If you manage a website or online service, there’s a security vulnerability lurking in your DNS settings that you might not even know exists. Subdomain takeover attacks have become one of the most overlooked yet dangerous threats in modern web security. The worst part? They’re surprisingly easy for attackers to execute, and they can give hackers complete control over parts of your domain without you realizing it until serious damage is done.
Understanding how these attacks work isn’t just for security professionals anymore. Whether you run an e-commerce site, a SaaS platform, or a corporate website, knowing how to identify and prevent subdomain takeovers could save you from brand damage, data breaches, and the trust of your customers.
What Exactly Is a Subdomain Takeover?
A subdomain takeover happens when an attacker gains control over a subdomain of your website by exploiting outdated or misconfigured DNS records. Here’s the simple version: imagine you once created a subdomain like blog.yourcompany.com that pointed to a third-party service like GitHub Pages, Heroku, or AWS S3. You used that service for a while, then stopped using it and deleted your account or project there. But here’s the critical mistake – you forgot to remove the DNS record that still points your subdomain to that now-defunct service.
An attacker can discover this dangling DNS record, claim the same username or resource on that third-party service, and suddenly your subdomain is serving their content. To anyone visiting blog.yourcompany.com, it looks completely legitimate because it’s still your domain.
The Real-World Impact of These Attacks
I learned about this vulnerability the hard way a few years back when auditing a client’s infrastructure. They had dozens of forgotten subdomains from old marketing campaigns, testing environments, and employee projects. One of them, promo.clientdomain.com, was pointing to an expired Heroku app. I demonstrated how easily someone could have claimed that Heroku app name and served malicious content under their trusted domain. They were shocked at how simple it was.
The consequences of a successful subdomain takeover can be severe. Attackers can use your subdomain to host phishing pages that look incredibly convincing because they’re actually on your domain. They can steal cookies and session tokens from your users. They can damage your SEO rankings and brand reputation. In some cases, they can even manipulate your domain’s security policies if the compromised subdomain has certain permissions.
How Attackers Find Vulnerable Subdomains
You might think your forgotten subdomains are safe because they’re obscure or not widely known. Unfortunately, attackers have automated tools that scan millions of domains looking for these exact vulnerabilities. They use DNS enumeration tools to discover all subdomains associated with a target domain, then check which ones point to services but return error messages indicating the resource no longer exists.
The process is disturbingly straightforward. An attacker runs a subdomain scanner, identifies a subdomain with a CNAME record pointing to something like old-project.herokuapp.com, visits that URL and sees a ”No such app” error, then simply creates a new Heroku app with that same name. Within minutes, they control your subdomain.
Common Services Vulnerable to Takeover
Certain platforms are more commonly exploited for subdomain takeovers than others. Cloud hosting services like AWS S3, Azure, and Google Cloud Storage are frequent targets because they allow users to claim bucket or resource names on a first-come, first-served basis. If your DNS points to a bucket you’ve deleted, someone else can create a bucket with the same name.
GitHub Pages, Heroku, Shopify stores, and various CDN services also present risks. Even services like Zendesk, Tumblr, and WordPress.com hosting can be vulnerable if you’ve deleted your account but left DNS records in place. The key factor is whether the service allows new users to claim resources that match your DNS configuration.
Step-by-Step Prevention Strategy
Preventing subdomain takeovers requires a systematic approach. Start by creating a complete inventory of all your subdomains. Don’t rely on memory or informal documentation – use DNS enumeration tools or subdomain discovery services to find every single subdomain associated with your domains. You’ll probably be surprised by what you find.
Next, verify each subdomain. Check where each one points and confirm that the resource on the other end is still under your control. If you find a subdomain pointing to a service you no longer use, you have two options: either reclaim or recreate the resource on that service to maintain control, or delete the DNS record entirely.
For ongoing protection, implement regular DNS audits. Set a schedule to review your DNS records monthly or quarterly. Document what each subdomain is for and who owns it within your organization. When someone leaves your company or a project ends, make DNS cleanup part of your offboarding checklist.
Automated Monitoring: Your Best Defense
Manual audits are good, but they’re not enough in dynamic environments where new subdomains get created frequently. Automated DNS monitoring can alert you immediately when something goes wrong. These systems continuously check that your DNS records point to resources you control and notify you if they detect vulnerable configurations.
The ideal monitoring solution will not only track your known subdomains but also discover new ones automatically as they’re created. It should verify that external services referenced in your DNS records are responding correctly and haven’t been abandoned.
Common Misconceptions About Subdomain Security
Many people believe that if they don’t actively use a subdomain, it can’t cause harm. This is dangerously wrong. An inactive subdomain with a misconfigured DNS record is actually more dangerous because you’re not watching it. Another myth is that only large companies need to worry about this. In reality, attackers often target smaller organizations specifically because they’re less likely to have robust DNS security practices.
Some teams think that SSL certificates protect against subdomain takeovers. They don’t. An attacker who controls your subdomain through a takeover can often get their own SSL certificate issued for it through services like Let’s Encrypt, making their malicious site look even more legitimate.
Quick Action Checklist
If you want to secure your infrastructure today, start with these immediate actions: Run a subdomain enumeration scan on your domains. Review every DNS record, especially CNAME records pointing to external services. Delete any records for services you no longer use. Document the purpose and owner of each remaining subdomain. Set up monitoring to alert you of DNS changes and potential vulnerabilities.
The security of your domain isn’t just about your main website. Every subdomain is a potential entry point for attackers, and subdomain takeovers are only becoming more common as organizations accumulate digital infrastructure over time. Taking control of your DNS configuration now will protect your brand, your users, and your reputation from an attack vector that’s hiding in plain sight.