What it looks like
A subdomain on your domain has a CNAME record pointing at a pantheonsite.io target, and loading that subdomain returns Pantheon's unclaimed page rather than a working site. The visible signature is a page containing text along the lines of "404 error unknown site".
Both signals are needed. A CNAME pointing at Pantheon 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 Pantheon site releases the name, and the custom domain can be attached to a different site.
What an attacker gets
- Content served from your subdomain, on your domain, usually with a valid certificate that Pantheon 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. Pantheon is common in agency-managed WordPress and Drupal estates, so check every client domain rather than only the one that flagged.
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.