CVE-2025-3902 is a Cross-Site Scripting (XSS) vulnerability identified in the Drupal Block Class module, specifically affecting version 4.0.0 prior to 4.0.1. The vulnerability arises due to improper neutralization of input during web page generation (CWE-79), which allows malicious actors to inject and execute arbitrary scripts in the context of a victim's browser. This flaw occurs because user-supplied input is not adequately sanitized or encoded before being rendered on web pages, enabling attackers to craft payloads that can bypass input validation mechanisms. The vulnerability is exploitable remotely without requiring authentication (AV:N/PR:N), but it does require user interaction (UI:R), such as a victim clicking a malicious link or visiting a compromised page. The scope is changed (S:C), meaning the vulnerability can affect resources beyond the vulnerable component, potentially impacting the confidentiality and integrity of data accessible through the Drupal instance. The CVSS v3.1 base score is 6.1 (medium severity), reflecting a low impact on confidentiality and integrity, no impact on availability, and relatively low attack complexity. No known exploits are currently reported in the wild, but the presence of this vulnerability in a widely used content management system module poses a risk for web applications relying on Drupal Block Class 4.0.0. Given Drupal's extensive use in European organizations for websites and intranets, this vulnerability could be leveraged for session hijacking, defacement, or phishing attacks if exploited.

For European organizations, the impact of CVE-2025-3902 can be significant, especially for those relying on Drupal-based websites and web applications that utilize the Block Class module version 4.0.0. Successful exploitation could lead to unauthorized script execution in users' browsers, resulting in theft of session cookies, redirection to malicious sites, or unauthorized actions performed on behalf of legitimate users. This undermines the confidentiality and integrity of user data and can damage organizational reputation. Sectors such as government, finance, education, and healthcare, which often use Drupal for public-facing portals and internal tools, are particularly at risk. The vulnerability could facilitate targeted phishing campaigns or social engineering attacks leveraging compromised web content. Although availability is not directly impacted, the indirect consequences include potential regulatory penalties under GDPR if personal data is compromised, and operational disruptions due to incident response efforts. The medium severity rating suggests that while the vulnerability is not critical, it should be addressed promptly to prevent exploitation, especially in high-value or sensitive environments.

To mitigate CVE-2025-3902 effectively, European organizations should: 1) Immediately upgrade the Drupal Block Class module from version 4.0.0 to 4.0.1 or later, where the vulnerability is patched. 2) Implement strict Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of potential XSS payloads. 3) Conduct thorough input validation and output encoding reviews in custom Drupal modules and themes to ensure no other vectors for XSS exist. 4) Employ web application firewalls (WAFs) with rules specifically targeting common XSS attack patterns to provide an additional layer of defense. 5) Educate web administrators and developers on secure coding practices related to input handling and sanitization within Drupal. 6) Monitor web server and application logs for unusual or suspicious requests that may indicate attempted exploitation. 7) Regularly scan Drupal installations with vulnerability assessment tools to detect outdated modules and known vulnerabilities. These steps go beyond generic advice by focusing on module-specific patching, layered defenses, and proactive monitoring tailored to the Drupal ecosystem.