What's the need ?🤔

What is subdomain enumeration?

It is one of the most crucial parts of the reconnaissance phase while performing a security assessment. Subdomain Enumeration is a process of finding sub-domains associated to the root domain. According to RFC 1034, "a domain is a subdomain of another domain if it is contained within that domain".

What's the need?

• Performing subdomain enumeration via various intensive techniques can help enlarge your attack surface, as you get more assets to find vulnerabilities on.

• A good subdomain enumeration will help you find those hidden/untouched subdomains, resulting lesser people finding bugs on that particular domain. Hence lesser duplicates.

• Finding applications running on hidden, forgotten(by the organization) sub-domains may lead to uncovering critical vulnerabilities.

• Discovering such strangely named subdomains is a critical skill, each bug hunter should possess in today's time.

• For large organizations, to find what services have they exposed to the internet while performing an internal pentest.

More the subdomains = More assets to look for vulnerabilities🐞

⚠️ Common Misconception about "subdomain"

A Fully Qualified Domain Name (FQDN) is the complete domain name for a specific computer, or host, on the internet.

An FQDN looks like this:-

myhost.example.com. ----> Fully Qualified Domain Name

myhost ----> is the host located within the domain example.com (subdomain)

Hence; https://example.com http://myhost.example.com https://internal.accounts.example.com http://internal.accounts.dashboard.example.com

The above-mentioned cannot be called as subdomains. They are the hyperlinks to web applications hosted the respective hosts. Most people have a misconception that these are subdomains of a particular target.

Let's consider an example, admin.example.com is a subdomain on which there isn't any web service hosted. This means that, when we send web probes to admin.example.com using httpx/httprobe (tools that check whether any web service is running on that host), it will not return any output.

This doesn't mean that admin.example.com is not a valid subdomain of root domain example.comThere may exists other services like SSH, SMTP, SMB, WinRM(non-web) hosted on that subdomain that cannot be accessed through your web browser. Surprisingly these services may be vulnerable and their exploits would be publicly available. So in such a case, it's always better that you DNS resolve the subdomains that are gathered from passive enumeration to get the valid ones. Later you can send the valid/alive subdomains for web probing and find out the hosted web applications on them.

Moral of the story:

The methodology of collecting subdomains from tools like amass, subfinder, findomain and directly sending them to httpx/httprobe is absolutely wrong❌. Instead, you should first DNS resolve them using tools like puredns or shuffledns.