Taking Countermeasures

In an ideal world, the DNS zone's administrators would immediately be notified upon completion of a project and subsequently would remove the CNAME record, ensuring these attacks couldn't happen. Unfortunately, this sometimes does not happen in real life. On a positive note, there are a number of tools to help admins to check active and passive DNS entries.

Punk Security's DNS Reaper [1], an actively maintained open source tool, is one such solution. You can download DNS Reaper from GitHub and install it on your system. Punk Security also offers a pre-built image for use in a Docker container, which lets you use DNS Reaper directly without managing the runtime environment and any dependencies yourself. To view DNS Reaper's options and download the Docker image, simply use the following command:

docker run -ti --rm punksecurity/dnsreaper --help

You can use the output to get oriented and identify potential arguments for your use case. Of course, DNS Reaper needs a way to check all the existing entries in your zone. A black box check (i.e., without further knowledge of your DNS zone's structure) is not possible. While there are techniques to determine different entries in the DNS, applying them will not give you all of the existing entries.

For a comprehensive check, your best bet is to export your entire zone from the DNS server and make it available to DNS Reaper. If this is available in a BIND DNS compatible format, the software can use it directly. To use a zone file named domain.zone in the Docker container, you need to mount it as a volume in the image. You can then start it using the following command:

docker run -it --rm -v ./domain.zone:/domain.zone punksecurity/dnsreaperbind --bind-zone-file /domain.zone

Alternatively, you also use providers supported by DNS Reaper, such as AWS or Azure, by passing in your access credentials with the call. Transferring the zone directly from a DNS server is also possible, but this requires intervention in the server's configuration to allow a zone transfer.

Conclusions

Poorly maintained DNS records are more dangerous than they might appear at first glance. Attackers can use dangling records to run attacks that look like they originated in your organization. DNS Reaper helps you monitor your DNS records to protect against attackers.