What it looks like
A subdomain on your domain has a CNAME record pointing at a bucket endpoint on amazonaws.com, and loading that subdomain returns Amazon S3's unclaimed page rather than a working site. The visible signature is a page containing text along the lines of "NoSuchBucket".
Both signals are needed. A CNAME pointing at Amazon S3 is completely normal when the service is in use. It only matters when the target is no longer claimed.
Why it can be taken over
S3 bucket names are globally unique and released on deletion. Anyone can create a bucket with the released name and serve content from your subdomain.
What an attacker gets
- Content served from your subdomain, on your domain, usually with a valid certificate that Amazon S3 issues automatically.
- A phishing address that passes every check a cautious person would run, because it genuinely is your domain.
- Access to any cookie your main site scopes to the parent domain, which can include session data.
How to fix it
Remove the record. If you need the subdomain, recreate the bucket under your account before pointing anything at it, and consider serving through CloudFront with an origin you control.
The full removal steps for any provider are in the dangling record guide.
Stopping the next one
These appear whenever a hosted service is trialled and dropped, so a single cleanup does not settle it. The habit worth building is deleting the DNS record in the same sitting as the cancellation, because those are separate actions and only one of them is on anyone's mind.