There are three general approaches to penetration testing: black-box, grey-box and white-box. What is the difference between these? Broadly speaking, it comes down to the amount of information, access, data and context provided to the tester at the beginning of the engagement. Each approach has its advantages, but choosing the right one can make a significant difference to the effectiveness of a penetration test, the vulnerabilities identified and ultimately the value an organisation receives from the engagement.

Black-Box Penetration Testing

For a black-box test, the information provided is typically limited to the specific scope of the engagement, for example URLs, IP addresses, CIDR ranges or binaries to be tested, with no other details, credentials, context or access provided. This is the type of test that most directly simulates a real-world scenario where an external threat actor or group targets a company, website or application and needs to perform initial reconnaissance to find a foothold in the form of credentials, an exploitable vulnerability or another flaw. Black-box testing can be particularly useful when an organisation wants to understand what an attacker could discover and exploit without prior knowledge of its internal systems. However, one limitation is that a significant portion of the allocated testing time may be spent on reconnaissance and attempting to gain initial access, potentially leaving less time to investigate vulnerabilities within authenticated areas or internal functionality.

Grey-Box Penetration Testing

A grey-box test is where the tester is provided with some additional information relevant to the test, for example low-level credentials, network diagrams, a copy of the binary to be tested (but not source code), etc. This type of test more closely aligns with a real-world scenario where an internal low-privilege user is compromised, either through a method like phishing, coercion or their own desire to act maliciously against their company. One of the key advantages of grey-box testing is that it allows testers to spend more time investigating security weaknesses rather than attempting to gain initial access. For web applications, providing authenticated user accounts can enable testers to assess vulnerabilities such as broken access controls, privilege escalation and insecure functionality that may not be accessible to an unauthenticated user. This is particularly relevant when testing applications that contain different user roles or permission levels, where vulnerabilities may only become apparent once a user has successfully authenticated.

White-Box Penetration Testing

Finally, in a white-box test, the tester is provided with as much information as possible about the target, typically including both low and high privilege credentials to the target application or network, network diagrams, firewall/WAF rules and, most importantly for an application, source code. This enables the tester to quickly narrow down their focus to specific weaknesses. White-box testing can provide significant value in terms of thoroughly testing and auditing the target, particularly where source code access allows testers to investigate vulnerabilities that may be difficult to identify through external testing alone. For example, reviewing source code can help identify insecure authentication logic, cryptographic weaknesses, flawed business logic and access control issues.

However, having greater access does not automatically guarantee more comprehensive testing. The depth and effectiveness of the assessment still depend on the agreed scope, testing methodology, available time and expertise of the testers. White-box testing can also introduce additional considerations for organisations, as it involves sharing potentially sensitive information with a third party. Appropriate confidentiality agreements, secure access arrangements and clear data-handling requirements should therefore be established before testing begins.

How Do You Decide Which Testing Approach Is Right for Your Organisation?

Deciding when to choose a particular type of testing (black, grey or white box) depends on many factors, including the specific goals of the test, the time available, cost, etc. However, in our experience, the best way to approach this decision is to focus primarily on the goals of the test. That is, why are you planning to conduct a penetration test at all?

Is it because of an audit requirement, because you genuinely want to identify and address security vulnerabilities in an application or network, to simulate known or unknown threat actors targeting your application, company or industry, or a combination of factors? If your goal is to simulate a real-world scenario involving an attacker with no prior knowledge or access, then a black-box test may be appropriate. Black-box testing may also satisfy certain audit requirements, although it is important to understand that compliance frameworks and assessment requirements can differ. Some may require authenticated testing, specific testing coverage or additional evidence. These can be very different reasons for conducting a black-box test. One may be focused on meeting an audit requirement, while the other is likely to be much more involved, take considerably longer and be significantly more expensive when simulating an adversary.

Time, cost and the desired outcome therefore all need to be considered.

The NIST SP 800-115 Technical Guide to Information Security Testing and Assessment provides additional guidance on planning security assessments, selecting testing techniques and aligning testing activities with organisational objectives.

An important distinction is that a conventional black-box penetration test is not necessarily the same as a full adversary simulation or red team engagement. While both may involve testing without prior access, adversary simulation generally focuses on achieving defined objectives by emulating attacker behaviour, potentially across multiple systems and attack stages.

When Does Grey-Box or White-Box Testing Provide Better Value?

If your goal is to identify and address security vulnerabilities and weaknesses in a specific application or network, either a grey-box or white-box test could be appropriate. It then really comes down to thoroughness, time and cost considerations.

A white-box test can provide more thorough coverage as testers have greater access, documentation and, crucially for an application test, source code. However, this may mean testers need more time to assess the additional information and functionality available, which can come at a greater cost. Alternatively, a time-boxed grey-box test can provide a compromise between black-box and white-box testing where time or cost is a concern, but the organisation still wants to identify as many vulnerabilities and weaknesses as possible within the given constraints. White-box and black-box tests can be time-boxed too (and usually are), but the time required will vary depending on the scope, complexity and objectives of the engagement.

When does grey-box or white-box testing provide better value?

If your goal is to identify and address security vulnerabilities and weaknesses in a specific application/network, either a grey or white-box test could be appropriate, and it then really comes down to the thoroughness, time and cost considerations. A white-box test can be more thorough as the tester(s) will have more access, documentation and crucially for an app test, source code. But this may mean they need more time, which comes at a greater cost. Alternatively, a grey time-boxed test can provide a compromise between a black and white-box test where time/cost is a concern but you want to identify as many vulnerabilities/weaknesses as possible within given constraints. White and black-box tests can be time-boxed too (and usually are), but the time required will depend on the scope and objectives. A black-box test focused on a limited external scope may be relatively short, whereas a white-box assessment may require more time to review the additional information and access provided. Black-box simulated adversary testing, however, can involve much longer engagements, sometimes extending over months or even years.

Final Recommendations

In general, we recommend that organisations planning to engage a security firm for penetration testing consider grey-box or white-box approaches, as in our experience these often provide the best value for the time allocated. That does not mean black-box testing is without value. It remains an important approach where the objective is to assess externally exposed weaknesses or understand what an unauthenticated attacker may be able to achieve. We also strongly encourage organisations conducting penetration testing of applications, including web and mobile applications, to provide testers with access to source code where possible. Access to source code can significantly improve the likelihood of identifying serious vulnerabilities efficiently, particularly when combined with practical testing to determine whether those weaknesses are exploitable. The most effective approach will always depend on the environment being tested, the risks involved and what the organisation wants to achieve..

Not Sure Which Testing Approach Is Right for Your Organisation?

At Red Cursor, we help organisations determine the most appropriate penetration testing methodology based on their environment, security objectives and available resources. Whether you are looking to meet compliance requirements, identify vulnerabilities or gain a deeper understanding of your security posture, choosing the right approach can make a significant difference to the value of your engagement.

If you are considering penetration testing, we would be happy to have a conversation about what approach would work best for you.

Learn more about Red Cursor’s penetration testing services.