Bug Bounty Recon — Horizontal Enumeration

5 min readJan 12, 2022

Hello guys and gals, it’s me again back with another article about horizontal enumeration.

While performing a security assessment our main goal is to map out all the domains owned by a single entity. This means knowing all the assets facing the internet of a particular organization. It is a bit trickier to find related domains/acquisitions of a particular organization as this step includes some tedious methods and doesn’t give accurate results always. One has to solely perform manual analysis.

From the below image you can get an idea of what a horizontal domain correlation is:



  1. Discovering the IP space:

ASN(Autonomous System Number) is a unique identifier of certain IP prefixes. Very large organizations such as Apple, Github, Tesla have their own significant IP space. To find an ASN of an organization https://bgp.he.net/ is a useful website where we can query.

Let’s find ASN for Apple Inc.


Now that we have found out the ASN number, next we need to figure out IP ranges within that ASN. For this, we will use whois tool.

whois -h whois.radb.net  -- '-i origin AS714' | grep -Eo "([0-9.]+){4}/[0-9]+" | uniq

2) Finding related domains/acquisitions:

a) WhoisXMLAPI

https://www.whoisxmlapi.com/ is an excellent source that provides a good amount of related domains & acquisitions based on the WHOIS record. Singing up on their platform will assign you 500 free credits which renew every month.

Visit https://tools.whoisxmlapi.com/reverse-whois-search . Now searching with the root domain name like dell.com will give all the associated domains.



These are not 100% accurate results, as they contain false positives.

b) Whoxy💰

https://www.whoxy.com/ is yet another great source to perform reverse WHOIS on parameters like Company Name, Registrant Email address, Owner Name. Whoxy has an enormous database of around 329M WHOIS records. But sadly this is a paid service :(

To effectively use Whoxy API there’s a command-line tool called https://github.com/MilindPurswani/whoxyrm.


c) Crunchbase💰

https://www.crunchbase.com/ is another great alternative for finding acquisitions but requires a paid subscription to view all the acquisitions. The trial version allows viewing some of the acquisitions.


3) PTR records (Reverse DNS):

Now since we have got to know the IP address ranges from ASN of an organization, we can perform PTR queries on the IP addresses and check for valid hosts.

What is reverse DNS?

When a user attempts to reach a domain name in their browser, a DNS lookup occurs, matching the domain name(example.com) to the IP address(such as A reverse DNS lookup is the opposite of this process: it is a query that starts with the IP address and looks up the domain name.

This means that, since we already know the IP space of an organization we can, we can reverse query the IP addresses and find the valid domains. Sounds cool?

But how?

PTR records (pointer record) helps us to achieve this. Using https://github.com/projectdiscovery/dnsx tool we can query a PTR record of an IP address and find the associated hostname/domain name.

Apple Inc.🍎 has ASN714 which represents IP range So, let’s see have to perform reverse DNS.


We will first need to install 2 tools:

GO111MODULE=on go get -v github.com/projectdiscovery/mapcidr/cmd/mapcidrGO111MODULE=on go get -v github.com/projectdiscovery/dnsx/cmd/dnsxecho | mapcidr -silent | dnsx -ptr -resp-only -o output.txt


When an IP range is given to mapcidr through stdin(standard input), it performs expansion spitting out each IP address from the range onto a new line:,,, .

Now when dnsx receives each IP address from stdin, it performs reverse DNS and checks for PTR record. If, found it gives us back the hostname/domain name.

dnsX demo

Note: We can also combine the step of discovering the IP space with reverse DNS lookup into one-liner like:

whois -h whois.radb.net  -- '-i origin AS714' | grep -Eo "([0-9.]+){4}/[0-9]+" | uniq | mapcidr -silent | dnsx -ptr -resp-only

4) Favicon Hashing:

What is a favicon?

The image/icon shown on the left-hand side of a tab is called as favicon.ico. This icon is generally fetched from a different source/CDN. Hence, we can find this favicon link from the source code of the website.


How to find the favicon.ico link?

Generating the MurmurHash value:

To generate the MurmurHash value which is unique to each favicon we will use a tool called MurMurHash.


MurMurHash is a simple tool used to generate hash for the given favicon.


1. git clone https://github.com/Viralmaniar/MurMurHash.git
2. cd MurMurHash/
3. pip3 install -r requirements.txt


  • Upon running the tool, it will ask you to enter the URL for the hash.
  • And after entering the favicon link it will provide you with a unique hash value (-2057558656).
python3 MurMurHash.py

Weaponizing through Shodan:

Now we query Shodan (https://www.shodan.io/) http.favicon.hash:<hash> with that favicon hash.

This gave us a whopping 162K assets/hosts. These all can be subdomains or related domains of the Microsoft organization.


🏁 That’s it !!! Done with Horizontal Enumeration 🏁

See ya folks in the next article.




I try to analyze ransomware attacks | Static Code Analysis | Privacy & Security Updates | Pen Testing | Bug Bounty