Black-box testing: simulating external attacks in practice
In cybersecurity assessments, penetration tests, it is essential to assess a system from multiple perspectives. One of the most common and most realistic approaches is black-box testing, which examines the availability and behaviour of a system by an external attacker. This method can also highlight problems that can be exploited without internal access or developer visibility. In this article, we explain what black-box testing is, when to use it, and its benefits and limitations.
What is a black-box test?
In black-box testing, the tester has no prior information about the internal workings, source code or architecture of the system. The test is based solely on the interactions that are available to an outside user or potential attacker.
The method is designed to simulate real user or attacker behaviour, for example:
- using public websites and forms,
- testing the available API endpoints,
- by analysing information indexed by search engines,
- or even mapping test sites that have been left out.
Black-box testing helps to find flaws that can be easily exploited by an attacker from the outside without any technical background.
In which cases should you use black-box testing?
This type of test should be used when the system:
- is mainly available through public platforms,
- or the goal is a quick, low-resource security survey.
The black-box approach is particularly useful for websites, webshops, campaign or marketing pages, but can also be used for external security audits of mobile apps and publicly available web APIs.
What types of errors can be detected by black box testing?
Black-box testing can reveal a number of faults that may be caused by the system's external operation, availability or incorrect responses. These are typically faults that can be detected or exploited by an average user or by an outside attacker.
Here are the most common failure categories that can be identified by black-box testing:
1. Security vulnerabilities
These are the most serious errors as they directly compromise data security and system integrity:
- SQL injection - unauthorised access to data through improperly managed database queries.
- Cross-site scripting (XSS) - injecting malicious scripts through the user interface.
- Cross-site request forgery (CSRF) - execute unauthorized commands on behalf of an authenticated user.
- Open redirect (open redirect) - redirect users to unsafe sites.
- Configuration gaps - default passwords, open ports, available debug or test pages.
2. Functional errors
Errors in the operation of the system that impair reliability or usability:
- Forms not working, errors handled badly
- Incomprehensible or incomplete user feedback
- Incorrect data processing (e.g. fields not validated)
- Incorrect error codes (e.g. error 500 for trivial input)
3. Information leakage
Situations where the system reveals too much information about itself:
- Detailed error output (e.g. stack trace)
- Technological data, version numbers (e.g. X-Powered-By HTTP header)
- Available sensitive files (.git, .env, backup.zip, log files)
- Routes to disable in Robots.txt
4. Performance and stability problems
Black-box testing may reveal that the system:
- does not scale properly under higher loads,
- slow response times (e.g. data queries, searches), crashes on erroneous or unexpected input (e.g. too long text, special characters).
5. Non-functional errors (user experience)
These may include problems affecting the user experience or the usability of the system:
- Feedback that is difficult to interpret or too technical
- Incorrect or missing data checks
- Non-responsive or incompatible display on different devices, browsers
6. Logical errors, business process gaps
These are not technical errors, but can pose serious business and security risks:
- Missing or incorrect process steps (e.g. purchase with invalid data)
- Repeatability of functions (e.g. multiple use of a coupon)
- Undocumented but exploitable behaviours
IMPORTANT: Black-box testing alone does not provide a complete picture of the security of the system, as it does not reveal all internal errors or configuration problems. It does, however, identify the very external flaws that a real attacker could detect. This is why it is a crucial part of any security audit or vulnerability testing.
Advantages and limitations of black-box testing
Now let's look at why it is useful and what the difficulties of black box testing might be!
The benefits of black-box testing
1. Realistic offensive approach
The analysis is done from the point of view of an external, unknown attacker. This will give us an accurate picture of the threats to publicly accessible interfaces.
2. Does not require internal access
Testing can be done without source code, documentation or internal permissions, so it requires minimal resources from the client and is easier to schedule.
3. Fast and cost-effective
Since no complex preparation is required, black-box testing can be done quickly. It can also be carried out using partly automated tools, which reduces costs.
4. Can be used in live or staging environments
No separate development environment is needed: black-box tests can be run safely in either live or test environments, with appropriate precautions.
Limitations of black-box testing
1. Limited coverage
Because the tester cannot see the inner workings of the system, many code-level or logical errors can remain hidden. This method is not suitable for static code analysis or configuration testing.
2. The source of the error is difficult to identify
When a fault is found, it is often not immediately clear what caused it. A white-box or grey-box approach may be necessary to identify the problem accurately.
3. Automation can be a challenge
Although there are excellent black-box tools (e.g. DAST), they do not always cover real user paths, especially if the system is dynamic.
4. Full functionality testing not ensured
The analysis covers only public interfaces. Closed functions, linked only to internal roles, may be omitted.
5. False negative option
A vulnerability may not be visible during external interactions, so the test may falsely assess the system as secure - even though a white-box analysis would have detected the problem.
The role of black-box testing in vulnerability testing
Black-box testing is only one possible approach to vulnerability testing. A grey-box testing testers have partial knowledge of the system - for example, access to a user account or documentation. White-box testing, on the other hand, provides a complete view of the inner workings of the system, including source code, configurations and architecture.
The different testing methods complement each other to provide a comprehensive picture of the security state of the system. The black-box approach is particularly important as it simulates a real attack scenario and illustrates the risks to be expected in case of external access.
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