CVE-2025-11876 identifies a stored Cross-Site Scripting vulnerability in the Mailgun Subscriptions plugin for WordPress, specifically affecting all versions up to and including 1.3.1. The vulnerability arises from improper neutralization of user-supplied input in the 'mailgun_subscription_form' shortcode, where insufficient sanitization and output escaping allow authenticated users with contributor-level permissions or higher to inject arbitrary JavaScript code. This malicious code is stored persistently and executed in the browsers of any users who visit the compromised pages, leading to potential session hijacking, credential theft, or other malicious activities. The attack vector requires authenticated access but no user interaction beyond page viewing. The CVSS 3.1 score of 6.4 reflects a medium severity, with a network attack vector, low attack complexity, and privileges required at the contributor level. The scope is changed (S:C) because the vulnerability affects resources beyond the attacker’s privileges, potentially impacting all users visiting the infected page. No patches or known exploits are currently available, but the vulnerability represents a significant risk to WordPress sites using this plugin, especially those with multiple contributors. The vulnerability is classified under CWE-79, indicating a classic stored XSS flaw. The lack of output escaping on user attributes in shortcode processing is the root cause, highlighting the need for secure coding practices in WordPress plugin development.

For European organizations, this vulnerability poses a moderate risk primarily to websites running WordPress with the Mailgun Subscriptions plugin installed. Exploitation could lead to unauthorized script execution in users’ browsers, enabling theft of session cookies, user impersonation, and potentially unauthorized access to sensitive information. This can damage organizational reputation, lead to data breaches, and violate GDPR requirements concerning personal data protection. Organizations with multiple contributors or editors on their WordPress sites are particularly vulnerable since the exploit requires contributor-level access. The vulnerability could be leveraged to target customers or employees visiting the compromised site, potentially facilitating phishing or further malware delivery. Although no known exploits exist yet, the medium CVSS score and the widespread use of WordPress in Europe make timely mitigation important. The impact on availability is minimal, but confidentiality and integrity of user data and sessions are at risk.

European organizations should immediately audit their WordPress installations to identify the presence of the Mailgun Subscriptions plugin and verify the version in use. Until an official patch is released, restrict contributor-level access to trusted users only and review user roles to minimize the number of users with permissions to inject shortcode attributes. Implement Web Application Firewall (WAF) rules to detect and block suspicious shortcode inputs or script tags in form submissions. Monitor website content for unexpected script injections, especially in pages using the 'mailgun_subscription_form' shortcode. Educate content contributors about the risks of injecting untrusted content and enforce strict content review processes. Once a patch is available, apply it promptly. Additionally, consider employing Content Security Policy (CSP) headers to limit the impact of any injected scripts. Regularly back up website data and monitor logs for unusual activity related to shortcode usage or contributor actions.