What it looks like
A subdomain on your domain has a CNAME pointing at a wordpress.com target, and loading it returns WordPress.com's unclaimed page rather than a working site. The visible signature is text along the lines of "Do you want to register".
Both signals are needed. A CNAME pointing at WordPress.com 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 WordPress.com site frees the address, which can be registered again by any account.
What an attacker gets
- Content served from your subdomain, on your domain, usually with a valid certificate WordPress.com 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. Blog subdomains outlive the blog more reliably than almost any other kind, because content gets migrated and DNS does not.
Full removal steps for any DNS provider are in the dangling record guide.
Stopping the next one
These appear whenever a hosted service is trialled and dropped, so one 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 is on anyone's mind.