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Chapter 4. Security |
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One feature of PHP that can be used to enhance security is configuring PHP with register_globals = off. By turning off the ability
for any user-submitted variable to be injected into PHP code, you can reduce the amount of variable
poisoning a potential attacker may inflict. They will have to take the additional time to forge
submissions, and your internal variables are effectively isolated from user submitted data.
While it does slightly increase the amount of effort required to work with PHP, it has
been argued that the benefits far outweigh the effort.
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Example 4-14. Working without register_globals=off
<?php
if ($username) { // can be forged by a user in get/post/cookies
$good_login = 1;
}
if ($good_login == 1) { // can be forged by a user in get/post/cookies,
fpassthru ("/highly/sensitive/data/index.html");
}
?>
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Example 4-15. Working with register_globals = off
<?php
if($HTTP_COOKIE_VARS['username']){
// can only come from a cookie, forged or otherwise
$good_login = 1;
fpassthru ("/highly/sensitive/data/index.html");
}
?>
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By using this wisely, it's even possible to take preventative measures to warn when forging is
being attempted. If you know ahead of time exactly where a variable should be coming from, you can
check to see if submitted data is coming from an inappropriate kind of submission. While it doesn't
guarantee that data has not been forged, it does require an attacker to guess the right kind of
forging.
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Example 4-16. Detecting simple variable poisoning
<?php
if ($HTTP_COOKIE_VARS['username'] ∓∓
!$HTTP_POST_VARS['username'] ∓∓
!$HTTP_GET_VARS['username'] ) {
// Perform other checks to validate the user name...
$good_login = 1;
fpassthru ("/highly/sensitive/data/index.html");
} else {
mail("admin@example.com", "Possible breakin attempt", $HTTP_SERVER_VARS['REMOTE_ADDR']);
echo "Security violation, admin has been alerted.";
exit;
}
?>
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Of course, simply turning off register_globals does not mean code is secure. For every piece of
data that is submitted, it should also be checked in other ways.
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