As it is, you find that your data is still tainted after with that regular expression - This is because of the 'valid' parsing of the '/' character by your regular expression. To correctly untaint data, you need to exclude everything except "word" characters (alphabetics, numerics and underscores), hyphens, at characters and a single dot - The inclusion of the forward-slash character allows data to remain tainted because it is exceptionally easy for parsed data to reference files outside the 'permitted scope' of your application - For example, if your routine was supposed to allow the deletion of trees under the directory /home/<user-entered-directory>, a maligned user could easily pervert this code by entering /etc and causing great harm to your system.

The recommended regular expression for untainting data (from perlsec) would be:

if ($data =~ /^([-\@\w.]+)$/) {
    $data = $1;                     # $data now untainted
} else {
    die "Bad data in $data";        # log this somewhere
}

[download]

This may however mean that you will have to modify your code to run under -T and possibly a chroot environment or else you could have potentially nasty users causing harm to your system.

Have a read through the "Laundering and Detecting Tainted Data" and "Cleaning Up Your Path" sections of perlsec.

Update - See davorg's post here for an update on this problem.

 

Ooohhh, Rob no beer function well without!

In reply to Re: Re: Re: Insecure dependency on perl module by rob_au
in thread Insecure dependency on perl module by Anonymous Monk

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