# Performing Subdomain Enumeration With Subfinder

> Use when enumerate subdomains of target domains using ProjectDiscovery's Subfinder passive reconnaissance tool to map the attack surface during security assessments. Use when working with performing subdomain enumeration with subfinder.

- Skill: `oyi77/performing-subdomain-enumeration-with-subfinder` (Agent Skill)
- Install (CLI): `npx skillmds add oyi77/performing-subdomain-enumeration-with-subfinder`
- Raw SKILL.md: https://api.skillmd.com/api/skills/oyi77/performing-subdomain-enumeration-with-subfinder/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- License: Apache-2.0
- Author: oyi77 (https://skillmd.com/u/oyi77)
- Updated: 2026-09-08
- Page: https://skillmd.com/skills/oyi77/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

```python
# 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
1. **Implement** — Write code following existing conventions, add tests
1. **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

| Rationalization | Reality |
|---|---|
| "We are too small to be targeted" | Automated attacks target everyone. Size does not matter. |
| "Security slows us down" | A breach slows you down 100x more. Build security in from the start. |
| "We will fix it after launch" | Vulnerabilities in production are exploited within hours. Fix before deploy. |