In a chatterbox discussion earlier this evening, Chrisf and myself were discussing potential security issues with CGI file upload scripts. After a bit of discussion involving the use of temporary files, the main points of which I have covered previously here, I put together the following block of code - Note that this code is not considered to be a complete example, but merely sufficient to highlight the elements of the CGI upload interface discussed.

#!/usr/bin/perl -Tw

use CGI;
use CGI::Carp qw/ fatalsToBrowser /;
use Fcntl;
use File::Basename;
use POSIX;
use strict;

$CGI::POST_MAX = 1048576;       #   Maximal file upload size
$CGI::DISABLE_UPLOADS = 0;

my $config = {
    'allowed_file_types'    =>  [ 
        'png', 
        'gif', 
        'jpg' 
    ],
    'upload_fields'         =>  [ 'image_file' ]
};

my $cgi = CGI->new;

foreach my $field ( @{ $config->{ 'upload_fields' } } ) {

    if ( defined $cgi->param( $field ) ) {

        my $fextension = ( fileparse( $cgi->param($field), '\..*' ) )[
+2];
        if ( grep { lc( $_ ) eq lc( $fextension ) } @{ $config->{ 'all
+owed_file_types' } } ) {

            my $fname;
            {
                local *FH;
                do {
                    $fname = tmpnam();
                } until sysopen ( FH, $fname, O_RDWR|O_CREAT|O_EXCL, 0
+666 );

                my $buffer;
                while ( read( $cgi->param( $field ), $buffer, 1024 ) )
+ {
                    syswrite FH, $buffer, length( $buffer );
                }
                close FH;
            }

            #   continue on and do stuff with uploaded file - file res
+ides in temporary directory with file name $fname

        } else {

            #   bad file extension

        }
    }
}

[download]

Now while any form of file system interaction via a CGI interface is going to come with a number of inherent security concerns, are there any other immediate concerns which should be addressed in a script similar to that above?

Any and all suggestions for the tightening and/or improvement of this code process, particularly from a security point of view, are welcomed.

 

perl -e 's&&rob@cowsnet.com.au&&&split/[@.]/&&s&.com.&_&&&print'

In reply to Security issues for CGI file upload by rob_au

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.