What it looks like
A subdomain on your domain has a CNAME record pointing at sitename.netlify.app, and loading that subdomain returns Netlify's unclaimed page rather than a working site. The visible signature is a page containing text along the lines of "Not Found".
Both signals are needed. A CNAME pointing at Netlify 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 deleted Netlify site frees its subdomain name, which can then be claimed and have your custom domain attached to it.
What an attacker gets
- Content served from your subdomain, on your domain, usually with a valid certificate that Netlify 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
Delete the DNS record. Netlify sites are frequently created for previews and client demos and then removed, which is why this one turns up often on agency-managed domains.
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.