CVE-2025-64203 is a reflected Cross-site Scripting (XSS) vulnerability identified in EverPress Mailster, a popular email marketing plugin. The flaw stems from improper neutralization of user-supplied input during web page generation, which allows attackers to inject malicious JavaScript code into web pages viewed by other users. Specifically, the vulnerability affects all versions of Mailster prior to 4.1.14. An attacker can craft a malicious URL containing the payload, which when clicked by a victim, executes the injected script within the victim's browser context. This can lead to theft of cookies, session tokens, or other sensitive information, as well as unauthorized actions performed on behalf of the victim. The CVSS 3.1 base score of 7.1 reflects a high severity, with an attack vector of network (remote), low attack complexity, no privileges required, but requiring user interaction. The scope is changed, indicating that the vulnerability affects components beyond the initially vulnerable module. Confidentiality, integrity, and availability impacts are all rated low to moderate but combined justify the high severity. No public exploits have been reported yet, but the vulnerability is publicly disclosed and should be considered exploitable. The vulnerability is particularly concerning for organizations relying on Mailster for customer communications, as successful exploitation could lead to phishing, credential theft, or defacement. The lack of official patches at the time of disclosure necessitates immediate attention to mitigation strategies.

For European organizations, the impact of CVE-2025-64203 can be significant, especially for those using EverPress Mailster in their marketing or customer engagement platforms. Exploitation can lead to unauthorized access to user sessions, theft of sensitive customer data, and potential compromise of internal networks if attackers leverage stolen credentials. This could result in reputational damage, regulatory penalties under GDPR due to data breaches, and operational disruptions. The reflected XSS nature means that attackers must trick users into clicking malicious links, which can be facilitated through phishing campaigns targeting employees or customers. Given the widespread use of email marketing tools in Europe, especially in sectors like retail, finance, and telecommunications, the vulnerability poses a risk to both the confidentiality and integrity of communications. Additionally, the scope change in the CVSS vector suggests that the vulnerability could affect multiple components or services, increasing the potential attack surface. Organizations failing to address this vulnerability promptly may face increased risk of targeted attacks, data leakage, and compliance violations.

1. Update Mailster to version 4.1.14 or later immediately once the patch is available to ensure the vulnerability is fully remediated. 2. Until patching is possible, implement Web Application Firewall (WAF) rules to detect and block typical XSS attack patterns targeting Mailster endpoints. 3. Employ strict input validation and output encoding on all user-supplied data rendered in web pages to prevent script injection. 4. Configure Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of potential XSS payloads. 5. Educate users and employees about phishing risks and the dangers of clicking suspicious links, as exploitation requires user interaction. 6. Monitor logs and network traffic for unusual activity or attempts to exploit the vulnerability. 7. Conduct regular security assessments and penetration testing focused on web application vulnerabilities, including XSS. 8. Review and limit the exposure of Mailster interfaces to only necessary users and networks, employing network segmentation where feasible.