Understanding and Mitigating XSS Vulnerabilities in WordPress Themes: A Deep Dive into CVE-2021-24316

What is XSS?

XSS (Cross-Site Scripting) is a common vulnerability in web applications. It occurs when user-controlled inputs are not properly sanitized, allowing the insertion of malicious code.

Without proper handling, an attacker can inject HTML tags, malicious scripts, or harmful HTML content, such as forms designed to capture credentials. These codes, reflected in the page's source code, enable harmful actions against both users and the company, such as:

• Obtaining users' session cookies from the website;

• Redirecting users to an arbitrary site, often a phishing site;

• Inserting forms to capture passwords.

An Introduction to WordPress

WordPress is a popular open-source CMS that simplifies the creation of websites such as blogs, forums, institutional sites, e-commerce platforms, and more. It streamlines web development, allowing users without technical experience to build and customize websites. With numerous plugins, themes, and features (often developed by third parties), WordPress becomes accessible to various user profiles, enabling easy implementation of modifications and functionalities.

CVE-2021-24316

As mentioned earlier, one of the features that simplify website development in WordPress is the theme system it offers. However, since many themes are created and managed by third parties, they can contain vulnerabilities, such as CVE-2021-24316.

This vulnerability is an XSS found in a theme called Mediumish, where a URL parameter (the "s" parameter) is not properly sanitized and is directly reflected on the page, exposing all websites using this theme to the risks of XSS.

This theme remains vulnerable in its most recent version (1.0.60), as of the date of this article (September 18, 2024).

PoC

Exploiting this vulnerability is relatively simple. All we need to do is inject an XSS payload into the aforementioned parameter.

The most common example would be: <script>alert("XSS PoC")</script>

By inserting it into the "s" parameter, which is responsible for content searches on a given site:

Mitigations and Best Practices

Now that we have discussed the exploitation scenario, it is important to understand how to protect ourselves in this case and what practices we need to adopt to enhance the security of our applications, especially when using WordPress for our site. Adopt measures such as:

• Keep WordPress plugins and themes updated, prioritizing versions that fix security flaws. Consider replacing vulnerable features that lack available patches;

• Consider using a WAF, such as CloudFlare’s, to reduce the chances of vulnerability exploitation, significantly increasing your site's security;

• Always research the theme or plugin you plan to install to ensure you are not installing something vulnerable or unreliable;

And finally, if you control the inputs:

• Always sanitize them, both what you allow a user to input and how the application returns it;

To reduce the impact of XSS exploitation:

• Always ensure cookies sent through your domain have the "HttpOnly" flag enabled. This prevents attackers from retrieving them, as they will not be accessible from the front-end;

• Implement CSP (Content Security Policy) headers on your site to ensure that scripts loaded on your site can only be loaded from a specific source (for example, a specific domain). This prevents injected scripts from running from unknown sources.