Dealing with Double File Extensions

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Due to security issues with Apache modules, it is recommended to leave the following setting enabled:
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{{#CUSTOMTITLE:Dealing with Double File Extensions}}
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Due to security issues with Apache modules it is recommended to leave the following setting enabled:
 
{{{code}}}
 
{{{code}}}
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How does it work? Suppose the following:
 
  
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If "php" is on the denied extensions list, a file named foo.php cannot be uploaded. If "rar" (or any other) extension is allowed, one can upload a file named foo.rar. The file foo.php.rar has "rar" extension so, in theory, it can be also uploaded.
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How does it work? Suppose the following scenario:
  
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In some conditions Apache can treat the foo.php.rar file just like any PHP script and execute it. If CheckDoubleExtension is enabled, each part of the file name after a dot is checked, not only the last part. In this way, uploading foo.php.rar would be denied, because "php" is on the denied extensions list.
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If <code>php</code> is added to the '''denied extensions''' list, a file named <code>foo.php</code> cannot be uploaded. If <code>rar</code> (or any other) extension is added to the '''allowed extensions''' list, one can upload a file named <code>foo.rar</code>. The file <code>foo.php.rar</code> has a <code>rar</code> extension so in theory, it can also be uploaded.
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Under some circumstances Apache can treat the <code>foo.php.rar</code> file just like any other PHP script and execute it. If <code>{{{config}}}</code> 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 <code>foo.php.rar</code> will be renamed into <code>foo_php.rar</code>.

Latest revision as of 15:20, 21 December 2012

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

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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This page was last edited on 21 December 2012, at 15:20.