Sanitize file path in PHP

Greetings, I'm hoping to make my tiny program secure so that potential malicious users cannot view sensitive files on the server.

    $path = "/home/gsmcms/public_html/central/app/webroot/{$_GET['file']}";


    if(file_exists($path)) {
        echo file_get_contents($path);
    } else {
        header('HTTP/1.1 404 Not Found');
    }

Off the top of my head I know that input such as '../../../../../../etc/passwd' would be trouble, but wondering what other malcious inputs I should expect and how to prevent them.

Solution 1:

realpath() will let you convert any path that may contain relative information into an absolute path...you can then ensure that path is under a certain subdirectory that you want to allow downloads from.

Solution 2:

Use basename rather than trying to anticipate all the insecure paths a user could provide.

Solution 3:

Solution by the OP:

$baseDir = "/home/gsmcms/public_html/central/app/webroot/"; 
$path = realpath($baseDir . $_GET['file']); 

// if baseDir isn't at the front 0==strpos, most likely hacking attempt 
if(strpos($path, $baseDir) !== 0) { 
   die('Invalid Path'); 
} elseif(file_exists($path)) { 
   echo file_get_contents($path); 
} else { 
   header('HTTP/1.1 404 Not Found'); 
   echo "The requested file could not be found"; 
} 

Solution 4:

If you can, use a whitelist like an array of allowed files and check the input against that: if the file asked by the user isn't present in that list, deny the request.