What it looks like
A subdomain on your domain has a CNAME pointing at a web.app or firebaseapp.com target, and loading it returns Firebase Hosting's unclaimed page rather than a working site. The visible signature is text along the lines of "Site Not Found".
Both signals are needed. A CNAME pointing at Firebase Hosting 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
Deleted Firebase projects release their hosting names, and custom domains left pointing at them can be reconnected elsewhere.
What an attacker gets
- Content served from your subdomain, on your domain, usually with a valid certificate Firebase Hosting 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. Firebase is common in prototypes and demos, so these records are usually left from something that was never meant to last.
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.