Safeguarding Against Subdomain Takeover
Subdomain takeovers are a growing threat in today’s cloud-first ecosystem. As organizations rely on third-party services, continuously launch digital assets, and manage sprawling DNS configurations, they often leave behind vulnerable subdomains ripe for exploitation.
In this article, we explore subdomain takeovers, why they pose such a serious risk, and most importantly, how to prevent them before threat actors strike.
What Is a Subdomain Takeover?
A subdomain takeover occurs when an attacker gains control over a subdomain of a target domain that is no longer in use or is misconfigured. This typically happens when a DNS entry (such as a CNAME record) still points to a third-party service (like GitHub Pages, Heroku, or AWS S3) that has been decommissioned but hasn’t been unlinked.
Once an attacker claims the resource originally intended for the subdomain, they can host malicious content, launch phishing campaigns, steal credentials or session cookies, or even deliver malware. This grants threat actors control over the content users see when they visit what appears to be a legitimate part of your site.
These subdomains were originally intended for an organization’s domain infrastructure or service delivery, but become easy targets if not properly maintained or monitored. Misconfigurations within the domain name system, especially forgotten or mismanaged DNS entries, often cause these vulnerabilities.
Common Examples of Vulnerable Scenarios
Subdomain takeovers can occur in various routine IT or DevOps workflows, especially when teams decommission services without fully cleaning up DNS records or verifying domain ownership status.
- A subdomain (e.g., dev.yourcompany.com) points to a deleted GitHub Pages site, but the DNS entry is still active.
- A CNAME record continues to reference an AWS S3 bucket that was removed during a cloud migration.
- A Heroku application is shut down during a product sunset, yet its associated subdomain remains publicly resolvable.
- A testing or staging environment is spun up temporarily for QA purposes and later forgotten, despite still being accessible via a subdomain.
- A team uses a third-party service to host microsites or landing pages, but after the service contract ends, the linked subdomains are never retired.
- A marketing campaign subdomain (like promo.yourcompany.com) is linked to a service like Unbounce or Webflow, and when the campaign ends, no one takes ownership to archive or remove the DNS pointer.
Lapses in oversight can greatly raise the risk of a compromised subdomain being exploited to deliver phishing attacks or distribute malware. These situations create an opening for attackers to register the now-unclaimed service and exploit the subdomain for malicious purposes.
Why Subdomain Takeover Is So Dangerous
The threat of a subdomain takeover goes far beyond simple embarrassment. Attackers exploit these misconfigurations to:
- Host phishing pages that appear legitimate
- Collect sensitive data like logins and payment info
- Redirect users to malicious websites
- Gain internal access via credential reuse or cookie harvesting
- Damage your brand reputation by leveraging your main domain
A hijacked subdomain can lead to a full-blown data breach, reputational damage, and regulatory consequences, especially if it’s used to exploit user trust or impersonate your services.
How Subdomain Takeover Happens
A typical subdomain takeover attack follows these steps:
- Discovery: The attacker identifies vulnerable subdomains through automation or reconnaissance
- Verification: They confirm that the DNS configuration is still live and pointing to an unclaimed external service
- Claiming: The attacker registers or claims the third-party service originally tied to the subdomain name
- Exploitation: Once in control, the attacker may host malicious content, set up credential harvesting forms, or launch phishing campaigns
Once the attacker has claimed the resource, they can use the hijacked subdomain to impersonate your services. This includes serving cloned login pages, injecting malicious scripts, or responding to an HTTP request as if it came from your legitimate infrastructure. Because the subdomain still appears to belong to your organization, users are far more likely to trust it and engage with it, making the impact of a takeover both stealthy and dangerous.
This entire process can happen in minutes if organizations don’t have a proper incident response plan or continuous monitoring in place.
Why Organizations Are Vulnerable to Takeover
Subdomain takeovers often stem from overlooked DNS entries, poor visibility into the DNS zone, or outdated infrastructure.
Common causes include:
- Untracked third-party service decommissions
- Gaps in cloud asset inventory
- No automation to detect potential subdomain takeover
- Siloed IT and security teams failing to monitor subdomain lifecycle
- Lack of control over DNS across distributed teams or subsidiaries
In many cases, no single team is clearly responsible for subdomain management, which means no one notices when a subdomain is left pointing to an inactive resource.
Without centralized tracking or cleanup workflows, decommissioned services are easy to miss. Attackers monitor for these lapses and quickly claim control of the subdomain, especially when it is still publicly resolvable and accessible via HTTP or HTTPS.
Companies that rely heavily on cloud-based development environments or frequently spin up temporary services are especially at risk. If cleanup procedures are not clearly defined, these environments can be forgotten and left exposed, increasing the overall threat of subdomain takeover.
How to Prevent Subdomain Takeover
Preventing subdomain takeovers requires vigilant monitoring, secure configurations, and streamlined asset management. Here are some best practices to help you stay protected:
Audit Your DNS Records Regularly
Manually and programmatically review all CNAME records, A records, and other relevant DNS entries. Ensure they are actively used and point to valid, claimed services. If they’re no longer needed, be sure to remove the DNS entry and reclaim control.
Monitor for Orphaned Subdomains
Use tools to identify vulnerable subdomain takeover scenarios and map your entire external attack surface, including subdomains tied to cloud services.
Automate Subdomain Detection
Automate scans for potential vulnerabilities in subdomains, especially when decommissioning a cloud asset. Alert your teams immediately when a misconfigured or unclaimed subdomain appears. This helps ensure an attacker can’t exploit an organization’s intended DNS entry.
Apply DNS Hygiene Best Practices
Avoid dangling CNAME records or stale redirects. Use verification processes when assigning third-party services, and clearly document domain ownership and lifecycle procedures. Prevent subdomain exposures by strictly controlling DNS record provisioning and decommissioning workflows.
Train Teams and Set Governance
Ensure developers, IT, and DevOps teams understand the risk of subdomain takeover. Build policies that require DNS cleanup during decommissioning and use a centralized registry for all domains and subdomains.
Consider adding a checkpoint in your process for creating a new subdomain to ensure it is not forgotten or left unmonitored.
Real-World Examples
Companies of all sizes have experienced real consequences when subdomains are left exposed or misconfigured.
- A major tech company had a subdomain takeover that allowed attackers to mimic its login page and steal credentials
- A financial institution was targeted by a phishing campaign via a compromised subdomain, resulting in fraud, data loss, and reputational damage
These attacks often go unnoticed until it’s too late—prevention is your best defense.
Take Control Before Attackers Do
Subdomain takeovers are stealthy, scalable, and increasingly common in an age of cloud-first, fast-moving organizations. If your DNS records aren’t tightly managed or you rely on outdated asset lists, you’re leaving the door open for attackers.
As you grow, continuously ask whether a subdomain is necessary; if not, decommission it properly.
Don’t Let Subdomain Takeover Be Your Blind Spot
Subdomain takeover is not just a theoretical risk. It is a real-world threat that attackers actively exploit to compromise organizations of every size. With the right visibility, automation, and hygiene practices, you can eliminate this risk before it becomes a crisis.
SecurityScorecard empowers you to:
- Discover vulnerable subdomains
- Monitor your external attack surface in real time
- Detect misconfigurations and prevent exploitation
- Collaborate across teams to secure your DNS zone
- Hold vendors accountable with supply chain risk insights
- Streamline remediation with automation and guided workflows
Take advantage of SecurityScorecard’s platform to stay ahead of subdomain threats and reduce risk across your digital ecosystem:
- Use Automatic Vendor Detection to uncover hidden third-party and fourth-party risks before attackers do
- Deploy MAX Managed Services to offload ongoing monitoring, issue resolution, and vendor communication to our security experts
- Leverage AI-driven insights with Scoring 3.0 to identify and prioritize the issues most likely to lead to breaches
- Strengthen your vendor risk program with integrated questionnaires, compliance frameworks, and continuous monitoring
- Generate executive-ready reports that translate technical findings into actionable business intelligence
