CVE-2025-11876 is a stored cross-site scripting vulnerability identified in the Mailgun Subscriptions plugin for WordPress, affecting all versions up to and including 1.3.1. The vulnerability arises from improper neutralization of user-supplied input within the 'mailgun_subscription_form' shortcode, where insufficient sanitization and output escaping allow authenticated users with contributor-level privileges or higher to inject arbitrary JavaScript code into pages. This malicious code is stored persistently and executed whenever any user accesses the compromised page, enabling potential attacks such as session hijacking, privilege escalation, or defacement. The vulnerability requires authentication but no user interaction, and the attack vector is network-based with low complexity. The CVSS 3.1 score of 6.4 reflects a medium severity impact, with partial confidentiality and integrity impacts but no availability impact. No public exploits are currently known, but the vulnerability poses a risk to WordPress sites using this plugin, especially those with multiple contributors. The issue stems from a failure to properly sanitize shortcode attributes before rendering them in the page output, violating secure coding practices related to CWE-79 (Improper Neutralization of Input During Web Page Generation).

For European organizations, this vulnerability could lead to unauthorized script execution on websites, potentially compromising user sessions, stealing credentials, or defacing web content. Organizations relying on WordPress sites with the Mailgun Subscriptions plugin, especially those allowing contributor-level access to multiple users, face increased risk. The confidentiality of user data and integrity of web content could be compromised, damaging organizational reputation and trust. While availability is not directly impacted, successful exploitation could facilitate further attacks or malware distribution. Given the widespread use of WordPress in Europe, especially in sectors like media, education, and e-commerce, the vulnerability could affect a broad range of organizations. Regulatory compliance under GDPR may also be impacted if personal data is exposed or manipulated via this vulnerability.

European organizations should immediately audit their WordPress installations to identify the presence of the Mailgun Subscriptions plugin and verify the version in use. Since no official patch links are currently available, organizations should consider the following mitigations: restrict contributor-level access strictly to trusted users, implement additional input validation and output escaping via custom code or security plugins, and monitor web logs for unusual activity or script injection attempts. Employing a Web Application Firewall (WAF) with rules targeting XSS payloads can help block exploitation attempts. Regularly update the plugin once a patch is released and apply WordPress core and plugin updates promptly. Additionally, educate content contributors about secure content practices and limit the use of shortcodes that accept user input. Conduct periodic security assessments and penetration tests focusing on XSS vulnerabilities in WordPress environments.