CVE-2025-40631: Icewarp Mail Server XSS Vulnerability

CVE-2025-40631 Overview

CVE-2025-40631 is an HTTP host header injection vulnerability affecting Icewarp Mail Server version 11.4.0. Attackers can modify the Host header and inject a payload that causes arbitrary JavaScript to execute when a victim loads the page. Exploitation requires user interaction — the victim must click a crafted link that triggers the redirect. The flaw is tracked under CWE-644: Improper Neutralization of HTTP Headers for Scripting Syntax.

Critical Impact
Successful exploitation enables client-side script execution in the context of the Icewarp Mail Server web interface, which can lead to session theft, phishing, or unauthorized actions on behalf of authenticated users.

Affected Products

Icewarp Mail Server 11.4.0
Vendor: icewarp
Component: icewarp:mail_server

Discovery Timeline

2025-05-16 - CVE-2025-40631 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-40631

Vulnerability Analysis

CVE-2025-40631 stems from improper neutralization of the HTTP Host header inside the Icewarp Mail Server 11.4.0 web application. The server reflects the value of the client-supplied Host header into generated responses without sanitization. An attacker who controls the header can inject JavaScript that executes when the browser renders the response.

Because the injection occurs in a location that influences URL generation or client-side rendering, exploitation typically involves a redirect flow. The victim clicks a link that reaches the vulnerable server with an attacker-controlled Host value, and the returned page executes the injected script on load. According to the CVSS 4.0 vector, the attack requires adjacent network access, high attack complexity, and user interaction, which limits the vulnerability's reach.

Root Cause

The root cause is trust in the incoming Host header. Web applications should treat this header as untrusted input, validate it against an allow-list of expected hostnames, and encode any reflected value. Icewarp Mail Server 11.4.0 fails to enforce these controls, allowing script content injected via the header to reach the client unescaped.

Attack Vector

Exploitation follows a client-side injection pattern:

The attacker crafts a URL or intermediary that forces the victim's browser or a proxy to send an HTTP request to the Icewarp server carrying a malicious Host header containing JavaScript syntax.
The victim, on an adjacent network, is lured into interacting with a malicious link.
The Icewarp server reflects the header value into the response body or a redirect target.
The browser parses the response and executes the injected script in the security context of the mail server origin.

For further technical context, see the INCIBE Security Notice for multiple vulnerabilities in Icewarp Mail Server. No public proof-of-concept or exploit code is currently available.

Detection Methods for CVE-2025-40631

Indicators of Compromise

HTTP requests to Icewarp Mail Server containing Host headers with non-standard values, script tags, or JavaScript event handlers such as onerror= or javascript:.
Web server access logs showing unusually long or malformed Host header values from external or adjacent-network sources.
Browser-side script errors or unexpected redirects reported by users accessing the mail web interface.

Detection Strategies

Enable verbose HTTP request logging on the Icewarp web front-end and inspect the Host header field for characters such as <, >, ", and '.
Deploy a Web Application Firewall (WAF) rule that rejects requests whose Host header does not match the canonical FQDN of the mail server.
Correlate web access logs with proxy logs to identify user-agents following crafted external links into the mail interface.

Monitoring Recommendations

Alert on any HTTP request to the Icewarp Mail Server with a Host header that differs from the configured production hostname.
Track outbound clicks from webmail sessions to unfamiliar external domains that could indicate phishing lures targeting this flaw.
Monitor for anomalous session behavior such as sudden mailbox rule changes, forwarding additions, or authentication events shortly after user link interaction.

How to Mitigate CVE-2025-40631

Immediate Actions Required

Inventory all Icewarp Mail Server instances and identify systems running version 11.4.0.
Restrict access to the Icewarp web interface to trusted networks until a vendor patch is applied.
Communicate awareness to end users about not clicking unverified links that reference the mail server.

Patch Information

At the time of publication, no vendor advisory URL is listed in the enriched data. Administrators should consult Icewarp directly for a fixed release addressing CVE-2025-40631 and monitor the INCIBE Security Notice for coordinated disclosure updates.

Workarounds

Configure the reverse proxy or load balancer in front of Icewarp Mail Server to validate the Host header against an allow-list of approved hostnames and drop non-matching requests.
Enforce a strict Content Security Policy (CSP) on the mail web interface to limit inline script execution.
Deploy WAF signatures that block script syntax such as <script, onerror=, and javascript: inside HTTP header fields.
Educate users to hover over links and verify the destination hostname before clicking, particularly for mail server URLs.