Security Strategies
The right way is not to make a list of what to disallow. The right way is to make a list of what to allow. This makes the solution much more manageable. If you start by saying that anything goes and looking for those things that cause problems, you will spend a long time looking. There are countless combinations to check. If you say that nothing goes and then slowly add things, you can check each of these as you add them and confirm that nothing will slip past you. If you miss something, you have disallowed something you should allow, and you can correct the problem by testing it and adding it. This is a much safer way to error.
— CGI Programming with Perl, 2nd Edition
The right way is not to make a list of what to disallow. The right way is to make a list of what to allow. This makes the solution much more manageable. If you start by saying that anything goes and looking for those things that cause problems, you will spend a long time looking. There are countless combinations to check. If you say that nothing goes and then slowly add things, you can check each of these as you add them and confirm that nothing will slip past you. If you miss something, you have disallowed something you should allow, and you can correct the problem by testing it and adding it. This is a much safer way to error.
— CGI Programming with Perl, 2nd Edition