Exploring Your Internal Network - The Importance of Internal Pentesting

While external penetration testing focuses on securing your perimeter, what happens if an attacker bypasses those defenses or if the threat originates from within your network? This is where Internal Penetration Testing becomes essential. It simulates attacks launched from inside the trusted network boundary, mimicking scenarios like a malicious insider, a compromised employee account, or malware that has gained an initial foothold.

An internal pentest assumes the attacker already has some level of access, perhaps equivalent to a standard user or a device connected to a guest Wi-Fi or corporate LAN port. The goal is to determine how far an attacker could move laterally, escalate privileges, and access sensitive data after breaching the perimeter.

Why Conduct Internal Penetration Tests?

1. Simulate Insider Threats: Malicious employees or contractors with existing access pose a significant risk. Internal tests assess the potential damage they could inflict.
2. Assess Post-Breach Scenarios: If an external defense fails (e.g., phishing attack, malware infection), internal testing reveals the vulnerabilities an attacker could exploit next.
3. Identify Lateral Movement Paths: Uncover how attackers can pivot from one compromised system to others, potentially reaching critical servers or data repositories.
4. Test Internal Segmentation: Evaluate the effectiveness of network segmentation in containing breaches and preventing attackers from moving freely across different network zones (e.g., user workstations, servers, development environments).
5. Uncover Misconfigurations: Identify internal systems with weak passwords, missing patches, excessive user permissions, or insecure protocols that aren't visible externally.
6. Validate Internal Security Controls: Test the effectiveness of internal monitoring, access controls, and security policies.
7. Compliance: Certain regulations and standards may require internal network security assessments.

The Internal Pentesting Process:

Internal tests often start with more initial information than external tests (sometimes called "grey box" testing). The typical phases include:

1. Planning & Scoping: Defining objectives, the scope of internal networks/segments to test, rules of engagement, and the assumed level of initial access (e.g., standard user credentials, network port access).
2. Internal Reconnaissance: Once "inside," testers map the internal network landscape. This involves identifying live hosts, network topology, running services, user accounts, shared drives, and domain structures (e.g., Active Directory enumeration).
3. Vulnerability Analysis: Scanning internal hosts for vulnerabilities, analyzing configurations (especially Active Directory group policies, permissions), checking for missing patches, and identifying weak protocols or credentials used internally.
4. Exploitation & Lateral Movement: Attempting to exploit identified vulnerabilities to gain access to additional systems. Common techniques include:
   • Pass-the-Hash / Pass-the-Ticket attacks in Windows environments.
   • Exploiting internal web applications or services.
   • Leveraging weak file share permissions.
   • Cracking or capturing credentials transmitted insecurely.
   • Exploiting trust relationships between systems.
5. Privilege Escalation: Attempting to elevate privileges from a standard user account to local administrator or domain administrator, granting broader control over the network.
6. Objective Achievement: Focusing on reaching the pre-defined "crown jewels" – accessing specific sensitive data, compromising domain controllers, or achieving other objectives outlined during scoping.
7. Reporting: Detailing the attack paths used, vulnerabilities exploited, evidence of access and privilege escalation, assessment of internal control effectiveness, and prioritized remediation recommendations.

Common Internal Pentest Findings:

• Weak/Default Passwords: Especially for service accounts or internal applications.
• Missing Patches: Critical systems lacking security updates.
• Excessive Permissions: Users or groups having more access than required (violating the principle of least privilege).
• Insecure Protocols: Use of protocols like LLMNR, NBT-NS, or SMBv1 that allow credential theft or relay attacks.
• Flat Network Architecture: Lack of proper segmentation, allowing easy lateral movement.
• Misconfigured Active Directory: Vulnerabilities in Group Policy Objects (GPOs), Kerberos delegation issues, etc.
• Sensitive Data Exposure: Unencrypted sensitive information stored on file shares or internal sites.

Conclusion

Internal Penetration Testing provides a crucial view of your security posture from the inside out. It highlights the risks posed by insider threats and demonstrates the potential impact if perimeter defenses are breached. By identifying and remediating internal vulnerabilities, organizations can significantly limit an attacker's ability to move laterally and access critical assets, thereby strengthening resilience against sophisticated attacks. Don't assume your network is secure just because the firewall is strong; test your internal defenses regularly.