So your wonderful little creation is finished, and it does exactly what it was designed to do. But, have you prevented it from doing what it’s not supposed to do?
Enter the forgotten art of negative testing. This is the safeguard from user error, malicious attacks, and blatant developer oversight. Negative testing is taking your calculator application and trying to add “Hello” and “Goodnight”. Negative testing is trying to supply an invalid email address–.email@example.com–into your mailing list form. Negative testing is trying to cause a buffer overflow on your lead-developer’s computer because you were able to sneak in a script injection.
The key word here is “try.”
If everyone has done their job, you will get nowhere. Unfortunately, rarely is this job done right. In 3 minutes I could considerably alter my best friend’s blog, and he doesn’t even know it. In 10 minutes I could corrupt the online database of a Fortune 500’s web site–both company and URL to remain anonymous. And, what scares me the most, in 20 minutes I could download the entire database of a certain benefits company, including the complete identity–SSN included–of a few thousand people.
For years, I have been paid to break things as much as build them. When that calculator finally adds 2 and 2 correctly, don’t be satisfied. Try to add “Hello” and “Goodnight”. Will it give you a neatly handled error message informing you that it couldn’t complete the procedure, or did it return a fatal exception and die a miserable death because it expected a Double and you gave it a String? Optimally, it shouldn’t allow you to even type characters into the input area unless you are working in hex; even then, only A-F.
If instructions tell you to do one thing, enter the opposite. If you see a value in the URL, change it. If a field asks for an integer between 0 and 5, try 0, 2, 5, -1, 9, 3.5, and “Q”, and see how it handles “unexpected inputs.” If a querystring is “?UserID=6″, change the 6 to a 7, to see if you get information on User 7, and try invalid items like 3.5 and “Q” to see if it fails on unexpected inputs. If a client-side cookie has a value of “User”, try changing it to “Admin” or “Administrator” and see if your access-level is increased.
Find the weaknesses, find the holes, and find the bugs so that they can get fixed. You are the demolition man. You get paid to blow things up. Do it. Do it with purpose. Pretend you are a hacker trying to get into the system. Pretend you are a teenager-hacker-wannabe trying to screw with the system. Pretend you are a grandma that doesn’t know what to do with the system. Do all of the things that you aren’t supposed to do to the application and do them on purpose, because if by ignorance or intelligence, your users will find what was missed.
The opinions expressed herein are my own personal opinions and do not represent
my employer's view in any way.