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Jan 12, 2010 
Okay, today should be the last day of Security topics (finally). The first one we’re going to cover is:
Remote Code Injection
Remote Code Injection is when a malicious user causes your PHP code to execute remote malicious code. The most obvious way is by an query string input. See this example:
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<p>If you were to pass <strong>page=http://www.example.com/malicious</strong> into that script, it would then include and execute that <strong>malicious.php</strong> script.  This is easily preventable by whitelist filtering your input.  Here is an example:</p>
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<p>
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<div style="padding-bottom: 0px; margin: 0px; padding-left: 0px; padding-right: 0px; display: inline; float: none; padding-top: 0px" id="scid:887EC618-8FBE-49a5-A908-2339AF2EC720:ed6d274d-2cd4-4bd5-907f-cc7c6b22931a" class="wlWriterEditableSmartContent"><pre lang="php">$valid = array('home', 'news', 'blog');
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if (in_array($_GET['page'], $valid))
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require_once "{$_GET['page']}.php";
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else
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echo "Invalid page.
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The alternate way to protect against this is by disabling allow_url_fopen. But that also prevents any script from accessing any remote content (images, files, etc).
Command Injection
This will be quick. When executing shell commands in PHP (using system, exec, passthru, or backtick), you can prevent any malicious code from being inserted by using the escapeshellcmd() and escapeshellarg() functions.
Shared Hosting
When you are running a shared hosting environment, you have to be careful about how various user's PHP scripts interact with eachother. The most important thing is you want to prevent one user from accessing another user's files. In PHP 6, safe_mode will no longer be an option. Therefore you can use the following php.ini directives:
- open_basedir: limits what folders PHP can open/read from
- disable_functions: disables specific functions (usually exec, passthru, system)
- disable_classes: disables specific classes (directory, etc.)
Okay, that's it about Security. Remember, NEVER TRUST THE USER!
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