Introduction: the security challenges of internal corporate networks
Enterprise networks are at the heart of today’s information systems. They interconnect users, servers, business applications, cloud environments and sometimes even external partners. This permanent interconnection considerably increases the attack surface and makes infrastructures more complex to secure.
In this context, penetration testing is an essential tool for assessing the actual security level of a corporate network. Unlike purely documentary audits or automated scans, it enables us to measure what an attacker could actually exploit. A well-conducted penetration test provides a concrete vision of risks, prioritizes critical vulnerabilities and helps IT teams to reinforce their security posture over the long term.
What is a network pentest?
A penetration test, or pentest, consists of simulating a computer attack in a controlled, authorized environment. The aim is to identify technical, organizational or human flaws that could lead to a compromise of the corporate network.
In the context of an internal network, the intrusion test may target :
- Network services (SMB, LDAP, DNS, RDP, FTP, etc.)
- Active Directory and identity management
- Authentication and access control mechanisms
- Network and system configurations
- Hybrid environments (on-premise and cloud)
The aim is not just to find vulnerabilities, but to demonstrate their exploitability and their real impact on the business.
Why carry out an intrusion test on an internal corporate network?
Performing a penetration test can answer some key questions for an ISSM or IT manager:
- Can an internal or external attacker access the network?
- How far can it move once it has gained initial access?
- Is it possible to compromise Active Directory or privileged accounts?
- Are detection and response mechanisms effective?
Beyond the technical aspect, network intrusion testing contributes to :
- Reduce the risk of a major compromise
- Prioritizing remedial actions
- Validate the effectiveness of existing safety controls
- Meet regulatory and standards requirements (ISO 27001, NIS2, etc.)
Key stages in an effective penetration test
1. Framing and preparing the penetration test
Every penetration test begins with a scoping phase. It defines the scope, objectives, constraints and rules of engagement. This stage is essential to avoid any ambiguity and guarantee a controlled test.
It generally includes :
- Defining the assets to be tested
- The expected level of stealth
- Operating hours
- Authorized and excluded scenarios
2. Network reconnaissance and mapping
The reconnaissance phase aims to understand the architecture of the corporate network. It identifies accessible machines, exposed services and the first potential weaknesses.
This step may include :
- Network discovery and host inventory
- Identifying open ports and services
- Version and configuration analysis
3. Identifying and exploiting vulnerabilities
Once the targets have been identified, penetration testing focuses on the controlled exploitation of vulnerabilities. This may involve :
- Known unpatched vulnerabilities (via obsolete software)
- Weak or reused passwords (due to a weak password policy)
- Configuration errors (due to employees’ lack of knowledge of cybersecurity)
- Weaknesses in the Active Directory (the attack surface is simply enormous, and errors are common).
The aim is to reproduce a realistic attack chain, without causing downtime.
4. Post-exploitation and elevation of privileges
Network penetration testing doesn’t stop with the first compromise. It evaluates an attacker’s ability to move laterally (steps 2 & 3 are repeated until the objective is reached), and thus :
- Access sensitive data
- Upgrade privileges to administrator accounts
- Reaching business-critical assets
5. Analysis, feedback and recommendations
The final step is to document precisely the vulnerabilities exploited, the attack paths and the impacts observed. The penetration test report must be clear, prioritized and usable by the technical teams.
Internal network penetration testing – The “gray box” format
Generally speaking, this type of test is also known as a “trainee test”, “insider threat” or “assumed breach”. The aim is to measure the extent of the impact in the event of one of the company’s users being compromised.
As a result, these tests are grey-boxed, since the customer provides partial information (user accounts, network diagrams).
Best practices and common mistakes to avoid
Best practices
- Define clear, measurable objectives
- Adapting penetration testing to the business context
- Test regularly, not just after an incident
- Involve IT and security teams in the feedback process
Common errors
- Limit yourself to a simple automated scan
- Neglecting the post-operation phase
- Producing overly technical or unusable reports
- Failure to follow up on post-test remediation actions
The business and strategic value of penetration testing
Intrusion testing is not just a technical exercise. It transforms abstract risks into concrete scenarios that management can understand. It helps to justify cybersecurity investments, improve IS governance and strengthen the company’s overall resilience.
Conclusion: taking action
Performing an effective penetration test on a corporate network means agreeing to look at security as it really is, not as we imagine it to be. By integrating penetration testing into a continuous process, organizations gain visibility, maturity and the ability to react to current threats.
A regularly tested network is better understood, better defended and, above all, less vulnerable.
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