scoped ability to encrypt or decrypt data.
• Key usage and ciphertext location: The data encrypted with a CMK in one account is typically decrypted with that same CMK (in the same or other account if permissions allow). The ciphertext is bound to the CMK that encrypted it, so cross account decryption requires that the CMK in the original account be usable by the decrypting party.
• Auditing and visibility: Enabling CloudTrail for KMS events is crucial in cross account kms to monitor who used the key and when. This makes it easier to detect unusual or unintended usage across accounts.
• Least privilege principle: Grant only the actions you need (Encrypt, Decrypt, GenerateDataKey, ReEncryptFrom, ReEncryptTo, etc.) and only to the principals that require access.

How permissions flow works

In a typical cross account kms scenario, access is granted through a combination of key policies and IAM policies, with optional grants. The general flow looks like this:

1. The CMK in the primary account (Account A) has a policy that explicitly allows the target account (Account B) to use the key for the required operations. This is the foundational permission for cross account kms.
2. A role or user in Account B is authorized via IAM to call KMS operations against the CMK, such as Encrypt and Decrypt, or to hold a grant for broader access.
3. When a client in Account B needs to encrypt data using the CMK in Account A, it sends a request to KMS with the CMK’s identifier (ARN) and the operation. If the permissions allow it, KMS performs the operation and returns the ciphertext.
4. When decrypting, the ciphertext can be sent back to KMS (in Account A) by a resource in Account B, provided the CMK policy and any grants permit the decryption operation.

Understanding this flow is essential for reliable cross account kms configurations. Misaligned policies can block encryption or decryption, leading to data access issues or operational outages.

Setting up cross account kms access

Below is a high-level, practical approach to enable cross account kms while keeping security tight. Adapt these steps to your organization’s governance model.

1. Choose or create a CMK in the owning account (Account A). For cross account kms, you typically use a CMK that is dedicated to the workload and not tied to a single service or project. Consider creating an alias for easier reference.
2. Update the CMK policy to allow the other account (Account B) to use the key. Add statements that permit the specific KMS actions you intend to allow (for example, Encrypt, Decrypt, GenerateDataKey) for the principals in Account B. Be explicit about resources (the CMK ARN) and principals to minimize risk.
3. Optionally create a grant for a specific principal in Account B. If you want to limit the duration or scope, grants provide fine-grained control. Include an expiration if you’re using temporary credentials.
4. In Account B, ensure IAM policies authorize the necessary KMS actions. The user or role that will perform encryption or decryption should have the corresponding kms:* permissions, within the scope defined by the CMK policy.
5. Test the cross account kms flow with a small test dataset. Start with a non-sensitive sample to verify encryption and decryption succeed across accounts. Validate that CloudTrail logs show the correct source account and principal for cross-account usage.
6. Implement monitoring and rotations. Enable CloudTrail for auditability and plan regular CMK rotates if policies permit, while keeping cross account access consistent with governance requirements.

In practice, many organizations rely on cross account kms when they share a data lake or data catalog across multiple AWS accounts. Keeping the encryption keys centralized in one account simplifies policy management, reduces key sprawl, and still supports secure cross-account data access. The term cross account kms captures this pattern, and it should be documented in your security playbooks to avoid drift between teams.

Security considerations and best practices

• Grant only the actions that are strictly necessary for the workload, and only to the actors in the other account that truly need them.
• The CMK policy is the ultimate source of truth for KMS usage. Narrow IAM policies in the other account to reduce risk.
• If a role needs short-term access, grants with expiration provide a safer alternative to open-ended permissions.
• Turn on KMS event logging in CloudTrail to track Encrypt, Decrypt, and other operations across accounts.
• Use separate CMKs per workload or data product when cross account kms are involved. This helps contain a breach and simplifies policy management.
• Plan rotation in a way that does not break cross account access. Rotation is supported, but you must ensure all dependent accounts and grants are updated accordingly.
• Regularly audit who has cross account kms permissions and adjust as teams evolve.

Common pitfalls and how to avoid them

• If you miss Encrypt or Decrypt in the policy, cross account kms usage will fail even if IAM permissions look correct.
• Avoid broad actions or wild-card declarations. Tighten permissions to specific operations and resources.
• If the ciphertext lands in a bucket or service outside the intended scope, encryption or decryption may fail or breach governance rules.
• Without CloudTrail visibility, you may miss anomalous cross account kms activity or misconfigurations.

Use cases for cross account kms

• Centralized encryption for a data lake where ingestion happens in one account but analytics happens across several accounts.
• Shared services such as a logging or encryption proxy that uses a CMK in a central account to protect data from multiple producers.
• Cross-account data sharing scenarios where data encryption keys must be controlled in one account but consumed across others.

Conclusion

Cross account kms is a powerful pattern that enables secure collaboration across AWS accounts when managed carefully. By correctly configuring CMK policies, leveraging grants where appropriate, and enforcing least privilege with robust auditing, teams can achieve centralized key management while maintaining clear boundaries and strong security controls. When you design cross account kms workflows, document the policy decisions, test thoroughly, and monitor continuously. With disciplined governance, cross account kms becomes a reliable backbone for cross-account data workflows rather than a source of risk. This is the essence of prudent cross account kms implementation.