What it looks like
A subdomain on your domain has a CNAME record pointing at a fastly.net target, and loading that subdomain returns Fastly's unclaimed page rather than a working site. The visible signature is a page containing text along the lines of "Fastly error: unknown domain".
Both signals are needed. A CNAME pointing at Fastly 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
A removed Fastly service leaves the domain unclaimed on their edge, where it can be attached to another account's service.
What an attacker gets
- Content served from your subdomain, on your domain, usually with a valid certificate that Fastly 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, or reattach the domain to a current Fastly service if the CDN is still in use elsewhere.
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.