FAQ 9.14 How do I make sure users can't enter values into a form that cause my CGI script to do bad


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9.14: How do I make sure users can't enter values into a form that cause my CGI script to do bad things?

(contributed by brian d foy)

You can't prevent people from sending your script bad data. Even if you
add some client-side checks, people may disable them or bypass them
completely. For instance, someone might use a module such as "LWP" to
access your CGI program. If you want to prevent data that try to use SQL
injection or other sorts of attacks (and you should want to), you have
to not trust any data that enter your program.

The perlsec documentation has general advice about data security. If you
are using the "DBI" module, use placeholder to fill in data. If you are
running external programs with "system" or "exec", use the list forms.
There are many other precautions that you should take, too many to list
here, and most of them fall under the category of not using any data
that you don't intend to use. Trust no one.


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