DMS Source Endpoint Connection Fail

This error indicates an authentication issue when connecting to your source DocumentDB cluster. Since you're using AWS Secrets Manager for authentication and experiencing a connection failure specifically with the "Authentication failed" message, there are several areas to check:

1. Verify the IAM role permissions:

• Ensure the IAM role used by the DMS replication instance has the necessary permissions to access the secret in AWS Secrets Manager (secretsmanager:GetSecretValue permission for the specific secret ARN).
• Double-check the policy attached to the role to confirm these permissions are granted.

2. Check network connectivity:

• Even though both clusters are in the same VPC and subnets, verify that the security group rules allow traffic between the replication instance and the source DocumentDB cluster.
• Check network ACLs if applicable.

3. Secrets Manager configuration:

• Confirm that the secret in Secrets Manager contains the correct username and password for the source DocumentDB cluster.
• Ensure the secret format matches what DMS expects.
• If your VPC doesn't have direct access to Secrets Manager, you may need a VPC endpoint for Secrets Manager.

4. Replication instance configuration:

• Verify that the replication instance has the correct subnet group and security group assignments.

Since your target endpoint works correctly but the source fails with an authentication error, it's likely that there's either an issue with the specific secret used for the source endpoint or a permission issue when accessing that particular secret.
Sources
Replication Instance Fails to Connect to DMS Migration Endpoint | AWS re:Post
How to Solve This Error in AWS DMS Serverless? | AWS re:Post