Web shell upload via Content-Type restriction bypass
https://portswigger.net/web-security/learning-paths/server-side-vulnerabilities-apprentice/file-upload-apprentice/file-upload/lab-file-upload-remote-code-execution-via-web-shell-upload
Description
This lab contains a vulnerable image upload function. It attempts to prevent users from uploading unexpected file types, but relies on checking user-controllable input to verify this.
To solve the lab, upload a basic PHP web shell and use it to exfiltrate the contents of the file /home/carlos/secret. Submit this secret using the button provided in the lab banner.
You can log in to your own account using the following credentials: wiener:peter
Solution
After login as Wiener user there's an upload option to upload avatar image:
with the avatar image at this path/location:
https://0a890067048524a181330c1d0073004e.web-security-academy.net/resources/images/avatarDefault.svg
We need to obtain Carlos's secret, we can do it upload a php shell (php_web_shell.php) that get secret file content
<?php echo file_get_contents('/home/carlos/secret'); ?>
there's a filtering and we can't upload php shell, seeing the output message file types allowed are: image/jpeg and image/png, so we can use the Burp Repeater to modify request updating Content-Type
Infact, inserting: image/jpeg we're able to upload our php web shell bypassing controls:
We know the file location and we can open it going to: https://0a890067048524a181330c1d0073004e.web-security-academy.net/files/avatars/php_web_shell.php
Obtaining Carlos's secret and solving the lab.
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