Performing Subdomain Enumeration With Subfinder

Performing Subdomain Enumeration With Subfinder

Overview

Cybersecurity skill for performing subdomain enumeration with subfinder. Follows industry best practices and security standards.

When to Use

Trigger phrases:

• "performing subdomain enumeration with subfinder"

• "Enumerate subdomains of target domains using ProjectDiscovery's Subfinder passiv"

• 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

When NOT to Use

• When you lack proper authorization for testing
• For production systems without change management
• When the task requires legal or compliance expertise beyond technical scope

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

# Example: IOC detection
import re

IOC_PATTERNS = {
    "ip": r"\b(?:\d{1,3}\.){3}\d{1,3}\b",
    "domain": r"\b[a-z0-9-]+\.[a-z]{2,}\b",
    "hash_md5": r"\b[a-f0-9]{32}\b",
    "hash_sha256": r"\b[a-f0-9]{64}\b",
}

def extract_iocs(text: str) -> dict:
    return {k: re.findall(v, text) for k, v in IOC_PATTERNS.items()}

1. Plan Operations — Define objectives, scope, and success criteria for subdomain enumeration operations.
2. Prepare Environment — Set up tools, access, and data sources required for subdomain enumeration.
3. Execute Core Workflow — Use subfinder to perform subdomain enumeration operations following established procedures.
4. Validate Results — Verify that results meet quality standards and objectives.
5. Report Findings — Document results, observations, and recommendations.
6. Follow Up — Track remediation actions and verify fixes where applicable.

Tools

• subfinder — Primary tool for this skill
• Analysis Platform — Data processing and visualization
• Collaboration Tools — Team coordination and knowledge sharing

Process

1. Design — Define interface, identify patterns, plan implementation
2. Implement — Write code following existing conventions, add tests
3. Verify — Run tests, check integration, validate behavior

Verification

• All subdomain enumeration procedures executed completely and documented
• Findings validated against multiple data sources
• False positives identified and filtered
• Results documented with evidence and timestamps
• Recommendations provided with risk-based prioritization

Anti-Rationalization Table