What it looks like
A subdomain on your domain has a CNAME pointing at a gitlab.io target, and loading it returns GitLab Pages's unclaimed page rather than a working site. The visible signature is text along the lines of "The page you're looking for could not be found".
Both signals are needed. A CNAME pointing at GitLab Pages 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 or renamed GitLab Pages project frees the namespace, which can be recreated by another account.
What an attacker gets
- Content served from your subdomain, on your domain, usually with a valid certificate GitLab Pages 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. Verify the domain in GitLab if you intend to keep using Pages, which prevents reclaiming.
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.