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External Recon Methodology

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Assets discoveries

So you were said that everything belonging to some company is inside the scope, and you want to figure out what this company actually owns.
The goal of this phase is to obtain all the companies owned by the main company and then all the assets of these companies. To do so, we are going to:
  1. 1.
    Find the acquisitions of the main company, this will give us the companies inside the scope.
  2. 2.
    Find the ASN (if any) of each company, this will give us the IP ranges owned by each company
  3. 3.
    Use reverse whois lookups to search for other entries (organisation names, domains...) related to the first one (this can be done recursively)
  4. 4.
    Use other techniques like shodan organd sslfilters to search for other assets (the ssl trick can be done recursively).


First of all, we need to know which other companies are owned by the main company. One option is to visit https://www.crunchbase.com/, search for the main company, and click on "acquisitions". There you will see other companies acquired by the main one. Other option is to visit the Wikipedia page of the main company and search for acquisitions.
Ok, at this point you should know all the companies inside the scope. Lets figure out how to find their assets.


An autonomous system number (ASN) is a unique number assigned to an autonomous system (AS) by the Internet Assigned Numbers Authority (IANA). An AS consists of blocks of IP addresses which have a distinctly defined policy for accessing external networks and are administered by a single organisation but may be made up of several operators.
It's interesting to find if the company have assigned any ASN to find its IP ranges. It will be interested to perform a vulnerability test against all the hosts inside the scope and look for domains inside these IPs. You can search by company name, by IP or by domain in https://bgp.he.net/. Depending on the region of the company this links could be useful to gather more data: AFRINIC (Africa), Arin(North America), APNIC (Asia), LACNIC (Latin America), RIPE NCC (Europe). Anyway, probably all the useful information (IP ranges and Whois) appears already in the first link.
#You can try "automate" this with amass, but it's not very recommended
amass intel -org tesla
amass intel -asn 8911,50313,394161
Also, BBOT's subdomain enumeration automatically aggregates and summarizes ASNs at the end of the scan.
bbot -t tesla.com -f subdomain-enum
[INFO] bbot.modules.asn: +----------+---------------------+--------------+----------------+----------------------------+-----------+
[INFO] bbot.modules.asn: | AS394161 | | 5 | TESLA | Tesla Motors, Inc. | US |
[INFO] bbot.modules.asn: +----------+---------------------+--------------+----------------+----------------------------+-----------+
[INFO] bbot.modules.asn: | AS16509 | | 4 | AMAZON-02 | Amazon.com, Inc. | US |
[INFO] bbot.modules.asn: +----------+---------------------+--------------+----------------+----------------------------+-----------+
[INFO] bbot.modules.asn: | AS394161 | | 3 | TESLA | Tesla Motors, Inc. | US |
[INFO] bbot.modules.asn: +----------+---------------------+--------------+----------------+----------------------------+-----------+
[INFO] bbot.modules.asn: | AS3356 | | 1 | LEVEL3 | Level 3 Parent, LLC | US |
[INFO] bbot.modules.asn: +----------+---------------------+--------------+----------------+----------------------------+-----------+
[INFO] bbot.modules.asn: | AS3356 | | 1 | LEVEL3 | Level 3 Parent, LLC | US |
[INFO] bbot.modules.asn: +----------+---------------------+--------------+----------------+----------------------------+-----------+
You can find the IP ranges of an organisation also using http://asnlookup.com/ (it has free API). You can fins the IP and ASN of a domain using http://ipv4info.com/.

Looking for vulnerabilities

At this point we known all the assets inside the scope, so if you are allowed you could launch some vulnerability scanner (Nessus, OpenVAS) over all the hosts. Also, you could launch some port scans or use services like shodan to find open ports and depending on what you find you should take a look in this book to how to pentest several possible services running. Also, It could be worth it to mention that you can also prepare some default username and passwords lists and try to bruteforce services with https://github.com/x90skysn3k/brutespray.


We know all the companies inside the scope and their assets, it's time to find the domains inside the scope.
Please, note that in the following purposed techniques you can also find subdomains and that information shouldn't be underrated.
First of all you should look for the main domain(s) of each company. For example, for Tesla Inc. is going to be tesla.com.

Reverse DNS

As you have found all the IP ranges of the domains you could try to perform reverse dns lookups on those IPs to find more domains inside the scope. Try to use some dns server of the victim or some well-known dns server (,
dnsrecon -r <DNS Range> -n <IP_DNS> #DNS reverse of all of the addresses
dnsrecon -d facebook.com -r #Using facebooks dns
dnsrecon -r -n #Using cloudflares dns
dnsrecon -r -n #Using google dns
For this to work, the administrator has to enable manually the PTR. You can also use a online tool for this info: http://ptrarchive.com/โ€‹

Reverse Whois (loop)

Inside a whois you can find a lot of interesting information like organisation name, address, emails, phone numbers... But which is even more interesting is that you can find more assets related to the company if you perform reverse whois lookups by any of those fields (for example other whois registries where the same email appears). You can use online tools like:
You can automate this task using DomLink (requires a whoxy API key). You can also perform some automatic reverse whois discovery with amass: amass intel -d tesla.com -whois
Note that you can use this technique to discover more domain names every time you find a new domain.


If find the same ID of the same tracker in 2 different pages you can suppose that both pages are managed by the same team. For example, if you see the same Google Analytics ID or the same Adsense ID on several pages.
There are some pages and tools that let you search by these trackers and more:


Did you know that we can find related domains and sub domains to our target by looking for the same favicon icon hash? This is exactly what favihash.py tool made by @m4ll0k2 does. Hereโ€™s how to use it:
cat my_targets.txt | xargs -I %% bash -c 'echo "http://%%/favicon.ico"' > targets.txt
python3 favihash.py -f https://target/favicon.ico -t targets.txt -s
favihash - discover domains with the same favicon icon hash
Simply said, favihash will allow us to discover domains that have the same favicon icon hash as our target.
Moreover, you can also search technologies using the favicon hash as explained in this blog post. That means that if you know the hash of the favicon of a vulnerable version of a web tech you can search if in shodan and find more vulnerable places:
shodan search org:"Target" http.favicon.hash:116323821 --fields ip_str,port --separator " " | awk '{print $1":"$2}'
This is how you can calculate the favicon hash of a web:
import mmh3
import requests
import codecs
def fav_hash(url):
response = requests.get(url)
favicon = codecs.encode(response.content,"base64")
fhash = mmh3.hash(favicon)
print(f"{url} : {fhash}")
return fhash
Search inside the web pages strings that could be shared across different webs in the same organisation. The copyright string could be a good example. Then search for that string in google, in other browsers or even in shodan: shodan search http.html:"Copyright string"

CRT Time

It's common to have a cron job such as
# /etc/crontab
37 13 */10 * * certbot renew --post-hook "systemctl reload nginx"
to renew the all the domain certificates on the server. This means that even if the CA used for this doesn't set the time it was generated in the Validity time, it's possible to find domains belonging to the same company in the certificate transparency logs. Check out this writeup for more information.

Passive Takeover

Apparently is common for people to assign subdomains to IPs that belongs to cloud providers and at some point lose that IP address but forget about removing the DNS record. Therefore, just spawning a VM in a cloud (like Digital Ocean) you will be actually taking over some subdomains(s).
โ€‹This post explains a store about it and propose a script that spawns a VM in DigitalOcean, gets the IPv4 of the new machine, and searches in Virustotal for subdomain records pointing to it.

Other ways

Note that you can use this technique to discover more domain names every time you find a new domain.
As you already know the name of the organisation owning the IP space. You can search by that data in shodan using: org:"Tesla, Inc." Check the found hosts for new unexpected domains in the TLS certificate.
You could access the TLS certificate of the main web page, obtain the Organisation name and then search for that name inside the TLS certificates of all the web pages known by shodan with the filter : ssl:"Tesla Motors" or use a tool like sslsearch.
โ€‹Assetfinder is a tool that look for domains related with a main domain and subdomains of them, pretty amazing.

Looking for vulnerabilities

Check for some domain takeover. Maybe some company is using some a domain but they lost the ownership. Just register it (if cheap enough) and let know the company.
If you find any domain with an IP different from the ones you already found in the assets discovery, you should perform a basic vulnerability scan (using Nessus or OpenVAS) and some port scan with nmap/masscan/shodan. Depending on which services are running you can find in this book some tricks to "attack" them. Note that sometimes the domain is hosted inside an IP that is not controlled by the client, so it's not in the scope, be careful.
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We know all the companies inside the scope, all the assets of each company and all the domains related to the companies.
It's time to find all the possible subdomains of each found domain.


Let's try to get subdomains from the DNS records. We should also try for Zone Transfer (If vulnerable, you should report it).
dnsrecon -a -d tesla.com


The fastest way to obtain a lot of subdomains is search in external sources. The most used tools are the following ones (for better results configure the API keys):
# subdomains
bbot -t tesla.com -f subdomain-enum
# subdomains (passive only)
bbot -t tesla.com -f subdomain-enum -rf passive
# subdomains + port scan + web screenshots
bbot -t tesla.com -f subdomain-enum -m naabu gowitness -n my_scan -o .
amass enum [-active] [-ip] -d tesla.com
amass enum -d tesla.com | grep tesla.com # To just list subdomains
# Subfinder, use -silent to only have subdomains in the output
./subfinder-linux-amd64 -d tesla.com [-silent]
# findomain, use -silent to only have subdomains in the output
./findomain-linux -t tesla.com [--quiet]
python3 oneforall.py --target tesla.com [--dns False] [--req False] [--brute False] run
assetfinder --subs-only <domain>
# It requires that you create a sudomy.api file with API keys
sudomy -d tesla.com
vita -d tesla.com
theHarvester -d tesla.com -b "anubis, baidu, bing, binaryedge, bingapi, bufferoverun, censys, certspotter, crtsh, dnsdumpster, duckduckgo, fullhunt, github-code, google, hackertarget, hunter, intelx, linkedin, linkedin_links, n45ht, omnisint, otx, pentesttools, projectdiscovery, qwant, rapiddns, rocketreach, securityTrails, spyse, sublist3r, threatcrowd, threatminer, trello, twitter, urlscan, virustotal, yahoo, zoomeye"
There are other interesting tools/APIs that even if not directly specialised in finding subdomains could be useful to find subdomains, like:
# Get list of subdomains in output from the API
## This is the API the crobat tool will use
curl https://sonar.omnisint.io/subdomains/tesla.com | jq -r ".[]"
curl https://jldc.me/anubis/subdomains/tesla.com | jq -r ".[]"
# Get Domains from rapiddns free API
curl -s "https://rapiddns.io/subdomain/$1?full=1" \
| grep -oE "[\.a-zA-Z0-9-]+\.$1" \
| sort -u
rapiddns tesla.com
# Get Domains from crt free API
curl -s "https://crt.sh/?q=%25.$1" \
| grep -oE "[\.a-zA-Z0-9-]+\.$1" \
| sort -u
crt tesla.com
  • โ€‹gau: fetches known URLs from AlienVault's Open Threat Exchange, the Wayback Machine, and Common Crawl for any given domain.
# Get subdomains from GAUs found URLs
gau --subs tesla.com | cut -d "/" -f 3 | sort -u
# Get only subdomains from SubDomainizer
python3 SubDomainizer.py -u https://tesla.com | grep tesla.com
# Get only subdomains from subscraper, this already perform recursion over the found results
python subscraper.py -u tesla.com | grep tesla.com | cut -d " " -f
# Get info about the domain
shodan domain <domain>
# Get other pages with links to subdomains
shodan search "http.html:help.domain.com"
export CENSYS_API_ID=...
python3 censys-subdomain-finder.py tesla.com
This project offers for free all the subdomains related to bug-bounty programs. You can access this data also using chaospy or even access the scope used by this project https://github.com/projectdiscovery/chaos-public-program-listโ€‹
You can find a comparison of many of these tools here: https://blog.blacklanternsecurity.com/p/subdomain-enumeration-tool-face-offโ€‹

DNS Brute force

Let's try to find new subdomains brute-forcing DNS servers using possible subdomain names.
For this action you will need some common subdomains wordlists like:
And also IPs of good DNS resolvers. In order to generate a list of trusted DNS resolvers you can download the resolvers from https://public-dns.info/nameservers-all.txt and use dnsvalidator to filter them. Or you could use: https://raw.githubusercontent.com/trickest/resolvers/main/resolvers-trusted.txtโ€‹
The most recommended tools for DNS brute-force are:
  • โ€‹massdns: This was the first tool that performed an effective DNS brute-force. It's very fast however it's prone to false positives.
sed 's/$/.domain.com/' subdomains.txt > bf-subdomains.txt
./massdns -r resolvers.txt -w /tmp/results.txt bf-subdomains.txt
grep -E "tesla.com. [0-9]+ IN A .+" /tmp/results.txt
  • โ€‹gobuster: This one I think just uses 1 resolver
gobuster dns -d mysite.com -t 50 -w subdomains.txt
  • โ€‹shuffledns is a wrapper around massdns, written in go, that allows you to enumerate valid subdomains using active bruteforce, as well as resolve subdomains with wildcard handling and easy input-output support.
shuffledns -d example.com -list example-subdomains.txt -r resolvers.txt
  • โ€‹puredns: It also uses massdns.
puredns bruteforce all.txt domain.com
  • โ€‹aiodnsbrute uses asyncio to brute force domain names asynchronously.
aiodnsbrute -r resolvers -w wordlist.txt -vv -t 1024 domain.com

Second DNS Brute-Force Round

After having found subdomains using open sources and brute-forcing, you could generate alterations of the subdomains found to try to find even more. Several tools are useful for this purpose:
  • โ€‹dnsgen: Given the domains and subdomains generate permutations.
cat subdomains.txt | dnsgen -
  • โ€‹goaltdns: Given the domains and subdomains generate permutations.
    • You can get goaltdns permutations wordlist in here.
goaltdns -l subdomains.txt -w /tmp/words-permutations.txt -o /tmp/final-words-s3.txt
  • โ€‹gotator: Given the domains and subdomains generate permutations. If not permutations file is indicated gotator will use its own one.
gotator -sub subdomains.txt -silent [-perm /tmp/words-permutations.txt]
  • โ€‹altdns: Apart from generating subdomains permutations, it can also try to resolve them (but it's better to use the previous commented tools).
    • You can get altdns permutations wordlist in here.
altdns -i subdomains.txt -w /tmp/words-permutations.txt -o /tmp/asd3
  • โ€‹dmut: Another tool to perform permutations, mutations and alteration of subdomains. This tool will brute force the result (it doesn't support dns wild card).
    • You can get dmut permutations wordlist in here.
cat subdomains.txt | dmut -d /tmp/words-permutations.txt -w 100 \
--dns-errorLimit 10 --use-pb --verbose -s /tmp/resolvers-trusted.txt
  • โ€‹alterx: Based on a domain it generates new potential subdomains names based on indicated patterns to try to discover more subdomains.

Smart permutations generation

  • โ€‹regulator: For more info read this post but it will basically get the main parts from the discovered subdomains and will mix them to find more subdomains.
python3 main.py adobe.com adobe adobe.rules
make_brute_list.sh adobe.rules adobe.brute
puredns resolve adobe.brute --write adobe.valid
  • โ€‹subzuf: subzuf is a subdomain brute-force fuzzer coupled with an immensly simple but effective DNS reponse-guided algorithm. It utilizes a provided set of input data, like a tailored wordlist or historical DNS/TLS records, to accurately synthesize more corresponding domain names and expand them even further in a loop based on information gathered during DNS scan.
echo www | subzuf facebook.com

Subdomain Discovery Workflow

Check this blog post I wrote about how to automate the subdomain discovery from a domain using Trickest workflows so I don't need to launch manually a bunch of tools in my computer:

VHosts / Virtual Hosts

If you found an IP address containing one or several web pages belonging to subdomains, you could try to find other subdomains with webs in that IP by looking in OSINT sources for domains in an IP or by brute-forcing VHost domain names in that IP.


You can find some VHosts in IPs using HostHunter or other APIs.
Brute Force
If you suspect that some subdomain can be hidden in a web server you could try to brute force it:
ffuf -c -w /path/to/wordlist -u http://victim.com -H "Host: FUZZ.victim.com"
gobuster vhost -u https://mysite.com -t 50 -w subdomains.txt
wfuzz -c -w /usr/share/wordlists/SecLists/Discovery/DNS/subdomains-top1million-20000.txt --hc 400,404,403 -H "Host: FUZZ.example.com" -u http://example.com -t 100
#From https://github.com/allyshka/vhostbrute
vhostbrute.py --url="example.com" --remoteip="" --base="www.example.com" --vhosts="vhosts_full.list"
VHostScan -t example.com
With this technique you may even be able to access internal/hidden endpoints.

CORS Brute Force

Sometimes you will find pages that only return the header Access-Control-Allow-Origin when a valid domain/subdomain is set in the Origin header. In these scenarios, you can abuse this behaviour to discover new subdomains.
ffuf -w subdomains-top1million-5000.txt -u -H 'Origin: http://FUZZ.crossfit.htb' -mr "Access-Control-Allow-Origin" -ignore-body

Buckets Brute Force

While looking for subdomains keep an eye to see if it is pointing to any type of bucket, and in that case check the permissions. Also, as at this point you will know all the domains inside the scope, try to brute force possible bucket names and check the permissions.


You can monitor if new subdomains of a domain are created by monitoring the Certificate Transparency Logs sublert does.

Looking for vulnerabilities

Check for possible subdomain takeovers. If the subdomain is pointing to some S3 bucket, check the permissions.
If you find any subdomain with an IP different from the ones you already found in the assets discovery, you should perform a basic vulnerability scan (using Nessus or OpenVAS) and some port scan with nmap/masscan/shodan. Depending on which services are running you can find in this book some tricks to "attack" them. Note that sometimes the subdomain is hosted inside an IP that is not controlled by the client, so it's not in the scope, be careful.


In the initial steps you might have found some IP ranges, domains and subdomains. Itโ€™s time to recollect all the IPs from those ranges and for the domains/subdomains (DNS queries).
Using services from the following free apis you can also find previous IPs used by domains and subdomains. These IPs might still be owned by the client (and might allow you to find CloudFlare bypasses)
You can also check for domains pointing a specific IP address using the tool hakip2hostโ€‹

Looking for vulnerabilities

Port scan all the IPs that doesnโ€™t belong to CDNs (as you highly probably wonโ€™t find anything interested in there). In the running services discovered you might be able to find vulnerabilities.
Find a guide about how to scan hosts.

Web servers hunting

We have found all the companies and their assets and we know IP ranges, domains and subdomains inside the scope. It's time to search for web servers.
In the previous steps you have probably already performed some recon of the IPs and domains discovered, so you may have already found all the possible web servers. However, if you haven't we are now going to see some fast tricks to search for web servers inside the scope.
Please, note that this will be oriented for web apps discovery, so you should perform the vulnerability and port scanning also (if allowed by the scope).
A fast method to discover ports open related to web servers using masscan can be found here. Another friendly tool to look for web servers is httprobe, fprobe and httpx. You just pass a list of domains and it will try to connect to port 80 (http) and 443 (https). Additionally, you can indicate to try other ports:
cat /tmp/domains.txt | httprobe #Test all domains inside the file for port 80 and 443
cat /tmp/domains.txt | httprobe -p http:8080 -p https:8443 #Check port 80, 443 and 8080 and 8443


Now that you have discovered all the web servers present in the scope (among the IPs of the company and all the domains and subdomains) you probably don't know where to start. So, let's make it simple and start just taking screenshots of all of them. Just by taking a look at the main page you can find weird endpoints that are more prone to be vulnerable.
To perform the proposed idea you can use EyeWitness, HttpScreenshot, Aquatone, Shutter or webscreenshot.
Moreover, you could then use eyeballer to run over all the screenshots to tell you what's likely to contain vulnerabilities, and what isn't.

Public Cloud Assets

In order to find potential cloud assets belonging to a company you should start with a list of keywords that identify that company. For example, a crypto for a crypto company you might use words such as: "crypto", "wallet", "dao", "<domain_name>", <"subdomain_names">.
You will also need wordlists of common words used in buckets:
Then, with those words you should generate permutations (check the Second Round DNS Brute-Force for more info).
With the resulting wordlists you could use tools such as cloud_enum, CloudScraper, cloudlist or S3Scanner.
Remember that when looking for Cloud Assets you should look for more than just buckets in AWS.

Looking for vulnerabilities

If you find things such as open buckets or cloud functions exposed you should access them and try to see what they offer you and if you can abuse them.


With the domains and subdomains inside the scope you basically have all what you need to start searching for emails. These are the APIs and tools that have worked the best for me to find emails of a company:

Looking for vulnerabilities

Emails will come handy later to brute-force web logins and auth services (such as SSH). Also, they are needed for phishings. Moreover, these APIs will give you even more info about the person behind the email, which is useful for the phishing campaign.

Credential Leaks

With the domains, subdomains, and emails you can start looking for credentials leaked in the past belonging to those emails:

Looking for vulnerabilities

If you find valid leaked credentials, this is a very easy win.

Secrets Leaks

Credential leaks are related to hacks of companies where sensitive information was leaked and sold. However, companies might be affected for other leaks whose info isn't in those databases:

Github Leaks

Credentials and APIs might be leaked in the public repositories of the company or of the users working by that github company. You can use the tool Leakos to download all the public repos of an organization and of its developers and run gitleaks over them automatically.
Leakos can also be used to run gitleaks agains all the text provided URLs passed to it as sometimes web pages also contains secrets.