IP Address: 172.25.200.200

Port 9001

Want to run this Docker container locally?

docker run -d -p 9001:80 --restart always --name ctf-mime-challenge-flag-n joshbeck2024/ctf-rev-shell-mime-flag-n

If you haven’t done Flag B yet, you should definitely do that first.

We’ll utilize the same code with a minor change.

If we try to upload our shell.php file from Flag_B to this web server, we get an error:

This time the web server is checking to see whether or not we are uploading a valid GIF file.

In this case, it checks whether the file signature (MIME Type) matches that of a GIF file. Fixing file signatures in a Hex editor is something all of the 9th-grade students have done with the hexed.it website, but we can also modify simple text files like our shell.php file just using Nano.

First, let’s look up what the file signature should be for a .gif file:

<https://www.garykessler.net/library/file_sigs_GCK_latest.html>

Use CTRL-F and search for ‘GIF’:

We can see that a file like this should start with GIF87a. Let’s add this to the top of our shell.php like this: