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Re: Is this safe??

by chromatic (Archbishop)
on Feb 07, 2001 at 00:27 UTC ( #56770=note: print w/replies, xml ) Need Help??


in reply to Is this safe??

On first glance, probably. On the other hand, are you willing to take the chance? Who knows what shell metacharacters may be used in the future? I'd rather deny everything I don't explicitly allow than allow everything I don't explicitly deny.

On the other hand, I would say it's fairly unlikely you'll have a query string AND posted data at the same time, for a normal operation. So it may be a moot point, as I don't expect this script to work very well.

I would recommend you read perlsec and enable Taint mode. Also enable warnings with perl -w and use the strict module. You should also use CGI for your form handling (see my home node or Ovid's CGI course for details).

If you want to be more paranoid, I would build up a hash of *allowed* files, then check the input against them:

my %files = ( '/var/www/index.html' => 1, '/var/www/home/index.html' => 1; ); my $request = $query->param('fulltxtpath'); if (exists($files{$request})) { open (FILE, "> " . $request) or dienice("couldn't open $request: $ +!"); # open and print file } else { # error }
Better yet, use opendir and readdir to build a list of allowed files. You might even use the keys of the hash as their values, and use $files{$request} in the open statement.

Finally, I'd replace the looping substitutions with a transliteration:

$var1 =~ tr/a-zA-Z0-9\/\.\-_//dc;

That's reasonably strict.

Replies are listed 'Best First'.
Re: Re: Is this safe??
by SilverB1rd (Scribe) on Feb 07, 2001 at 01:08 UTC
    I was trying to copy the way this other cgi that my company was using, that stoped working for who knows why, this is way it uses file names passed in by the user.

    "I would say it's fairly unlikely you'll have a query string AND posted data at the same time"
    The mail program uses a template for the email it sends, this txt file is specified in the action line like so action="https://www.mydomain.com/cgi-bin/mail.cgi?/myfolder/template.txt" The form data is sent via the post method.

    This script works fine, I just dont want to miss anything. What do I need to check to make sure I'm not letting anything slip by? is it the data being used to open files? or all the data being sent?
      What do I need to check to make sure I'm not letting anything slip by?

      As described above, use both taint mode and CGI.pm as follows:

      #!/usr/bin/perl -wT #warnings and taint mode now enabled use CGI; #you may now use the CGI methods.
      It would scare me greatly if you copied this code from a company web page since those pages are now vunerable to a variety of DoS and hack attacks. CGI has tested-true functions for nearly everything you do above. It looks like you could reduce that program to 10 lines or less OF SECURE CODE!

      On CGI security: If you're not using the inputted data in shell calls, then you're relatively safe. Taint mode specifically checks for this. Of course, some loser can still input some random schlop, so work your CGI with "valid entry or reject"/deny-allow in mind. As I said earlier, CGI.pm provides numerous functions for EXACTLY what you are doing (and doing incompletely), including character escaping, so click that link and read the docs on it.

      AgentM Systems nor Nasca Enterprises nor Bone::Easy nor Macperl is responsible for the comments made by AgentM. Remember, you can build any logical system with NOR.
        I'm working on a new scritp with the CGI stuff and its giving me this error when I run it on my pc that says "Too late for the "-T" option at mailer.cgi line 1." Is this just because on running it on a pc?
        Thanks I'll do that and let you all know how it works! I wrote that script.

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