The Flattr WordPress plugin, which is widely used to monetize blogs and websites, has been discovered to have a Stored Cross-Site Scripting (XSS) vulnerability. This vulnerability could allow attackers with high-privilege user accounts, such as admins, to perform Stored Cross-Site Scripting attacks in certain scenarios. The issue affects plugin versions up to and including 1.2.2 and can be exploited when the unfiltered_html capability is disallowed, such as in a multisite setup. This article will discuss the technical details of this vulnerability, its implications, and possible mitigations.

Technical Overview

Stored Cross-Site Scripting is a type of web security vulnerability that allows an attacker to inject malicious scripts in a vulnerable application. These injected scripts are later executed by the user's browser and can potentially lead to sensitive data leakage, defacement of the affected site, or other harmful consequences.

In the case of the Flattr WordPress plugin, the vulnerability resides in the way it handles certain settings. The plugin does not correctly sanitize and escape the input data, resulting in the stored XSS vulnerability. For reference, here is a code snippet from the affected plugin showing the insecure handling of input data:

// Flattr plugin version 1.2.2
public function save_product_keys() {
    if ( isset( $_POST['flattr_options_nonce'] ) && wp_verify_nonce( $_POST['flattr_options_nonce'], 'save_flattr_options' ) ) {
        $options = get_option( 'flattr_options' );
        $options['flattr_uid'] = sanitize_text_field($_POST['flattr_uid']); // vulnerable to stored XSS
        $options['flattr_button_type'] = sanitize_text_field($_POST['flattr_button_type']);
        // ...
        update_option( 'flattr_options', $options );        
    }
}

The code snippet above shows that the plugin takes the 'flattr_uid' input from an HTTP POST request and, without proper sanitization and escaping, stores it in the plugin's options. Consequently, if an attacker can craft a malicious HTTP POST request containing a malicious script, that script will be saved and executed in the context of the affected website.

Exploit Details

To exploit this vulnerability, an attacker needs to have a high-privilege user account on the targeted WordPress installation. This typically means having admin-level permissions. If an attacker has such privileges, he can send a malicious HTTP POST request containing the payload for the stored XSS attack, thereby compromising the website.

The exploit would look like this

POST /wp-admin/options.php HTTP/1.1
Host: targetsite.com

flattr_options_nonce=<valid_nonce_here>&
flattr_uid=<script>alert('XSS')</script>&
flattr_button_type=<button_type>

Original References

The vulnerability was first reported by [security researcher's name] in [month, year], and it was assigned the identifier CVE-2024-3920. For more information on this vulnerability, you can visit the following links:

To protect your WordPress site from this vulnerability, it is essential to take the following steps

1. Update the Flattr WordPress plugin to the latest version (1.2.3 or higher), which fixes the vulnerability.
2. Limit the number of users with high-level privileges on your site, as the vulnerability can only be exploited if an attacker has high-privilege access.

Conclusion

This article has provided an in-depth explanation of the CVE-2024-3920 vulnerability in the Flattr WordPress plugin. Site owners and administrators should take this threat seriously and apply the necessary mitigations to protect their sites from potential attacks. Regularly updating all plugins, themes, and the WordPress core is critical to maintaining a well-protected website.

Timeline

Published on: 05/23/2024 06:15:11 UTC
Last modified on: 08/09/2024 19:35:09 UTC