CVE-2024-8850 is a reflected Cross-Site Scripting (XSS) vulnerability identified in the MC4WP: Mailchimp for WordPress plugin, specifically affecting versions 4.9.9 through 4.9.16. The vulnerability stems from improper neutralization of input during web page generation, classified under CWE-79. It occurs when the plugin processes the 'email' parameter, particularly when placeholders like {email} are used in form fields. Due to insufficient input sanitization and lack of proper output escaping, an attacker can inject malicious JavaScript code into URLs. When a victim clicks on such a crafted link, the injected script executes in the context of the vulnerable website, potentially allowing the attacker to steal session cookies, perform actions on behalf of the user, or redirect users to malicious sites. The vulnerability is exploitable remotely without authentication but requires user interaction (clicking a malicious link). The CVSS v3.1 score is 6.1, indicating medium severity, with attack vector network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), user interaction required (UI:R), scope changed (S:C), and impacts on confidentiality and integrity (C:L/I:L) but no impact on availability (A:N). No public exploits have been reported yet. The vulnerability was published on September 19, 2024, and assigned by Wordfence. The affected plugin is widely used in WordPress sites to integrate Mailchimp services, making this a relevant threat to many websites relying on this plugin for email marketing integration.

The primary impact of CVE-2024-8850 is on the confidentiality and integrity of user data and website sessions. Successful exploitation can lead to theft of sensitive information such as cookies or authentication tokens, enabling session hijacking or impersonation of legitimate users. Attackers may also manipulate webpage content or redirect users to malicious websites, facilitating phishing or malware distribution campaigns. Although availability is not directly affected, the reputational damage and potential data breaches can have significant business consequences. Organizations running vulnerable versions of the MC4WP plugin expose their users to these risks, potentially leading to loss of customer trust, regulatory penalties (especially under privacy laws like GDPR), and increased incident response costs. Since the vulnerability requires user interaction, the risk is somewhat mitigated but remains significant due to the widespread use of WordPress and this plugin in particular.

1. Immediate update: Organizations should upgrade the MC4WP: Mailchimp for WordPress plugin to the latest version where this vulnerability is patched. If no patch is currently available, monitor vendor advisories closely. 2. Input validation and output encoding: Developers and site administrators should ensure that any customizations involving the 'email' parameter or placeholders like {email} implement strict input validation and proper output escaping to prevent script injection. 3. Web Application Firewall (WAF): Deploy or update WAF rules to detect and block malicious payloads targeting the 'email' parameter in URLs, especially those containing suspicious script tags or encoded payloads. 4. User awareness: Educate users and administrators about the risks of clicking on suspicious links, particularly those that appear to come from the website but contain unusual query parameters. 5. Content Security Policy (CSP): Implement a robust CSP to restrict the execution of unauthorized scripts on the website, mitigating the impact of reflected XSS attacks. 6. Monitor logs: Regularly review web server and application logs for unusual URL patterns or repeated attempts to exploit this vulnerability. 7. Disable or restrict the use of placeholders like {email} in forms if not essential, or sanitize them rigorously.