AWS Security Blog

Enhanced Domain Protections for Amazon CloudFront Requests

Over the coming weeks, we’ll be adding enhanced domain protections to Amazon CloudFront. The short version is this: the new measures are designed to ensure that requests handled by CloudFront are handled on behalf of legitimate domain owners.

Using CloudFront to receive traffic for a domain you aren’t authorized to use is already a violation of our AWS Terms of Service. When we become aware of this type of activity, we deal with it behind the scenes by disabling abusive accounts. Now we’re integrating checks directly into the CloudFront API and Content Distribution service, as well.

Enhanced Protection against Dangling DNS entries
To use CloudFront with your domain, you must configure your domain to point at CloudFront. You may use a traditional CNAME, or an Amazon Route 53 “ALIAS” record.

A problem can arise if you delete your CloudFront distribution, but leave your DNS still pointing at CloudFront, popularly known as a “dangling” DNS entry. Thankfully, this is very rare, as the domain will no longer work, but we occasionally see customers who leave their old domains dormant. This can also happen if you leave this kind of “dangling” DNS entry pointing at other infrastructure you no longer control. For example, if you leave a domain pointing at an IP address that you don’t control, then there is a risk that someone may come along and “claim” traffic destined for your domain.

In an even more rare set of circumstances, an abuser can exploit a subdomain of a domain that you are actively using. For example, if a customer left “images.example.com” dangling and pointing to a deleted CloudFront distribution which is no longer in use, but they still actively use the parent domain “example.com”, then an abuser could come along and register “images.example.com” as an alternative name on their own distribution and claim traffic that they aren’t entitled to. This also means that cookies may be set and intercepted for HTTP traffic potentially including the parent domain. HTTPS traffic remains protected if you’ve removed the certificate associated with the original CloudFront distribution.

Of course, the best fix for this kind of risk is not to leave dangling DNS entries in the first place. Earlier in February, 2018, we added a new warning to our systems. With this warning, if you remove an alternate domain name from a distribution, you are reminded to delete any DNS entries that may still be pointing at CloudFront.

We also have long-standing checks in the CloudFront API that ensure this kind of domain claiming can’t occur when you are using wildcard domains. If you attempt to add *.example.com to your CloudFront distribution, but another account has already registered www.example.com, then the attempt will fail.

With the new enhanced domain protection, CloudFront will now also check your DNS whenever you remove an alternate domain. If we determine that the domain is still pointing at your CloudFront distribution, the API call will fail and no other accounts will be able to claim this traffic in the future.

Enhanced Protection against Domain Fronting
CloudFront will also be soon be implementing enhanced protections against so-called “Domain Fronting”. Domain Fronting is when a non-standard client makes a TLS/SSL connection to a certain name, but then makes a HTTPS request for an unrelated name. For example, the TLS connection may connect to “www.example.com” but then issue a request for “www.example.org”.

In certain circumstances this is normal and expected. For example, browsers can re-use persistent connections for any domain that is listed in the same SSL Certificate, and these are considered related domains. But in other cases, tools including malware can use this technique between completely unrelated domains to evade restrictions and blocks that can be imposed at the TLS/SSL layer.

To be clear, this technique can’t be used to impersonate domains. The clients are non-standard and are working around the usual TLS/SSL checks that ordinary clients impose. But clearly, no customer ever wants to find that someone else is masquerading as their innocent, ordinary domain. Although these cases are also already handled as a breach of our AWS Terms of Service, in the coming weeks we will be checking that the account that owns the certificate we serve for a particular connection always matches the account that owns the request we handle on that connection. As ever, the security of our customers is our top priority, and we will continue to provide enhanced protection against misconfigurations and abuse from unrelated parties.

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