CVE-2025-40631 is a security vulnerability identified in Icewarp Mail Server version 11.4.0, classified under CWE-644: Improper Neutralization of HTTP Headers for Scripting Syntax. This vulnerability arises from insufficient sanitization of the HTTP Host header, allowing an attacker to inject malicious scripting code. Specifically, by manipulating the Host header and embedding a crafted payload, an attacker can cause arbitrary JavaScript code to execute when a user loads a page. However, exploitation requires user interaction, as the victim must click on a malicious link that triggers a redirection to the vulnerable server with the manipulated Host header. The vulnerability is categorized as a low-severity issue with a CVSS 4.0 base score of 2.0, reflecting factors such as attack vector (adjacent network), high attack complexity, no privileges required, and mandatory user interaction. The impact is limited to client-side script execution, which could lead to cross-site scripting (XSS)-like effects, potentially enabling session hijacking, phishing, or other client-targeted attacks. There are no known exploits in the wild at the time of publication, and no patches have been released yet. The vulnerability is specific to Icewarp Mail Server 11.4.0, a product used for email and collaboration services.

For European organizations utilizing Icewarp Mail Server 11.4.0, this vulnerability poses a risk primarily to end-users who interact with maliciously crafted links. Successful exploitation could lead to client-side script execution, enabling attackers to perform actions such as stealing session cookies, conducting phishing attacks, or manipulating user interactions within the mail server's web interface. While the server's core confidentiality and integrity are not directly compromised, the user experience and trustworthiness of the mail service could be degraded. This may result in reputational damage, potential data leakage through user session compromise, and increased risk of targeted social engineering attacks. Given the requirement for user interaction and the high attack complexity, the threat is somewhat mitigated; however, organizations with high user exposure or those lacking robust user awareness training may be more vulnerable. Additionally, organizations in regulated sectors (e.g., finance, healthcare) may face compliance risks if user data is compromised via this vector.

To mitigate this vulnerability, European organizations should: 1) Immediately monitor for updates or patches from Icewarp and apply them as soon as they become available. 2) Implement web application firewall (WAF) rules to detect and block suspicious Host header manipulations and malformed HTTP requests targeting the mail server. 3) Educate users about the risks of clicking on unsolicited or suspicious links, emphasizing caution with links received via email or other communication channels. 4) Conduct regular security assessments and penetration testing focusing on HTTP header injection and client-side scripting vulnerabilities. 5) Where possible, restrict access to the Icewarp Mail Server web interface to trusted networks or VPNs to reduce exposure to adjacent network attackers. 6) Employ Content Security Policy (CSP) headers and other browser-based mitigations to limit the impact of injected scripts. 7) Monitor logs for unusual HTTP Host header values or redirection patterns that could indicate exploitation attempts.