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Black Box versus White Box testing and when to use them

I have recently been speaking to many security professionals and asking them about black box and white box testing. I have used it as an interview question on many occasions as well. People's answers are varied and interesting, but I thought I would share my views briefly here.

Firstly, what are black box testing and white box testing, or grey box testing for that matter? Simply put, a black box test is one where the tester has no knowledge of the internal structure or workings of the system and will usually test with security protections in place. They may not even be given credentials to a system that requires authentication. This would be equivalent to what a hacker would have access to.

The opposite extreme is a white box test, where the tester has full knowledge of the system and access to the code, system settings and credentials for every role, including the administrator. The tester will likely be testing from inside the security perimeter. Grey box testing sits somewhere in the middle, where the tester will have knowledge of the functionality of the system and the overall components, but not detailed knowledge. They will usually have credentials, but may still test with some security controls in place.

So, when would you use the different levels of testing? Personally, I think that grey box testing is neither one thing nor the other and holds little value. For me, the motivation behind black box testing is compliance, whereas the motivation behind white box testing is security. With a white box test you are far more likely to find security issues, understand them and be able to fix or mitigate them effectively, so why wouldn't you do it? The black box test is supposedly what a hacker would see, but they have far more time, so it isn't even representative. The only reason to perform a black box test is to pass some audit that you are afraid you might fail if you perform a full white box test, in my opinion.

If you actually want to be secure, then make sure you always commission white box tests from your security testers.

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