Lets say I have an EC2 instance with attached IAM Role Role A which
posses one policy AmazonS3ReadOnlyAccess, correct me if i'm wrong but
it means this particular instance only allow to perform S3 Read only
operation.
Yes
Now says I created a User with Programmatic access and
AmazonS3FullAccess policy. If this user SSH into the EC2 instance, can
he write file to s3 ?
IAM users cannot SSH to EC2 instances using IAM user credentials. After provisioning a EC2 instance, you need to use regular Operating System User constructs, to SSH to the Server (Default user keys created by AWS).
In addition if a user SSH to EC2 instance and use a Programatic Access Credentials of a EC2 User through AWS CLI, REST API or SDKs (Doesn't have to be a EC2 instance, it also can be your on-premise server) then if the IAM User has a S3 write policy, the CLI commands or API calls or the code using SDK is able to write files to S3.
So in a summary
- Use IAM roles if you are running a EC2 instance, for your CLI commands, Code with SDK, or REST API calls to access AWS Resources.
- If you are using a server on-premise or outside AWS, use IAM User's Programatic Access keys to do the same.
Insight on how IAM roles work internally with EC2
- When you attach a IAM role to a EC2 instance, AWS periodically updates the EC2 instance with temporal Access Credentials to that EC2 instance (Which is a good security practice).
- These credentials are accessible through the Metadata URLs for the CLI, REST API and Code using SDKs inside EC2 instance.
Note: When using Roles, its much secure since it uses Temporal access credentials vs IAM Users Programatic Access uses Long lived access credentials.
