Perl has a tainted mode where any outside data is marked as tainted. You can only detained it in specific ways. Copying the data from one variable to another ke

Perl has a tainted mode where any outside data is marked as tainted. You can only detained it in specific ways. Copying the data from one variable to another keeps the tainted flag. Pretty much only explicit checks are able to remove the tainted flag from the data. As such you limit the security problems to faulty checks. It is impossible to forget to check something. Above is used in Bugzilla. Despite all of that, it's still pretty easy to make mistakes. IMO it's rather unfortunate that not more languages have such a feature. This as Perl code if often pretty difficult to read (it doesn't have to be!).