security - Sanitize file path in PHP

Question

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.

Answer 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.