CVE-2025-64203 is a reflected cross-site scripting (XSS) vulnerability identified in EverPress Mailster, a popular email marketing plugin. The vulnerability stems from improper neutralization of user-supplied input during the generation of web pages, allowing malicious scripts to be injected and executed in the context of a victim's browser. Specifically, versions of Mailster prior to 4.1.14 are affected, with no lower bound version specified. Reflected XSS typically occurs when input from HTTP requests is immediately included in the response without adequate sanitization or encoding. An attacker can exploit this by crafting a malicious URL or form that, when visited by a user, executes arbitrary JavaScript code. This can lead to session hijacking, theft of cookies or credentials, defacement, or unauthorized actions performed on behalf of the user. The vulnerability does not require authentication, increasing its risk profile, and does not require user interaction beyond clicking a malicious link or visiting a compromised site. Although no active exploits have been reported in the wild, the vulnerability is publicly disclosed and patched in Mailster version 4.1.14. The lack of a CVSS score necessitates an independent severity assessment. Given the potential for significant confidentiality and integrity impacts, ease of exploitation, and broad scope of affected systems, this vulnerability is considered high severity. The absence of known exploits provides a window for proactive mitigation. European organizations relying on Mailster for email campaigns or customer communications should prioritize patching and review input handling and output encoding practices. Additional defenses such as Content Security Policy (CSP) headers can mitigate the impact of potential XSS attacks.

For European organizations, exploitation of this reflected XSS vulnerability could lead to unauthorized access to user sessions, theft of sensitive information such as authentication tokens or personal data, and manipulation of email marketing content. This can result in reputational damage, regulatory non-compliance (e.g., GDPR violations due to data leakage), and financial losses from phishing or fraud. Organizations using Mailster to manage large subscriber bases or sensitive customer data are at higher risk. Attackers could leverage this vulnerability to conduct targeted phishing campaigns or spread malware through compromised email content. The impact extends beyond the immediate Mailster environment as compromised user sessions could lead to broader network access or lateral movement. The lack of authentication requirement and ease of exploitation increase the likelihood of successful attacks, especially if users are tricked into clicking malicious links. The vulnerability also undermines trust in digital communications, which is critical for European businesses operating in competitive markets.

1. Immediately update EverPress Mailster to version 4.1.14 or later, where the vulnerability is patched. 2. Implement strict input validation and output encoding on all user-supplied data to prevent script injection. 3. Deploy Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts in browsers. 4. Educate users and administrators about the risks of clicking untrusted links and encourage cautious behavior. 5. Monitor web server logs and application behavior for suspicious requests that may indicate attempted exploitation. 6. Conduct regular security audits and penetration testing focused on web application vulnerabilities, including XSS. 7. Use web application firewalls (WAFs) configured to detect and block XSS attack patterns targeting Mailster endpoints. 8. Review and harden email templates and subscriber input forms to minimize injection vectors. 9. Ensure incident response plans include procedures for handling XSS incidents and potential data breaches. 10. Coordinate with EverPress support and security advisories for ongoing updates and best practices.