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Since January 5th Amazon S3 encrypts all new objects by default with AES-256 to protect data at rest. S3 automatically applies server-side encryption using Amazon S3-managed keys for each new object, unless a different encryption option is specified.

The cloud provider claims that the change puts a security best practice into effect without impacts on performance: S3 buckets that do not use default encryption will now apply SSE-S3 as the default setting. Server-side encryption with customer-provided keys (SSE-C) and server-side encryption with AWS Key Management Service (SSE-KMS) are not affected by the change.

Since 2017 the S3 Default Encryption feature was already an optional setting available to enforce encryption for every object uploaded. Going forward, S3 will automatically apply SSE-S3 for all buckets without any customer-configured encryption setting. Sébastien Stormacq, principal developer advocate at AWS, explains why the change is significant:

While it was simple to enable, the opt-in nature of SSE-S3 meant that you had to be certain that it was always configured on new buckets and verify that it remained configured properly over time. For organizations that require all their objects to remain encrypted at rest with SSE-S3, this update helps meet their encryption compliance requirements without any additional tools or client configuration changes.

The encryption status for new object uploads and S3 Default Encryption configuration is available in CloudTrail logs providing an option to validate that all new data uploaded to S3 is encrypted. To explain the changes, AWS published a Default encryption FAQ, clarifying that S3 only encrypts new object uploads. To encrypt existing objects, the cloud provider suggests using S3 Batch Operations. While no changes are required to access objects, it is no longer possible to disable encryption for new uploads and client-side encrypted objects will now have an additional layer of encryption. Angelica Phaneuf, CISO of Army Software Factory, writes:

This is an amazing release by AWS and will progress the security posture of everyone using their cloud.

Segev Eliezer, penetration tester at LIFARS, comments:

Now they should configure IMDSv2 by default on EC2 instances and update GuardDuty’s IAM findings.

Security blogger Mellow Root thinks that disk encryption in AWS is close to useless and potentially harmful, claiming it is security theater:

I suggest spending your time on IAM permissions, backups, disaster recovery, appsec, or pretty much anything else before disk encryption.

Corey Quinn, chief cloud economist at The Duckbill Group, writes:

This is a clear win for customers. Personally, I find the idea of encrypting objects in S3 at rest to be something of a checkbox requirement and nothing more, but if that box gets checked by default for the rest of time I’m not going to complain any.

The S3 change applies to all AWS regions and there are no costs associated with using server-side encryption with SSE-S3.