What is subdomain hijacking?
Subdomain hijacking is a cybersecurity risk where attackers exploit abandoned DNS records to take control of legitimate subdomains. This can lead to phishing attacks, credential theft, and malware distribution. Organizations must regularly audit DNS records, remove outdated entries, and strengthen cloud security policies to prevent these vulnerabilities.
We all hate falling victim to phishing emails sent by our IT departments to ensure we keep a watchful eye on suspicious communications. But hackers are getting sneakier, and the ability to replicate domains that appear authentic is becoming easier. As organizations continue their rapid migration to cloud-based infrastructures, cybersecurity threats are evolving just as quickly. One such emerging risk that demands attention is subdomain hijacking, a vulnerability that, if left unaddressed, can provide attackers with a backdoor into trusted domains.
Understanding subdomain hijacking
Subdomains play a vital role in web infrastructure, allowing businesses to segment services, host applications, and streamline operations. However, when organizations make changes to their hosting environment and fail to clean up outdated DNS entries, they create dangling DNS records: Subdomains that remain in configurations but no longer point to active resources.
This oversight presents an opportunity for attackers. By identifying these orphaned subdomains, bad actors can re-register the associated cloud resources, effectively seizing control of a legitimate-looking domain. The consequences? Phishing campaigns, credential theft, malware distribution, and brand reputation damage, all delivered under the guise of a trusted entity.
The risks are real
Historically, this attack vector has flown under the radar, but security researchers are now shining a spotlight on its prevalence. A recent report from Certitude, a group of ethical hackers, revealed that numerous high-profile organizations, including government agencies and financial institutions, have been susceptible to subdomain hijacking. In some cases, attackers successfully compromised subdomains belonging to entities like the Australian Department of Foreign Affairs and Trade, the UK Meteorological Office, and multiple U.S. state agencies.
These findings reinforce the sobering reality that no organization is immune. The larger and more complex the web infrastructure, the greater the risk of an overlooked vulnerability.
How attackers exploit subdomain hijacking
Understanding the mechanics of this exploit highlights why it is such a potent threat. Attackers typically follow these steps:
- Identify vulnerable subdomains: Using automated tools, cybercriminals scan public DNS records and cloud service configurations to locate orphaned subdomains.
- Reclaim the cloud resource: If expired cloud services are still tied to a dangling DNS entry, attackers often can re-register these services under their control.
- Deploy malicious content: Once control is established, the attacker can host phishing pages, distribute malware, or engage in other fraudulent activities, all under a domain that appears to be a legitimate source.
- Exploit user trust: Because the subdomain remains under the original parent domain, unsuspecting users may trust the site, increasing the success rate of phishing and credential theft campaigns.
Mitigating the risk of subdomain hijacking
Given the potential consequences, organizations must take proactive steps to eliminate this attack vector. A robust security strategy includes the following best practices:
- Conduct regular DNS audits: Maintain a comprehensive inventory of all DNS records and ensure that subdomains actively point to valid resources.
- Eliminate orphaned DNS entries: Immediately remove DNS records linked to deprecated services to prevent attackers from taking control.
- Monitor subdomain activity: Leverage security tools to detect unauthorized changes to subdomains and flag potential vulnerabilities.
- Rethink wildcard certificates: While wildcard SSL certificates simplify certificate management, they also extend trust to all subdomains, including those that may be hijacked. Where possible, opt for individual certificates.
- Strengthen cloud security policies: Implement rigorous domain management procedures, enforce strict access controls, and ensure cloud configurations follow security best practices.
- Move to shorter-lived certificates: If a dangling subdomain still has an active certificate, that is one less barrier for the attacker to overcome. Shorter-lived certificates minimize the attack window for this and other exploits.
The future of subdomain security
With heightened awareness and increased visibility into subdomain hijacking, cybersecurity professionals are developing automated solutions to detect and mitigate these risks before they can be exploited. However, organizations cannot afford to wait for a silver-bullet solution.
By adopting a proactive security mindset, businesses can close the gaps in their DNS configurations, safeguard digital assets, and reinforce customer trust. As cyber threats continue to evolve, securing the foundation of your web infrastructure is not optional, it’s imperative.
Conclusion
Subdomain hijacking is a preventable yet critical security risk that you should not ignore. Through diligence, proactive monitoring, and strategic security measures, organizations can effectively mitigate this threat and maintain a resilient digital presence. Want to learn more? Check out Tim Callan and Jason Soroko's podcast, Root Causes.