How Enterprises Can Mitigate the Risk of Subdomain Takeover

What is subdomain takeover?

Cybercriminals diligently monitor the internet for publicly available information on DNS zone records to carry out subdomain takeover, also known as subdomain hijacking. It’s a cyber threat executed when an attacker gains control of an authentic subdomain that’s no longer in use, then cleverly leverages the forgotten or improperly configured “dangling” DNS to host their own content on the previously used zone.

Innocent web users land on these subdomains loaded with the criminal’s harmful content, all without the criminal infiltrating an organization’s infrastructure or third-party service account. Aside from reputation damage and loss in consumer confidence, a subdomain takeover could lead to more damaging data and security breaches.

What creates a subdomain takeover vulnerability?

Large organizations with diverse brand portfolios and international operations are often unaware of the scale of their digital footprint. Over time, digital records accumulate, creating "noise" that complicates basic cyber hygiene and housekeeping. This lack of oversight makes organizations more vulnerable to cybercriminals.

The issue is further compounded with decentralized management or staff turnover. For example, marketers may retire a brand or campaign and deactivate a website but leave associated DNS records intact. These records, which are entries in the DNS that provide instructions on how domain names should be handled, are often left untouched by administrators who fear deleting something critical. This results in dangling DNS—inactive zones that no longer point to valid content and are at risk of subdomain hijacking.

Subdomain hijacking vs. other types of DNS attacks

While subdomain takeover targets specific subdomains, other DNS-related threats differ in scope and method.

• DNS hijacking involves unauthorized changes to DNS resolution paths, typically by compromising a DNS provider, nameserver, or network infrastructure. Attackers use this technique to redirect traffic, intercept communications, or disrupt domain function—often affecting an entire domain rather than just a subdomain.

• DNS poisoning manipulates DNS resolver caches to store false DNS records, redirecting users to malicious sites or causing service disruption until the cache is cleared or expired.

Risks of subdomain takeover

Subdomain hijacking can lead to several serious risks for businesses, including:

1. Data breaches. Once in control, cybercriminals can access confidential information such as customer data, business secrets, or login credentials. This may result in financial losses, legal penalties, and regulatory issues, especially in industries handling personally identifiable information or financial data.

2. Phishing campaigns. Hijacked subdomains can host phishing sites that look genuine, tricking users into revealing sensitive details like passwords or payment information. This compromises both the business and its customers.

3. Brand damage. Malicious use of a subdomain, such as hosting schemes or malware, can harm your standing, causing customers and partners to lose trust.

4. Search engine optimization (SEO) and traffic manipulation. Hijackers may redirect traffic to harmful sites or promote deceptive content. This hurts sales, user engagement, and SEO efforts, leading to long-term visibility issues.

5. Legal and compliance issues. If hijacked subdomains are used for illegal activities, businesses may face fines or penalties for inadequate security, especially if customer data is involved.

6. Financial loss. Reputation damage, legal fees, and remediation efforts can be costly, impacting overall business performance.

Restoring trust, fixing security gaps, and addressing the aftermath of a security event can all be costly endeavors.

How enterprises can prevent subdomain takeover

There are some steps you can take to mitigate the risk of subdomain hijacking.

1. Regular subdomain audits. Frequently review and remove unused subdomains, especially those linked to cloud services or external services that are no longer in use.

2. DNS record management. Identify and eliminate dangling CNAME or A records that point to decommissioned resources.

3. Access control and decommissioning policies. Restrict who can create and manage subdomains and establish a formal decommissioning process to ensure subdomains and their associated records are properly removed when they’re no longer needed.

4. Subdomain monitoring. Implement a robust monitoring tool that tracks DNS changes, detects vulnerabilities, and flags unauthorized use in real time.

Subdomain monitoring