Unrestricted File Upload

Severity: High
Test name: Unrestricted File Upload

Summary

Unrestricted File Upload vulnerability allows an attacker to upload malicious files to a web server without proper validation. The web application takes the uploaded file and saves it in the file system, data storage, or in a database.

If a malicious file has been uploaded into the file system, the attacker is able to request this file using the URL of the victim’s site. As a result, the attacker can execute any command on the web server. In this case, the attacker should know the uploaded file path.

Impact

This vulnerability allows an attacker to:

Execute code on the server side and the client side.
Gain sensitive information. Browse server folders and files.
Crash the server. Overload the file system or the database.
Send attacks to other servers. Create spam content.
Overwrite critical files or personal data.

Example of uploading a web-shell script

Consider that you have a web application which allows uploading an account photo without proper validation.
Instead of a picture, an attacker uploads a prepared PHP script ("malicious-file.php") using the file picker for the account photo. The content of the PHP script is the following:

<?php 
exec($_GET['c']); // Execute an external command passed to the script as entry parameter

AAfter the script has been successfully uploaded, the attacker can execute any system command on the web server using the following URL:

https://www.{your_web_site}.com/account-photos/malicious-file.php?c=<any-command>

As a result, the attacker can send a large number of messages anonymously. The attacker may also send phishing emails, where the recipient believes that these messages are originating from a trusted source (your website).

Location

The issue can be found in the source code on the server side.
The issue can be found in the server configuration.

Remedy suggestions

The uploaded file types must be restricted. Use a whitelist of accepted, non-executable file extensions.
Validate the uploaded files. Limit the uploaded files amount and maximum file size.

Make sure that your web server uses only one extension in the filename.

Apache

Insecure:

<FilesMatch ".+\.php">SetHandler application/x-httpd-php</FileMatch>

Secure:

<FilesMatch ".+\.php$">SetHandler application/x-httpd-php</FileMatch>

Nginx

Insecure:

location ~* \.php$ {
    fastcgi_pass backend;
    # [...]
}

Secure:

location ~* \.php$ {
 try_files $uri = 404;
    fastcgi_pass backend;
    # [...]
}

The directory with the uploaded files should not have any "execute" permission, and all the script handlers should be removed from these directories.
When running on a web server that supports case insensitive names, apply the whitelist rule to filter extensions such as .exe or .EXE to disallow a situation where a bypass could be applied to the rules that state what type of file could be uploaded to the server.
Consider saving the uploaded files in a specially designed data storage rather than in the file system of your web server.
It is recommended to verify that the uploaded files are not saved with their original name (for example, text.txt will be saved as 2504sgbrys3f.txt - thereby decreasing the chance that an attacker will access the file he has uploaded).

Classifications

CWE-434
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:L

References

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