CVE-2025-3929 is a medium-severity Cross-site Scripting (XSS) vulnerability identified in MDaemon Email Server versions 25.0.1 and below. The vulnerability arises from improper neutralization of input during web page generation (CWE-79). Specifically, an attacker can craft a malicious HTML email containing JavaScript embedded within an img tag. When a webmail user views this email through the MDaemon webmail interface, the malicious script executes in the context of the user's browser session. This can lead to unauthorized access to user data, session hijacking, or other malicious actions that leverage the victim's browser privileges. The vulnerability does not require any authentication or privileges on the server, and exploitation only requires user interaction in the form of opening or previewing the crafted email. The CVSS 4.0 base score is 5.3, reflecting network attack vector, low attack complexity, no privileges required, but requiring user interaction and resulting in low confidentiality and integrity impact. No known exploits are currently reported in the wild, and no patches or vendor mitigation links are provided yet. The vulnerability affects the webmail component of MDaemon Email Server, which is used by organizations for email communication and management.

For European organizations, exploitation of this XSS vulnerability could lead to unauthorized disclosure of sensitive email content, session hijacking, and potentially further compromise of user accounts within the MDaemon webmail environment. This can disrupt business communications, lead to data leakage, and facilitate phishing or lateral movement within networks. Organizations relying on MDaemon Email Server for critical communications, especially those handling sensitive or regulated data (e.g., finance, healthcare, government), face increased risk of data breaches and compliance violations under GDPR. The impact is heightened in sectors where email is a primary communication tool and where webmail access is common. While the vulnerability does not directly compromise server availability or integrity, the ability to execute arbitrary JavaScript in users' browsers can be leveraged for targeted attacks, credential theft, or spreading malware. Given the medium CVSS score and the requirement for user interaction, the threat is moderate but significant enough to warrant prompt attention.

1. Immediate mitigation should include educating users to avoid opening suspicious or unexpected emails, especially those containing HTML content with images. 2. Disable HTML rendering or image loading in the webmail client if possible, or configure the webmail interface to sanitize or block potentially malicious HTML tags and attributes. 3. Implement Content Security Policy (CSP) headers on the webmail server to restrict execution of inline scripts and limit script sources. 4. Monitor email traffic for suspicious messages containing unusual HTML or embedded scripts. 5. Apply network-level protections such as web application firewalls (WAF) configured to detect and block XSS payloads targeting the MDaemon webmail interface. 6. Engage with the vendor for patches or updates addressing this vulnerability and plan for timely deployment once available. 7. Review and tighten email filtering rules to quarantine or block emails with suspicious HTML content. 8. Conduct regular security awareness training emphasizing risks of interacting with untrusted email content. These steps go beyond generic advice by focusing on configuration changes, user behavior, and layered defenses specific to the MDaemon webmail environment.