Imagine visiting a company's website, sayfiles.example.com, expecting to see important documents. Instead, you find a page that says, "This subdomain has been hacked!" How did this happen? The answer is a common security mistake called a Subdomain Takeover.

Let's break down what this is, why it happens, and how someone can actually do it.

The Story of a Forgetful Developer

To understand this, let's follow a simple story:

  1. Setting Up Shop: A company wants to store files online. They use a popular storage service, like Google Drive. After setting it up, the service gives them a special web address, something like xyz.storage.com.
  2. Pointing Home: The company wants this storage to have a professional name, so they create their own subdomain files.example.comand point it to xyz.storage.com. Now, when you visitfiles.example.com, you see their files on Google Drive. Great!
  3. The Big Move: Later, the company switched to a different storage service, like Amazon S3. They delete their old Google Drive account and move all their files.
  4. The Critical Mistake: Here's where things go wrong. The company forgets one crucial step: they don't remove the pointer from files.example.com to the old xyz.storage.com.

This is like moving out of a house but forgetting to return your old address. Your mail is still being sent to a place you no longer own.

How the Hacker Steps In

Now, the old Google Drive address (xyz.storage.com) is up for grabs. It's like a username that became available after the original owner deleted their account.

A hacker can come along, create a new account on that same storage service, and claim that same address: xyz.storage.com.

Since the company's subdomain files.example.com is still pointing to this address; it will now display whatever content the hacker puts there. Just like that, the hacker has taken over the subdomain.

Why Is This So Dangerous?

You might think, "It's just a subdomain, how bad could it be?" The answer is: pretty bad.

  1. Fake Logins: A hacker could create a fake login page that looks exactly like the real company's site, tricking employees and customers into handing over their passwords.
  2. Stolen Data: The subdomain might receive sensitive information like user cookies, which the hacker can intercept.
  3. Damaged Reputation: It makes the company look irresponsible and damages trust.

How to Actually Find and Take Over a Subdomain (An Example)

So, how do you find these vulnerable subdomains? Let's walk through the steps:

  1. Find a Candidate First, you need to find a subdomain that shows a specific error. A common one is for Amazon's S3 storage service. You might see an error like "404 NoSuchBucket" or "The specified bucket does not exist." This is a big red flag that the company might have left a subdomain pointing to a deleted S3 storage "bucket."
  2. Confirm It's Vulnerable You don't have to guess. There's a fantastic online resource or a repo on GitHub called "canitakeoverxyz". This repo has a list of all the services (like AWS, Google, etc.) that are vulnerable to this attack. It tells you exactly what error messages to look for. If your subdomain's error message matches one on the list, it's likely vulnerable.
  3. Claim the Territory If it's vulnerable, the exploit is straightforward. In the case of an abandoned S3 bucket:
  • Create an account on AWS (Amazon's cloud service).
  • Go to the S3 service and create a new storage bucket.
  • Here's the key: Give the new bucket the exact same name as the vulnerable subdomain (e.g., files.example.com).
  • Upload a simple file (like an index.html page) and configure the bucket to be publicly accessible and to act as a website.

4. The Magic: Now, when you or anyone else visits the company's subdomain, it will load the file from your S3 bucket instead of showing an error. You've successfully taken it over!

The Bottom Line

Subdomain takeover is a simple yet powerful vulnerability caused by a simple oversight: forgetting to clean up old digital pointers. For companies, it's a critical reminder to manage their DNS records carefully. For security learners, it's a fascinating and accessible way to understand how small mistakes can lead to significant security holes.