What it does. Enumerate subdomains of target domains using ProjectDiscovery's Subfinder Part of mukul975/Anthropic-Cybersecurity-Skills (817 security skills) (mukul975/Anthropic-Cybersecurity-Skills).

Install

• npx skills add mukul975/Anthropic-Cybersecurity-Skills --skill performing-subdomain-enumeration-with-subfinder, or copy the skill folder into ~/.claude/skills/performing-subdomain-enumeration-with-subfinder/.
• Raw file: curl -sL https://raw.githubusercontent.com/mukul975/Anthropic-Cybersecurity-Skills/HEAD/skills/performing-subdomain-enumeration-with-subfinder/SKILL.md

SKILL.md (verbatim)

name: performing-subdomain-enumeration-with-subfinder
description: Enumerate subdomains of target domains using ProjectDiscovery's Subfinder
  passive reconnaissance tool to map the attack surface during security assessments.
domain: cybersecurity
subdomain: web-application-security
tags:
- subdomain-enumeration
- reconnaissance
- bug-bounty
- attack-surface
- subfinder
- passive-recon
- osint
version: '1.0'
author: mahipal
license: Apache-2.0
nist_csf:
- PR.PS-01
- ID.RA-01
- PR.DS-10
- DE.CM-01
mitre_attack:
- T1190
- T1059.007
- T1505.003
- T1083
- T1595

Performing Subdomain Enumeration with Subfinder

When to Use

• During the reconnaissance phase of penetration testing or bug bounty hunting
• When mapping the external attack surface of a target organization
• Before performing vulnerability scanning on discovered subdomains
• When building an asset inventory for continuous security monitoring
• During red team engagements requiring passive information gathering

Prerequisites

• Go 1.21+ installed for building from source
• Subfinder v2 installed (go install -v github.com/projectdiscovery/subfinder/v2/cmd/subfinder@latest)
• API keys configured for passive sources (Shodan, Censys, VirusTotal, SecurityTrails, Chaos)
• Provider configuration file at $HOME/.config/subfinder/provider-config.yaml
• Network access to passive DNS and certificate transparency sources
• httpx or httprobe for validating discovered subdomains

Workflow

Step 1 — Install and Configure Subfinder

# Install subfinder
go install -v github.com/projectdiscovery/subfinder/v2/cmd/subfinder@latest

# Verify installation
subfinder -version

# Configure API keys for enhanced results
mkdir -p $HOME/.config/subfinder
cat > $HOME/.config/subfinder/provider-config.yaml << 'EOF'
shodan:
  - YOUR_SHODAN_API_KEY
censys:
  - YOUR_CENSYS_API_ID:YOUR_CENSYS_API_SECRET
virustotal:
  - YOUR_VT_API_KEY
securitytrails:
  - YOUR_ST_API_KEY
chaos:
  - YOUR_CHAOS_API_KEY
EOF

Step 2 — Run Basic Subdomain Enumeration

# Single domain enumeration
subfinder -d example.com -o subdomains.txt

# Multiple domains from a file
subfinder -dL domains.txt -o all_subdomains.txt

# Use all passive sources (slower but more thorough)
subfinder -d example.com -all -o subdomains_all.txt

# Silent mode for piping to other tools
subfinder -d example.com -silent | httpx -silent -status-code

Step 3 — Filter and Customize Source Selection

# Use specific sources only
subfinder -d example.com -s crtsh,virustotal,shodan -o filtered.txt

# Exclude specific sources
subfinder -d example.com -es github -o results.txt

# Enable recursive subdomain enumeration
subfinder -d example.com -recursive -o recursive_subs.txt

# Match specific patterns
subfinder -d example.com -m "api,dev,staging" -o matched.txt

Step 4 — Control Rate Limiting and Output Format

# Rate limit to avoid API throttling
subfinder -d example.com -rate-limit 10 -t 5 -o rate_limited.txt

# JSON output for programmatic processing
subfinder -d example.com -oJ -o subdomains.json

# Output with source information
subfinder -d example.com -cs -o subdomains_with_sources.txt

# Collect results in a directory per domain
subfinder -dL domains.txt -oD ./results/

Step 5 — Validate Discovered Subdomains with httpx

# Pipe subfinder output to httpx for live validation
subfinder -d example.com -silent | httpx -silent -status-code -title -tech-detect -o live_hosts.txt

# Check for specific ports
subfinder -d example.com -silent | httpx -ports 80,443,8080,8443 -o web_services.txt

# Resolve IP addresses
subfinder -d example.com -silent | dnsx -a -resp -o resolved.txt

Step 6 — Integrate with Broader Recon Pipeline

# Chain with nuclei for vulnerability scanning
subfinder -d example.com -silent | httpx -silent | nuclei -t cves/ -o vulns.txt

# Combine with amass for comprehensive enumeration
subfinder -d example.com -o subfinder_results.txt
amass enum -passive -d example.com -o amass_results.txt
cat subfinder_results.txt amass_results.txt | sort -u > combined_subdomains.txt

# Screenshot discovered hosts
subfinder -d example.com -silent | httpx -silent | gowitness file -f - -P screenshots/

Key Concepts

Tools & Systems

Common Scenarios

1. Bug Bounty Reconnaissance — Enumerate all subdomains of a target program scope to identify forgotten or misconfigured assets that may contain vulnerabilities
2. Attack Surface Mapping — Build a comprehensive inventory of externally accessible subdomains for ongoing security monitoring and risk assessment
3. Cloud Asset Discovery — Identify subdomains pointing to cloud services (AWS, Azure, GCP) that may be vulnerable to subdomain takeover
4. CI/CD Integration — Automate subdomain monitoring in pipelines to detect new subdomains and alert on changes to the attack surface
5. Merger & Acquisition Due Diligence — Map the complete external footprint of an acquisition target during security assessment

Output Format

## Subdomain Enumeration Report
- **Target Domain**: example.com
- **Total Subdomains Found**: 247
- **Live Hosts**: 183
- **Unique IP Addresses**: 42
- **Sources Used**: crt.sh, VirusTotal, Shodan, SecurityTrails, Censys

### Discovered Subdomains
| Subdomain | IP Address | Status Code | Technology |
|-----------|-----------|-------------|------------|
| api.example.com | 10.0.1.5 | 200 | Nginx, Node.js |
| staging.example.com | 10.0.2.10 | 403 | Apache |
| dev.example.com | 10.0.3.15 | 200 | Express |

### Recommendations
- Remove DNS records for decommissioned subdomains
- Investigate subdomains with CNAME pointing to unclaimed services
- Restrict access to development and staging environments

Other files in this skill

• LICENSE
• assets/template.md
• references/api-reference.md
• references/standards.md
• references/workflows.md
• scripts/agent.py
• scripts/process.py

assets/template.md (verbatim)

Subdomain Enumeration Report Template

Engagement Details

• Target Domain: [domain]
• Date: [date]
• Assessor: [name]
• Scope: Passive subdomain enumeration only

Executive Summary

Performed passive subdomain enumeration against [domain] using Subfinder and complementary tools. Discovered [N] unique subdomains, of which [M] are live and responding to HTTP requests.

Methodology

1. Passive subdomain enumeration using Subfinder with all available sources
2. DNS resolution and validation using dnsx
3. HTTP probing with httpx for live host identification
4. CNAME analysis for subdomain takeover risk assessment

Results Summary

Live Hosts

Subdomain Takeover Risks

Recommendations

1. Remove dangling DNS records for decommissioned services
2. Claim or remove CNAME records pointing to unclaimed cloud resources
3. Restrict access to development and staging subdomains
4. Implement continuous subdomain monitoring for new asset detection
5. Review cloud service configurations for publicly accessible resources

references/api-reference.md (verbatim)

API Reference: Subdomain Enumeration with Subfinder

Subfinder CLI Options

httpx CLI Options

dnsx CLI Options

Passive Sources

Python Libraries

References

• Subfinder GitHub: https://github.com/projectdiscovery/subfinder
• httpx GitHub: https://github.com/projectdiscovery/httpx
• dnsx GitHub: https://github.com/projectdiscovery/dnsx
• Subfinder Config: https://docs.projectdiscovery.io/tools/subfinder/install

references/standards.md (verbatim)

Standards & References — Subdomain Enumeration with Subfinder

Industry Standards

• OWASP Testing Guide v4.2 — OTG-INFO-004: Enumerate applications on web server through subdomain discovery
• PTES (Penetration Testing Execution Standard) — Intelligence Gathering phase requiring comprehensive asset enumeration
• NIST SP 800-115 — Technical Guide to Information Security Testing and Assessment, passive reconnaissance methods
• MITRE ATT&CK T1596 — Search Open Technical Databases for target infrastructure information

Tool References

• Subfinder GitHub: https://github.com/projectdiscovery/subfinder
• ProjectDiscovery Documentation: https://docs.projectdiscovery.io/tools/subfinder/overview
• Certificate Transparency RFC 6962: https://www.rfc-editor.org/rfc/rfc6962
• crt.sh Certificate Search: https://crt.sh/

API Provider Documentation

• Shodan API: https://developer.shodan.io/api
• Censys Search API: https://search.censys.io/api
• VirusTotal API v3: https://docs.virustotal.com/reference/overview
• SecurityTrails API: https://docs.securitytrails.com/reference
• Chaos ProjectDiscovery: https://chaos.projectdiscovery.io/

Regulatory Considerations

• Passive subdomain enumeration does not involve active scanning and is generally legal
• Always verify scope and authorization before proceeding to active enumeration
• Bug bounty programs define specific scope for subdomain testing
• GDPR may apply when collecting data that reveals organizational structure

references/workflows.md (verbatim)

Workflows — Subdomain Enumeration with Subfinder

Standard Enumeration Workflow

1. Configure API keys in provider-config.yaml for maximum source coverage
2. Run subfinder with -all flag against target domain(s)
3. Deduplicate results and remove out-of-scope entries
4. Validate live hosts with httpx including status codes and technologies
5. Resolve DNS records with dnsx to map IP infrastructure
6. Screenshot live hosts with gowitness for visual review
7. Feed live hosts into vulnerability scanner (nuclei) for automated checks

Continuous Monitoring Workflow

1. Schedule subfinder runs via cron (daily or weekly)
2. Compare new results against baseline subdomain list
3. Alert on newly discovered subdomains via webhook or email
4. Automatically scan new subdomains for known vulnerabilities
5. Update asset inventory with new discoveries

Bug Bounty Recon Pipeline

1. Collect target domains from bug bounty program scope
2. Run subfinder + amass + findomain for maximum coverage
3. Merge and deduplicate all results
4. Filter results to in-scope assets only
5. Probe for live HTTP services with httpx
6. Run nuclei templates for quick wins
7. Manually investigate interesting subdomains (dev, staging, api, admin)

Integration Commands

# Full pipeline example
subfinder -d target.com -all -silent | \
  httpx -silent -status-code -title -tech-detect | \
  tee live_hosts.txt | \
  nuclei -t cves/ -t exposures/ -t misconfigurations/ -o findings.txt

# Delta monitoring
subfinder -d target.com -silent > today_subs.txt
comm -13 <(sort baseline_subs.txt) <(sort today_subs.txt) > new_subs.txt

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