DVWA 1.9: File Inclusion Medium and High

7:52 AM 0 Comments

Although I've studied and practiced secure coding standards for some time now, I had yet to try my hand at the offensive approach before last Friday when I downloaded DVWA and started working on the exercises.

File Inclusion

The file inclusion exercises were unexpectedly eye opening. Initially, I thought: "Directory traversal, get the etc/passwd file, etc., etc., not much here I don't already know." Then, I stumbled into Ashfaq Ansari's walkthrough of File Inclusion and Log Poisoning on DVWA Low which showed to my astonishment how one could use this security hole to poison logs and subsequently upload a php shell to the DVWA server.

Clever. Not bad for a day's work, right?

Medium Level

Thanks to Mr. Ansari, I learned a lot more than I thought I would about the dangers of file inclusion security holes; however, there was more to come. On the medium level, the same directory traversal attack initially seems defended against with the following code:

$file = str_replace( array("http://", "https://"), "", $file);
$file = str_replace( array("../", "..\""), "", $file);

Now, the url parameter value "../../../../../etc/passwd" will instead be transformed into etc/passwd and nothing will show:


Blacklisting is hard, though, and a single-pass search and replace cannot remove all ills. Consider, for example, what would happen when performing a str_replace on "hthttp://tp://". You, of course, would be left with "http://", the thing you were trying to prevent from being in the string in the first place!

So, of course, if all one is going to do is remove the "../" instances from the string, we simply need to construct a string that will leave "../" instances in the wake of a search and replace, e.g. "....//....//....//....//....//etc/passwd" or "..././..././..././..././..././etc/passwd" will both do fine.


Now, the same steps of log poisoning and shell uploading can again be performed with relative ease.

The right way to defend against this is whitelisting, which the higher levels of this exercise employ.

High Level

Actually, I'm not certain quite how to leverage this, yet, but I thought I'd post some of my initial thoughts. The defense against file inclusion in the high level is incomplete because unintended patterns can get passed it:

if ( !fnmatch("file*", $file) || $file != "include.php" ) {
    echo "ERROR: File not found!";
    exit;
}

Here, the regex allows for the file protocol, e.g. page=file:///etc/passwd. Since this would simply serve files from the user's local machine, I'm not sure what could be done with it, but I found it interesting.

Josh Cummings

"I love to teach, as a painter loves to paint, as a singer loves to sing, as a musician loves to play" - William Lyon Phelps

0 comments: