full-disclosure - file_exists() bypassing , critical problem ?

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From: m.esco at wp.pl (m.esco@...pl)
Subject: file_exists() bypassing , critical problem ?

>  It's a very critical bug because a malicious user can use it to bypassing
>  the file_exist() protection on a include() script for example.

I don't think that is critical problem. It is rather bad-programmers
problem. There are various methods for securing file_exists() like e.g.

<?php

    // allowing files only from directory /test
    $sFileName = $_GET['page'];

    // is file name equals XXXXX.XXX pattern
    if(preg_match("|[\w\-]+\.[\w]+|i",$sFileName))
    {
        // check if file exists
        if(file_exists("./test/".$sFileName))
        {
            // open file and stuff
        } else
        {
            // sorry
        }
    } else
    {
        // mail the webmaster that someone has tampered the url's
    }

?>

That is just one of the sollutions, far from the best one.

There is an article about this here:
http://www.zend.com/zend/art/art-oertli.php