Dealing with Double File Extensions

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Revision as of 09:47, 29 March 2011

Due to security issues with Apache modules it is recommended to leave the following setting enabled: 

$config['CheckDoubleExtension'] = true ;

How does it work? Suppose the following scenario:

If php is added to the denied extensions list, a file named foo.php cannot be uploaded. If rar (or any other) extension is added to the allowed extensions list, one can upload a file named foo.rar. The file foo.php.rar has a rar extension so in theory, it can also be uploaded.

Under some circumstances Apache can treat the foo.php.rar file just like any other PHP script and execute it. If {{{config}}} is enabled, each part of the file name after a dot is checked, not only the last part. If extension is disallowed, the dot (.) is replaced with an underscore (_). So the uploaded file foo.php.rar will be renamed into foo_php.rar.

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