Hi Readers,It's quite possible that a sub-domain has a "loop back" address, i.e. 127.0.0.1. Many security researchers and developers may not be aware of this lesser-known vulnerability.
Imagine a scenario where a user has to access "subdomain.example.com". If the sub domain is configured with address 127.0.0.1 and, if the user is already running a service on their localhost (Eg. Xamp/Wamp server running), he/she will obviously be redirected to the localhost services. He/she will never be able to visit "subdomain.example.com" unless the user has stopped the services on the localhost.
Run a Simple Test
Simply send a ping request to the sub domain to find the IP address:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Many organization have the DNS misconfigured with the address 127.0.0.1 (but, we won't disclose all those websites). We will share that the famous Bug Bounty Program, HackerOne, had this issue. They were notified by a security researcher and fixed the vulnerability.
Let's Fix the Issue
1. Change the IP address from 127.0.0.1 to a random address.
2. Simply remove the sub-domain entry if it's not needed.