Composr 10.0.36 - Remote Code Execution

# Exploit Title: Composr 10.0.36 - Remote Code Execution
# Date: 04/06/2021
# Exploit Author: Orion Hridoy
# Vendor Homepage: https://compo.sr/
# Software Link: https://compo.sr/download.htm
# Version: 10.0.36
# Tested on: Windows/Linux
# CVE : CVE-2021-30149

A RCE on Composr CMS has been discovered by BugsBD Private LTD. We have a galleries security issue which allows us to upload a PHP file. Whenever we upload an image from galleries, Composr allows us to upload only images. If we tried to upload a PHP file from galleries uploader it will say someone attempting hacking activities. But we have a security issue on the Upload In Bulk section. Whenever we check allowed extension in Upload in bulk function we can see PHP is completely prohibited. But whenever we tamper the request and change the extension we can see it will upload the PHP file without other or server side verification. This allows a user to upload malicious file even when they restricted it.

Steps To Reproduce:
1. Go to upload galleries.
2. Upload a image and tamper the request and change the extension from .jpg to .php
3. It will say hacking attempts, check the allowed extension and you can see it's not accepting PHP extension.
4. Now go to upload in bulk option.
5. Upload a image with PHP codes and tamper the request.
6. Change extension from .jpg to .php
7. It will get uploaded with the blocked PHP extension.