If you run a website, manage infrastructure, or work in DevOps — there's a good chance you have at least one forgotten subdomain pointing at a service you no longer use.

It's not negligence. It's just how the web works. Teams spin up services constantly — staging environments, marketing landing pages, support portals, documentation sites — and when those services get shut down, DNS records are rarely the first thing anyone thinks about.

The problem is that a forgotten DNS record isn't just clutter. It can be a serious security vulnerability.

What's the risk?

When a subdomain points to an external service via a CNAME record, and that service is no longer configured, an attacker can register the unclaimed service and take control of your subdomain.

This is called a subdomain takeover .

staging.yourcompany.com → CNAME → old-app.herokuapp.com → ❌ unclaimed

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An attacker who notices this can register old-app.herokuapp.com and immediately control what appears at…