Subdomain Takeover: A Google Vulnerability Guide

Hey guys! Ever heard of a subdomain takeover ? It sounds like something out of a hacker movie, right? Well, in a nutshell, it’s when someone manages to gain control over a subdomain that’s supposed to belong to someone else. And yes, even tech giants like Google aren’t immune. So, let’s dive deep into what subdomain takeovers are, how they happen, especially in the context of Google’s services, and what you can do to protect yourself or your organization. Trust me; this is one area of cybersecurity you’ll want to understand.

What is a Subdomain Takeover?

So, what exactly is a subdomain takeover ? Imagine a company, let’s call them “Example Corp,” owns the domain example.com . They might create subdomains like blog.example.com or store.example.com to organize different parts of their website. Now, sometimes, companies change their services or move things around. Maybe Example Corp used to have their blog hosted on a third-party platform like Medium or Tumblr , pointing the blog.example.com subdomain to their Medium or Tumblr account. If they stop using that Medium or Tumblr account but forget to remove the DNS record pointing blog.example.com to it, that’s where the trouble begins. An attacker can then claim that abandoned Medium or Tumblr account, and suddenly, they control blog.example.com . Boom, subdomain takeover! This means they can host malicious content, phish users, or even damage Example Corp’s reputation. The core issue is a dangling DNS record – a record that points to a service that’s no longer in use or controlled by the original owner. Subdomain takeovers are a serious threat because they exploit trust. Users trust the original domain (like example.com ), and that trust extends to its subdomains. Attackers capitalize on this trust to carry out their nefarious activities. Always remember to properly manage and monitor your DNS records to avoid falling victim to such attacks. Doing regular audits and keeping your records clean are essential cybersecurity hygiene practices.

How Subdomain Takeovers Happen on Google

Alright, let’s get specific about how subdomain takeovers can occur within the Google ecosystem. Google offers a vast array of services, and companies often integrate these services with their own domains and subdomains. This integration, while convenient, can also introduce vulnerabilities if not handled carefully. One common scenario involves Google Cloud Storage (GCS). Companies might use GCS to host static assets like images, videos, or documents. They can then point a subdomain, such as assets.example.com , to a specific GCS bucket. If the company later decides to migrate those assets to a different location or discontinue the project altogether, they might forget to delete the GCS bucket or update the DNS record. This leaves the assets.example.com subdomain pointing to a non-existent or abandoned GCS bucket. An attacker can then create a GCS bucket with the same name, effectively taking control of the assets.example.com subdomain. Another area where Google services can be exploited is through Google App Engine. App Engine allows developers to build and deploy web applications. Similar to the GCS scenario, a company might map a subdomain to an App Engine application. If the application is later decommissioned but the DNS record remains, an attacker can potentially deploy their own application to that subdomain. Google Sites is another potential target. If a company creates a Google Site and maps a subdomain to it, forgetting to unmap the subdomain when the site is no longer in use can lead to a takeover. The attacker simply recreates the Google Site with the same name, and they’re in control. The key takeaway here is that any Google service that allows mapping a custom domain or subdomain is a potential entry point for a subdomain takeover. Regular monitoring and meticulous record-keeping are crucial to prevent these vulnerabilities.

Identifying Potential Subdomain Takeover Vulnerabilities

Okay, so how do you go about finding these potential subdomain takeover vulnerabilities? The first step is reconnaissance. You need to identify all the subdomains associated with your target domain. Tools like Sublist3r , Amass , and Assetfinder can help you discover these subdomains. Once you have a list of subdomains, the next step is to check which services they’re pointing to. You can use tools like dig or nslookup to query the DNS records for each subdomain. Look for CNAME records, as these often indicate that the subdomain is pointing to a third-party service. Once you’ve identified the services, you need to determine if the subdomain is still in use. If the service is no longer active or if the content is generic and doesn’t seem to belong to the original domain owner, it could be a sign of a potential takeover vulnerability. Many automated tools can help with this process. Tools like SubOver and Takeover scan subdomains for common takeover vulnerabilities. These tools typically check for specific error messages or responses that indicate a vulnerable service. For example, if a subdomain points to an inactive Amazon S3 bucket, the tool might detect an