What it looks like
A subdomain on your domain has a CNAME record pointing at yourcompany.zendesk.com, and loading that subdomain returns Zendesk's unclaimed page rather than a working site. The visible signature is a page containing text along the lines of "Help Center Closed".
Both signals are needed. A CNAME pointing at Zendesk 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 cancelled Zendesk account releases the subdomain, which can be registered again by another account and have your help centre domain pointed at it.
What an attacker gets
- Content served from your subdomain, on your domain, usually with a valid certificate that Zendesk 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. Help centre subdomains such as help. or support. are frequently left behind after a support platform change, and they are the most convincing possible phishing address.
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.