Six tips to improve the security of your AWS Transfer Family server

AWS Transfer Family is a secure transfer service that lets you transfer files directly into and out of Amazon Web Services (AWS) storage services using popular protocols such as AS2, SFTP, FTPS, and FTP. When you launch a Transfer Family server, there are multiple options that you can choose depending on what you need to do. In this blog post, I describe six security configuration options that you can activate to fit your needs and provide instructions for each one.

Use our latest security policy to help protect your transfers from newly discovered vulnerabilities

By default, newly created Transfer Family servers use our strongest security policy, but for compatibility reasons, existing servers require that you update your security policy when a new one is issued. Our latest security policy, including our FIPS-based policy, can help reduce your risks of known vulnerabilities such as CVE-2023-48795, also known as the Terrapin Attack. In 2020, we had already removed support for the ChaCha20-Poly1305 cryptographic construction and CBC with Encrypt-then-MAC (EtM) encryption modes, so customers using our later security policies did not need to worry about the Terrapin Attack. Transfer Family will continue to publish improved security policies to offer you the best possible options to help ensure the security of your Transfer Family servers. See Edit server details for instructions on how to update your Transfer Family server to the latest security policy.

Use slashes in session policies to limit access

If you’re using Amazon Simple Storage Service (Amazon S3) as your data store with a Transfer Family server, the session policy for your S3 bucket grants and limits access to objects in the bucket. Amazon S3 is an object store and not a file system, so it has no concept of directories, only prefixes. You cannot, for example, set permissions on a directory the way you might on a file system. Instead, you set session policies on prefixes.

Even though there isn’t a file system, the slash character still plays an important role. Imagine you have a bucket named DailyReports and you’re trying to authorize certain entities to access the objects in that bucket. If your session policy is missing a slash in the Resource section, such as arn:aws:s3:::$DailyReports*, then you should add a slash to make it arn:aws:s3:::$DailyReports/*. Without the slash (/) before the asterisk (*), your session policy might allow access to buckets you don’t intend. For example, if you also have buckets named DailyReports-archive and DailyReports-testing, then a role with permission arn:aws:s3:::$DailyReports* will also grant access to objects in those buckets, which is probably not what you want. A role with permission arn:aws:s3:::$DailyReports/* won’t grant access to objects in your DailyReports-archive bucket, because the slash (/) makes it clear that only objects whose prefix begins with DailyReports/ will match, and all objects in DailyReports-archive will have a prefix of DailyReports-archive/, which won’t match your pattern. To check to see if this is an issue, follow the instructions in Creating a session policy for an Amazon S3 bucket to find your AWS Identity and Access Management (IAM) session policy.

Use scope down policies to back up logical directory mappings

When creating a logical directory mapping with a role that has more access than you intend to give your users, it’s important to use session policies to tailor the access appropriately. This provides an extra layer of protection against accidental changes to your logical directory mapping opening access to files you didn’t intend.

Details on how to construct a session policy for an S3 bucket can be found in Creating a session policy for an Amazon S3 bucket, and Create fine-grained session permissions using IAM managed policies provides additional context. Amazon S3 also offers IAM Access Analyzer to assist with this process.

Don’t place NLBs in front of a Transfer Family server

We’ve spoken with many customers who have configured a Network Load Balancer (NLB) to route traffic to their Transfer Family server. Usually, they’ve done this either because they created their server before we offered a way to access it from both inside their VPC and from the internet, or to support FTP on the internet. This not only increases the cost for the customer, it can cause other issues, which we describe in this section.

If you’re using this configuration, we encourage you to move to a VPC endpoint and use an Elastic IP. Placing an NLB in front of your Transfer Family server removes your ability to see the source IP of your users, because Transfer Family will see only the IP address of your NLB. This not only degrades your ability to audit who is accessing your server, it can also impact performance. Transfer Family uses the source IP to shard your connections across our data plane. In the case of FTPS, this means that instead of being able to have 10,000 simultaneous connections, a Transfer Family server with an NLB in front of it would be limited to only 300 simultaneous connections. If you have a use case that requires you to place an NLB in front of your Transfer Family server, reach out to the Transfer Family Product Management team through AWS Support or discuss issues on AWS re:Post, so we can look for options to help you take full advantage of our service.

Protect your API Gateway instance with WAF

If you’re using the custom identity provider capability of Transfer Family, you connect your identity provider through Amazon API Gateway. As a best practice, Transfer Family recommends use AWS Web Application Firewall (WAF) to help protect your API Gateway. This will allow you to create access control lists (ACLs) for your API Gateway instance to allow access for only AWS and anyone in the ACL. To help protect your API Gateway instance, see Securing AWS Transfer Family with AWS Web Application Firewall and Amazon API Gateway.

FTPS customers should use TLS session resumption

One of the security challenges with FTPS is that it uses two separate ports to process read/write requests. An analogy to this in the physical world would be going through a drive-thru window where you pay for your food and someone else can cut in front of you to receive your order at the second window. For this reason, security measures have been added to the FTPS protocol over time. In a client-server protocol, there are server-side configurations and client-side configurations.

TLS session resumption helps protect client connections as they hand off between the FTPS control port and the data port. The server sends a unique identifier for each session on the control port, and the client is meant to send that same session identifier back on the data port. This gives the server confidence that it’s talking to the same client on the data port that initiated the session on the control port. Transfer Family endpoints provide three options for session resumption:

1. Disabled – The server ignores whether the client sends a session ID and doesn’t check that it’s correct, if it is sent. This option exists for backward compatibility reasons, but we don’t recommend it.
2. Enabled – The server will transmit session IDs and will enforce session IDs if the client uses them, but clients who don’t use session IDs are still allowed to connect. We only recommend this as a transitional state to Enforced to verify client compatibility.
3. Enforced – Clients must support TLS session resumption, or the server won’t transmit data to them. This is our default and recommended setting.

To use the console to see your TLS session resumption settings:

1. Sign in to the AWS Management Console in the account where your transfer server runs and go to AWS Transfer Family. Be sure to select the correct AWS Region.
2. To find your Transfer Family server endpoint, find your Transfer Family server in the console and choose Main Server Details.
3. Select Additional Details.
4. Under TLS Session Resumption, you will see if your server is enforcing TLS session resumption.
5. If some of your users don’t have access to modern FTPS clients that support TLS, you can choose Edit to choose a different option.

Conclusion

Transfer Family offers many benefits to help secure your managed file transfer (MFT) solution as the threat landscape evolves. The steps in this post can help you get the most out of Transfer Family to help protect your file transfers. As the requirements for a secure, compliant architecture for file transfers evolve and threats become more sophisticated, Transfer Family will continue to offer optimized solutions and provide actionable advice on how you can use them. For more information, see Security in AWS Transfer Family and take our self-paced security workshop.

If you have feedback about this post, submit comments in the Comments section below. If you have questions about this post, start a new thread on the AWS Transfer Family re:Post or contact AWS Support.