CVE-2025-58430 is a high-severity vulnerability affecting listmonk, a standalone, self-hosted newsletter and mailing list manager, in versions up to and including 1.1.0. The core issue involves improper neutralization of script-related HTML tags (CWE-80) leading to a basic Cross-Site Scripting (XSS) vulnerability, compounded by Cross-Site Request Forgery (CSRF) weaknesses (CWE-352). Specifically, every HTTP request to listmonk includes a session cookie named 'session' and an additional parameter 'nonce'. However, the backend does not validate or check the 'nonce' value, and removing it allows requests to be processed correctly. This behavior may appear benign but can be exploited when chained with other vulnerabilities. The combination of XSS and CSRF can enable attackers to perform unauthorized actions such as improper admin account creation. This chain attack could allow an attacker to inject malicious scripts that execute in the context of an authenticated administrator's browser, bypassing authentication and authorization controls. As of the publication date, no patches or fixed versions are available, increasing the risk for users of affected versions. The CVSS 4.0 score of 8.6 reflects the high impact on confidentiality, integrity, and availability, with network attack vector, low attack complexity, no privileges or authentication required, but user interaction needed. The vulnerability does not require prior authentication, making it accessible to remote attackers. The lack of validation on the 'nonce' parameter and the presence of XSS and CSRF vulnerabilities together create a critical attack surface in listmonk installations.

For European organizations using listmonk for managing newsletters and mailing lists, this vulnerability poses significant risks. Exploitation could lead to unauthorized administrative access, allowing attackers to manipulate mailing lists, send phishing or malware-laden emails to subscribers, or exfiltrate sensitive subscriber data. This could damage organizational reputation, violate GDPR regulations due to data breaches, and result in financial penalties. The ability to create admin accounts improperly undermines trust in the system's integrity and could facilitate further lateral movement within organizational networks. Since listmonk is self-hosted, organizations with less mature patch management or security monitoring may be particularly vulnerable. The impact extends beyond confidentiality to integrity and availability of communication channels, potentially disrupting marketing, customer engagement, or internal communications. Given the lack of a patch, organizations must assume active risk and prepare mitigations accordingly.

1. Immediate mitigation should include disabling or restricting access to listmonk instances from untrusted networks to reduce exposure. 2. Implement Web Application Firewall (WAF) rules to detect and block common XSS and CSRF attack patterns targeting listmonk endpoints, particularly filtering requests with suspicious 'nonce' parameter manipulations. 3. Enforce strict Content Security Policy (CSP) headers to limit script execution and reduce XSS impact. 4. Conduct thorough input validation and output encoding on all user-supplied data within listmonk interfaces, especially in administrative functions. 5. Monitor logs for unusual administrative account creation or suspicious HTTP requests containing manipulated 'nonce' parameters. 6. Where possible, isolate listmonk instances in segmented network zones with limited access. 7. Prepare for rapid patch deployment once vendor releases a fix; consider contributing to or monitoring vendor communications for updates. 8. Educate administrators and users about phishing risks that could leverage this vulnerability. 9. Consider temporary alternative mailing list management solutions if risk is unacceptable until a patch is available.