Glossary Glossary

Subdomain Enumeration




Subdomain enumeration is the process of identifying and mapping the subdomains associated with a given domain name. It allows security teams to get a complete layout of their organizations’ websites.

Since subdomains are generally Internet-facing assets, they are often targeted in cyber attacks and can serve as potential attack entry points. For this reason, subdomain enumeration is a crucial part of external attack surface management (EASM), particularly of the asset discovery process.

Table of Contents

Subdomain Enumeration: A Deep Dive

Why Is Subdomain Enumeration Important?

Subdomain enumeration is a critical security process because it can contribute to the following:

  • Attack surface reduction: Each subdomain represents a potential attack vector, contributing to attack surface expansion. As such, security teams need to ensure that all subdomains are necessary and well secured. The first step to achieve that goal is to get a list of all the subdomains under a given domain name.
  • Hidden application or shadow IT discovery: Subdomains are often used for internal applications, development environments, and testing purposes. Some of them may not be approved by security teams and potentially lack in security controls.
  • Vulnerability detection: Once subdomains are enumerated, security teams can proceed with vulnerability scanning and uncover issues that could lead to subdomain takeover, sensitive data exposure, or unauthorized system access.
  • Forgotten asset discovery: Organizations sometimes forget about old subdomains that are no longer in use. They can become security risks if not adequately secured. Enumerating subdomains can help uncover forgotten and unprotected subdomains.
Subdomain enumeration

What Are the Types of Subdomain Enumeration?

There are two main types of subdomain enumeration—passive and active. Some organizations combine both techniques to get the most out of security processes.

What Is Active Subdomain Enumeration?

Active subdomain enumeration involves directly interacting with a target domain or its associated systems to identify subdomains. For example, the technique called “DNS brute forcing” involves systematically guessing the names of possible subdomains and checking if they resolve to valid IP addresses.

This type of subdomain enumeration is resource-intensive and may have legal and ethical implications since sending large requests to a target domain is required. Because of its intrusive nature, active subdomain enumeration may trigger security alerts.

What Is Passive Subdomain Enumeration?

Passive subdomain enumeration involves gathering information from publicly available sources to identify subdomains. It is a less intrusive approach than active enumeration, as it does not involve directly interacting with a target domain.

Some passive enumeration techniques involve obtaining subdomain information from DNS records and certificate transparency logs, among other data sources.

Subdomains constitute a significant part of an organization’s infrastructure. Subdomain enumeration can help complete an organization’s network view, enabling security teams to develop targeted security strategies to protect their assets.

Key Takeaways

  • Subdomain enumeration is the process of identifying and mapping the subdomains associated with a domain name.
  • It helps reduce attack surfaces, discover hidden applications, detect vulnerabilities, and find forgotten assets.
  • Active enumeration directly interacts with a target domain, can be resource-intensive, and has legal and ethical implications.
  • Passive enumeration is less intrusive and gathers information from publicly available sources.

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