Lately, I've spotted a few examples where user's haven't been parsing input before displaying in a web page (katgirl!), allowing malicious users (ie me) to enter javascript in the inputs that then gets displayed on the web page.

Most people are aware of this, but I haven't seen a formal way of taking precautions that's globally accepted.

My thought was to create a module (CGI::Taint) that when used, overloaded print to add taint checks to items sent to it - currently, print is ignored as a factor in untainting data.

I'm not 100% sure this is the best approach, but would something like the following work? 

use CGI;
use CGI::Taint;

my $q=CGI->new();

my $tainted_var = $q->param('form_input');

# first case - dies to browser with 
# "attempt to print tainted var at line 10"
print $q->header.
      "Tainted: $tainted_var";
exit(0);

# second case - no error
my $untainted_var = '';
$tainted_var =~ /(\w\s+)/ and $untainted_var=$1;
print $q->header.
      "Untainted: $untainted_var";
exit(0);

[download]

So, my questions are (I guess):

  1. is this the right approach
  2. is it feasible, given how complicted CGI.pm is?

If it could work, I don't think it would ba that much work, or are there other functions that would need overloading too? printf?

Hmmm. Thoughts welcomed.

cLive ;-)

In reply to writing a "CGI::Taint" module by cLive ;-)

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