Domain & ASN Enumeration

When to Use

• Phase 1 of External Penetration Tests / Bug Bounty hunting.
• When mapping the Attack Surface of a large enterprise or conglomerate.
• When tracking mergers and acquisitions (discovering newly acquired, potentially insecure infrastructure).
• Before conducting vulnerability scanning, to ensure you know every asset the target owns.

Prerequisites

• Target organization name, domain, or individual identifier
• OSINT framework tools installed (theHarvester, Maltego, Recon-ng)
• Understanding of operational security to avoid alerting the target
• Legal authorization for the intelligence gathering scope

Workflow

Phase 1: ASN (Autonomous System Number) Discovery

# Concept: Large organizations have their own ASNs, representing huge blocks of IP addresses.
# We want to map these to find infrastructure they host themselves vs cloud hosting.

# 1. Start with the main domain or company name via BGPView API or Hurricane Electric (bgp.he.net)
curl -s "https://api.bgpview.io/search?query_term=tesla" | jq

# 2. Extract ASNs provided in the response (e.g., AS394380)

# 3. Use Amass to map ASNs to IP ranges
amass intel -asn 394380

# 4. Alternatively, use whois to lookup the organization name
whois -h whois.radb.net -- '-i origin AS394380' | grep -Eo "([0-9.]+){4}/[0-9]+"

# This gives you the CIDR blocks (e.g., 216.239.32.0/19) owned directly by the company.

Phase 2: Reverse WHOIS and Domain Discovery

# Concept: We have the main domain (target.com). Now we want to find OTHER root domains 
# owned by the same company (target.net, target-holdings.com, acquired-startup.com).

# 1. Extract WHOIS registrant email or organization name from the main domain
whois target.com | grep "Registrant Organization\|Registrant Email"
# Result: Registrant Organization: Target Corp

# 2. Perform Reverse WHOIS (who owns domains matching this Org/Email?)
# Use Amass Intel module
amass intel -org "Target Corp"
amass intel -whois -d target.com

# 3. Use Crunchbase and Wikipedia to manually search for Subsidaries and Acquisitions.
# Add these root domains to your target list.

Phase 3: Subdomain Enumeration (Passive)

# Concept: Find thousands of subdomains (dev.target.com, api.target.com) without sending
# a single packet to the target's actual servers. Rely on third-party data.

# 1. Certificate Transparency Logs (crt.sh)
# Organizations must register TLS/SSL certs publicly.
curl -s "https://crt.sh/?q=%25.target.com&output=json" | jq -r '.[].name_value' | sed 's/\*\.//g' | sort -u > subdomains_crt.txt

# 2. Use Subfinder (Rapid passive enumeration using multiple APIs)
subfinder -d target.com -all -silent > subdomains_subfinder.txt

# 3. Use theHarvester for search engine scraping
theHarvester -d target.com -b google,bing,linkedin,twitter -f target_harvest.html

Phase 4: Subdomain Enumeration (Active & Bruteforce)

# Concept: Actively query a massive list of potential subdomains against DNS resolvers
# to see which ones actually resolve to an IP address.

# 1. Powerful active enumeration mapping with Amass
amass enum -active -d target.com -brute -w /usr/share/wordlists/SecLists/Discovery/DNS/subdomains-top1million-110000.txt -o amass_active.txt

# Note: Amass can take a long time. For pure speed, Puredns or Massdns is preferred.
# 2. Fast resolution with Puredns
puredns resolve combined_subdomains.txt -r public_resolvers.txt -w resolved_subdomains.txt

Phase 5: Result Compiling & Port Scanning Prep

# We now have a list of all root domains, combined subdomains, and ASNs (CIDR blocks).

# 1. Clean and deduplicate lists
cat subdomains_*.txt | sort | uniq > final_subdomains.txt

# 2. Resolve final subdomains to IP addresses
cat final_subdomains.txt | httpx -silent -rt -ip

# These IPs and CIDR bounds are now ready for Phase 2: Mass Port Scanning (e.g., using Naabu or Masscan).

🔵 Blue Team / Defensive Perspective

• Attack Surface Management (ASM): The blue team must continuously run these exact same procedures to know their exposure. You cannot secure what you do not know you own.
• Dangling DNS: Periodically review DNS records. If a subdomain points to an expired AWS S3 bucket or unrenewed Heroku app, it is a critical Subdomain Takeover vulnerability.
• WHOIS Privacy: Use WHOIS Privacy protection where legally applicable to slow down Reverse WHOIS reconnaissance.

Key Concepts

Concept	Description
ASN	Autonomous System Number; uniquely identifies a network routing domain on the internet
CIDR	Classless Inter-Domain Routing; a method for allocating IP addresses and IP routing (e.g., a "/24" block)
Certificate Transparency	Publicly accessible logs of all issued SSL/TLS certificates
Active vs Passive Recon	Active touches the target's servers. Passive asks third parties about the target without touching them.

Output Format

OSINT Infrastructure Intel Report
=================================
Target Organization: MegaCorp Industries

1. Autonomous Systems Identified: 
- AS12345 (MegaCorp US) -> Maps to 192.168.0.0/22
- AS67890 (MegaCorp EU) -> Maps to 10.0.0.0/24

2. Root Domains Identified (Via Reverse WHOIS):
- megacorp.com
- megacorp.net
- acquired-startup.io
- megacorp-staging.com

3. Subdomain Discovery Statistics:
- Certificate Logs (crt.sh): 432
- Passive Scraping (Subfinder): 856
- Active DNS Bruteforce (Amass): 120
- Total Unique Resolvable Subdomains: 1,024

Next Steps Approved: Initiate targeted port scanning against the 1,024 resolved endpoints and 2 identified CIDR blocks.

🛡️ Remediation & Mitigation Strategy

• Input Validation: Sanitize and strictly type-check all inputs.
• Least Privilege: Constrain component execution bounds.

📚 Shared Resources

For cross-cutting methodology applicable to all vulnerability classes, see:

• _shared/references/elite-chaining-strategy.md — Exploit chaining methodology and high-payout chain patterns
• _shared/references/elite-report-writing.md — HackerOne-optimized report writing, CWE quick reference
• _shared/references/real-world-bounties.md — Verified disclosed bounties by vulnerability class

References

• OWASP: OSINT Framework
• ProjectDiscovery: Subfinder
• OWASP Amass: Amass GitHub