BackHow We Discover Subdomains and Why It Matters
5 minIntroduction
Subdomains often serve as entry points to an organization’s infrastructure, hosting web applications, APIs, or development environments. Attackers frequently target subdomains because they can be forgotten, misconfigured, or insufficiently secured.
In External Attack Surface Management (EASM), subdomain discovery is a critical first step in mapping an organization’s external footprint. This article explores how we discover subdomains, why it's essential, and how it improves security posture.
Why Subdomain Discovery is Important
Hidden Attack Vectors
Subdomains may host:
- Staging or development environments with weaker security controls.
- Forgotten legacy applications that are no longer maintained.
- Internal tools mistakenly exposed to the internet.
Increased Attack Surface
A single overlooked subdomain can provide attackers with a foothold into an organization’s network. Detecting and managing subdomains helps to:
- Reduce exposure by identifying forgotten or outdated assets.
- Improve response time to fix misconfigurations before attackers exploit them.
- Enhance compliance with security frameworks by ensuring full asset visibility.
How We Discover Subdomains
1. DNS Enumeration
By querying DNS records, we identify subdomains registered under a parent domain. This includes:
- A records (IP mappings)
- CNAME records (alias mappings)
- MX records (email servers)
2. Certificate Transparency Logs
Whenever an SSL certificate is issued, it is logged publicly. We analyze these logs to extract previously unknown subdomains.
3. Passive DNS Data
Historical DNS lookups and third-party intelligence feeds provide additional subdomains that might not appear in active DNS records.
4. Web Crawling & OSINT
We scan public resources, search engines, and indexed metadata to uncover subdomains referenced on external sites.
5. Brute-force and Wordlist Attacks
Using curated wordlists of common subdomains, we systematically test for unlisted but valid subdomains.
Risk Factors in Unsecured Subdomains
Even a seemingly harmless subdomain can introduce significant risk. Common threats include:
- Subdomain Takeover: When an expired or unmaintained subdomain is hijacked by an attacker.
- Exposed API Endpoints: Misconfigured subdomains may expose sensitive data.
- Forgotten Admin Panels: Legacy admin pages may have weak or no authentication.
- Insecure Development Environments: Staging environments may run unpatched software.
How to Secure Subdomains
Organizations should take proactive steps to manage subdomains:
✅ Regularly audit DNS records to identify unexpected or unused subdomains.
✅ Monitor SSL certificate transparency logs to catch unauthorized subdomain registrations.
✅ Enforce strict access controls for admin panels and APIs exposed via subdomains.
✅ Decommission unused subdomains to prevent exploitation.
Conclusion
Subdomain discovery is a fundamental part of EASM, enabling organizations to identify, manage, and secure their full attack surface. Unsecured subdomains can expose critical assets to attackers, making continuous monitoring essential.
In the next article, we’ll explore how mapping IPs, ports, and services provides deeper insights into an organization’s security posture.
Frequently Asked Questions
Can I ask follow-up questions about vulnerabilities?
Absolutely! The 'Dig Deeper' chat is interactive, allowing you to:
- Request more details about how a vulnerability works.
- Ask for additional remediation methods.
- Clarify attack vectors and potential risks.
This ensures security teams get deeper insights beyond just a vulnerability description.
What is the 'Dig Deeper' chat, and how does it help?
The 'Dig Deeper' AI chat provides instant expert-level insights on detected issues.
- It explains vulnerabilities in a clear, actionable way.
- It helps users understand why an issue is risky and how attackers might exploit it.
- It suggests remediation steps tailored to the asset and issue type.
This allows security teams to quickly grasp risks and respond effectively without deep security expertise.
Does this tool comply with security frameworks like NIST or CIS?
Yes, the EASM tool aligns with frameworks such as NIST, CIS, and ISO 27001 by identifying security gaps in publicly accessible infrastructure. While it does not enforce compliance, it helps security teams detect misconfigurations and vulnerabilities that could impact regulatory requirements.
What sources does the scanner use to find vulnerabilities?
The scanner leverages multiple intelligence sources to detect vulnerabilities, including:
- Public vulnerability databases (CVE, NVD, MITRE, etc.).
- Security research feeds and threat intelligence sources.
- Passive reconnaissance techniques such as OSINT and fingerprinting.
By combining these sources, the scanner provides real-time risk assessments without intrusive scans.
Does the scanner perform active exploitation or just passive mapping?
The scanner performs only passive mapping, meaning it identifies vulnerabilities but does not exploit them.
This approach:
- Avoids system disruptions while gathering security intelligence.
- Provides safe, real-world attacker perspectives without breaching legal boundaries.
- Ensures compliance with ethical scanning practices.
Organizations can use these findings to proactively patch weaknesses before attackers do.
How does the EASM scanner find subdomains?
The scanner discovers subdomains using:
- DNS enumeration (brute-force, dictionary-based, wildcard resolution).
- Certificate transparency logs that expose registered subdomains.
- Passive DNS records and web crawling to identify related assets.
This helps map the full attack surface, including shadow IT and forgotten subdomains.