Greetings, Monks.

OK, I've been reading here on the Monestary and elsewhere that it's a Good Idea to use placeholders in an SQL query where information for the query is derived from CGI form input, in case someone were to enter "; drop table foo;" or some such. So I'll go rewrite bits of my code using placeholders. No problems there.

What I want to do is write "bad" code and submit "bad" data so that I can see for myself the effects of using bad code, and its fix. But, in this instance, I can't do it.

From what I've heard, the DBI->prepare() method will only take a single statement, as will the DBI->do() method. So, how can these be exploited? I keep getting SQL syntax errors, even though the resulting multiple-statement string works just fine when entered directly into MySQL. (For instance, if I end up passing "select object_id, object_title from testac where object_title = 'test'; insert into testad values (49, 'blarg');" to do() or prepare(), it complains.) I just can't quite figure out what exactly *not* to do, and how such a badly-written script can be exploited.

So, can anybody write some "bad" code for me, and explain just how it can be taken advantage of? I have the following test code, assuming the form has already been submitted, and left untainted: 

my $data = $cgi->param('userdata');
my $sql = 'select col1, col2 from tablename';

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What comes next? Thanks for any replies, folks.

In reply to CGI/DBI and placeholders, revisited. by mephit

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