The Domain Name System (DNS), comparable to an address book, forms the backbone of today's communication on the Internet. With IPv4, IT administrators could memorize their most important servers' IPv4 addresses, but there is no alternative to DNS with IPv6. In this article, I will show you how attackers exploit old DNS records to hijack parts of your domain, as well as show you how to protect yourself against these attacks.

DNS is divided into zones (a portion of the DNS namespace) managed by individual organizations or administrations. New entries are quickly created, and many projects sometimes result in very large zone files. Often, remnants of test setups or projects will remain in a zone even after they are no longer in use. Combine this with external services, such as external hosting or cloud service providers, and outdated entries can become dangerous.

Subdomain Hijacking

A simple example involves an A or AAAA entry from a subdomain on a cloud provider's IP address. When a project ends, all paid services are canceled with no further access to the cloud server. If this DNS entry does not result in explicit costs, users often simply forget to delete the entry. Because the entry still points to the provider's IP address, this entry becomes a dangling record – at least as long as the resource is not accessible. If a potential attacker gets the newly freed IP address for their server, the address can also be reached through your company's subdomain, but the services or content offered at the IP address are no longer under your control.

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