Amazon S3 Security

With any shared storage system, the biggest question is whether unauthorized users can
access information either intentionally or by mistake.  To ensure that customers have the
utmost in flexibility to determine how, when, and to whom they wish to expose the
information they store in AWS, Amazon S3 APIs provide both bucket- and object-level
access controls, with defaults that only permit authenticated access by the bucket and/or
object creator.  Write and Delete permission is controlled by an Access Control List
(ACL) associated with the bucket.  Permission to modify the  bucket  ACLs is itself
controlled by an ACL, and it defaults to creator-only access.  Therefore, the customer
maintains full control over who has access to their data.  Amazon S3 access can be
granted based on AWS Account ID, DevPay Product ID, or open to everyone.

Data Management  

Another potential concern is whether or not data can be intercepted while “in transit”
from one node on the Internet to AWS.  For maximum security, Amazon S3 is accessible
via SSL encrypted endpoints.  The encrypted endpoints are accessible from both the
Internet and from within Amazon EC2, ensuring that data is transferred securely both
within AWS and to and from sources outside of AWS.  

Customers may wish to secure data even when it is being stored within Amazon S3.  Data
stored within Amazon S3 is not encrypted at rest by AWS.  However, users can encrypt
their  data before it is uploaded to Amazon S3 so that the data cannot be accessed or
tampered with by unauthorized parties.

When an object is deleted from Amazon S3, removal of the mapping from the public
name to the object starts immediately, and is generally processed across the distributed
system within several seconds.  Once the mapping is removed, there is no external access
to the deleted object.  That storage area is then made available only for write operations
and the data is overwritten by newly stored data. 


From www.amazon.com