What is an Arbitrary File Upload Vulnerability?

The Arbitrary File Upload vulnerabilities are among some of the most dangerous security flaws in the WordPress ecosystem. These vulnerabilities allow malicious users to upload files that can execute harmful code on a WordPress website.

In 2023, Arbitrary File Upload ranked 7 as one of the most common vulnerability type found in the ecosystem. It’s also a commonly mass-exploited vulnerability.

What Causes a Arbitrary File Upload Vulnerability?

Depending on a theme or plugin’s purpose, you very well may have a requirement to allow users to upload images or other content. For example, Arbitrary File Upload occurs when a file uploaded by a user does not have its file extension checked properly. This in turn could allow a .php file to be uploaded which could result in a full compromise of the website.

The most common mistakes that can cause Arbitrary File Upload vulnerability are:

• Lack of File Type Validation: When plugins or themes allow any file type to be uploaded without checking if it’s safe or necessary. For example, allowing uploads of executable files like .php instead of restricting them to harmless formats such as .jpg or .pdf.
• Improper Handling of File Names: Sometimes, developers trust the file extension provided by the user instead of verifying it after the upload, which allows attackers to disguise dangerous files as safe ones (e.g., renaming a .php file to .jpg).
• Insufficient User Permissions Checks: If users without administrative privileges are allowed to upload files in sensitive areas, attackers may exploit this flaw to gain further control over the site.

What Can Happen If Your WordPress Website Has a Arbitrary File Upload Vulnerability?

When a WordPress website has an Arbitrary File Upload vulnerability, the consequences can be very severe. Arbitrary File Upload can lead to Remote Code Execution (RCE), Privilege Escalation and ultimately to a complete website takeover.

When a website is being taken over by the attackers, they commonly use the website and server resources to host malware, phishing pages, SEO spam and can redirect traffic to scam websites and more.

With any vulnerability that can lead to a full compromise, what can happen to a website is entirely up to the imagination of the attacker. Commonly, websites are also added into a botnet aka a network of backdoored websites in a way that nothing seems to be wrong for the user, which allows the attacker to keep the backdoor on the site and use the website later when they need to.

How Can WordPress Developers Prevent Arbitrary File Upload Vulnerabilities?

To plugin developers, I always recommend checking out if WordPress has hooks for specific usecases. They often provide some WordPress specific security out of the box. In fact, WordPress has its own file uploading functionality so native PHP functions should never be used to handle the upload of a file. Use the wp_handle_upload function to upload files. This function will automatically check the file extension and mime type of the uploaded file.

In case you’re a web developer building sites with WordPress. Ask yourself do you even need to have upload functions at all. Is it absolutely necessary that the users/visitors can upload files to your server?

If you use a plugin that provides this functionality, make sure to check if they have any unfixed security vulnerabilities and set up alerts for future vulnerability fixes. Meanwhile, if possible, make sure to restrict file uploads based on whitelist to only the format you expect the users to upload (such as .pdf).

Conclusion

Arbitrary File Upload vulnerability is a common security issue found in WordPress plugins and themes. It’s a severe vulnerability which is often mass-exploited and leads to a full website takeover.

Consider avoiding plugins that come with file upload capabilities if you don’t need it. If you do – make sure it’s tightly restricted and matches with only your specific usecase.