This challenge consisted of a path traversal vulnerability and something similar to a MIME type confusion attack. While I was not able to solve this during the CTF due to a skill issue, I decided to attempt to solve it after the CTF in hopes of being able to better understand what’s going on.


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The source code for the challenge can be obtained from here.

Initial Overview

While clicking around on the page, we see that the page changes the theme based on a parameter theme.


If we see how the theme parameter is handled in the backend, we can see that it changes the theme if the value is either dark or light. However, it defaults to light if the value is neither, and it takes in any arbitrary value.

function theme() {
    return $_SESSION['theme'] == "dark" ? "dark" : "light";

$_SESSION['flag'] = FLAG; /* Flag is in the session here! */
$_SESSION['theme'] = $_GET['theme'] ?? $_SESSION['theme'] ?? 'light';

Unfortunately, my dumbass decided to turn a blind eye to this during the competition, which turned out to be fatal mistake.

Down the Rabbit Hole

Before we delve any further into how the theme parameter can be exploited, let’s take a look at possible ways to find the flag.

There’s another file called audio.php which seems to be vulnerable to path traversal attacks. However, it is protected by a MIME type check.

$file = 'audio/' . $_GET['f'];  // Vulnerable to path traversal (audio/../flag.php)
$mime = mime_content_type($file);  // MIME check

if (!$mime || !str_starts_with($mime, 'audio')) {  // Checks if MIMEtype is an audio
    http_response_code(403); die;

Initially, I thought that we could somehow bypass the MIME check and force audio.php to view flag.php, as the flag was stored there. However, that would not be possible as we are not able to control flag.php, and thus would not be able to spoof its MIMEtype.

PHP Sessions

While I was writing this, I realized that the organizers left a really helpful hint in index.php, which was to say that the flag is hidden in the session.

$_SESSION['flag'] = FLAG; /* Flag is in the session here! */

It would have been really nice if I knew of this during the CTF, and not after, but I found out that PHP’s session data is stored on the filesystem itself, and not as a database that spawns when PHP runs.

Now that I think about it, it certainly makes more sense to place it as on the filesystem as a persistent file rather than and in-memory database which is volatile and would lose all information when the server crashes.

If you’d like to read up more about how PHP determines where the files are saved, do check this StackOverflow post and the PHP documentation.

Putting Two and Two Together

Now that we know PHP session variables are stored as file, and as the flag is also stored as a session variable as evidenced by $_SESSION['flag'] = FLAG;, we can conclude that the right direction to go is to somehow spoof the session variable file to act as a audio MIMEtype.

If we look up the documentation for mime_content_type here , we see that the MIME content type for a file is given by magic.mime.

When we google for magic.mime, we see that Apple has kindly open-sourced their magic.mime file which can be found over here.

After doing a bit of searching, I found out that magic.mime is a file full of byte patterns used to determine a file’s MIMEtype. To illustrate this, if we refer to Apple’s magic.mime file, there is one entry that says at offset 1080, a file that contains the string M!K! will be considered as a audio/x-mod.

# Magic data for KMimeMagic (originally for file(1) command)
# The format is 4-5 columns:
#    Column #1: byte number to begin checking from, ">" indicates continuation
#    Column #2: type of data to match
#    Column #3: contents of data to match
#    Column #4: MIME type of result
#    Column #5: MIME encoding of result (optional)


1080    string    FLT4       audio/x-mod


Now that we know how to spoof MIMEtypes, let’s put it into action. Firstly, we have to figure out where the PHP session is stored. The StackOverflow post mentioned above reveals that it is at /tmp/sess_<ID>.

Next, we have to somehow inject the string M!K! into the file at offset 1080. As we do not know how many bytes are written before the theme variable is written into the temp file, we have to brute force this by adding an arbitrary number of random characters before M!K!.

After the string gets injected, we can simply request for the file and the flag should be inside.


import requests

s = requests.Session()

for n in range(1000, 1080):  # Brute force offset
    s.get(f"{'a'*n + "M!K!"}")  # Insert M!K! after `n` characters

    # Get the contents of the file
    r = s.get(f"{s.cookies['PHPSESSID']}")

    # If we don't get a 403 Unauthorized response, we got the flag
    if (r.status_code != 403):
        print("Offset found:", n)

Flag: DUCTF{koo-koo-koo-koo-koo-ka-ka-ka-ka-kaw-kaw-kaw!!}