CVE-2025-40630: Icewarp Mail Server Open Redirect Flaw

CVE-2025-40630 Overview

CVE-2025-40630 is an open redirection vulnerability [CWE-601] in IceWarp Mail Server version 11.4.0. Attackers can craft a malicious URL that appears to point to a trusted IceWarp domain but redirects victims to an attacker-controlled site. The flaw abuses URL path handling combined with encoded traversal sequences such as %2e%2e to bypass origin validation. Researchers confirmed the behavior in Firefox during testing. Threat actors typically weaponize open redirects for phishing, credential harvesting, and malware distribution campaigns that rely on trusted-domain reputation.

Critical Impact
Attackers can redirect authenticated IceWarp users to arbitrary malicious domains through crafted URLs, enabling phishing and credential theft campaigns that leverage the trusted mail server hostname.

Affected Products

IceWarp Mail Server 11.4.0
Deployments exposing the IceWarp web interface to untrusted networks
Environments where IceWarp URLs are shared via email or messaging

Discovery Timeline

2025-05-16 - CVE-2025-40630 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-40630

Vulnerability Analysis

The vulnerability resides in the URL parsing logic of the IceWarp Mail Server web interface. The server accepts URLs where the path begins with multiple forward slashes followed by an external hostname and encoded parent-directory traversal sequences. A representative payload takes the form https://icewarp.domain.com//<MALICIOUS_DOMAIN>/%2e%2e. When the browser processes the response, it treats the injected host as the redirect target rather than a path segment on the IceWarp origin. The double-slash pattern combined with %2e%2e decoding causes the browser to interpret the second segment as an authority component. This behavior was verified in Firefox, though other browsers may exhibit similar parsing quirks.

Root Cause

The root cause is insufficient validation of redirect targets and path normalization inconsistencies between the server and the client. IceWarp does not enforce a strict allowlist of internal redirect destinations. The application also fails to canonicalize path segments before issuing redirects, permitting encoded traversal characters to alter the effective URL authority when rendered by the browser.

Attack Vector

Exploitation requires user interaction. An attacker sends the crafted URL through email, chat, or social engineering, using the legitimate IceWarp hostname to build trust. Once clicked, the victim's browser navigates to the attacker-controlled destination, which typically hosts a phishing page mimicking the IceWarp login. Because the initial hostname is genuine, security-aware users and mail gateway URL scanners may fail to flag the link. The attack does not require authentication on the IceWarp server.

Detection Methods for CVE-2025-40630

Indicators of Compromise

Web server access logs containing requests with double-slash prefixes followed by external domains, for example //attacker.example/%2e%2e
Outbound HTTP referrer headers from IceWarp URLs pointing to non-corporate domains
Phishing reports from users describing IceWarp-branded links that led to unexpected login pages

Detection Strategies

Deploy web application firewall (WAF) rules that block requests containing // in the path followed by external hostname characters
Inspect email gateway logs for outbound URLs matching the IceWarp hostname combined with encoded traversal sequences such as %2e%2e
Correlate proxy telemetry for redirects originating from the IceWarp domain that terminate on unknown external hosts

Monitoring Recommendations

Enable verbose logging on the IceWarp web front end and forward events to a centralized SIEM for URL pattern analysis
Monitor DNS queries and HTTP referrer chains that begin at the IceWarp mail domain and pivot to newly registered domains
Track user-reported phishing incidents that reference the mail server hostname for anomalous click-through patterns

How to Mitigate CVE-2025-40630

Immediate Actions Required

Apply the vendor patch or upgrade to a fixed IceWarp Mail Server release once available from the vendor
Deploy WAF or reverse-proxy rules that reject request paths containing consecutive slashes followed by external authority components
Notify users about phishing campaigns that may abuse the IceWarp domain and reinforce URL inspection guidance

Patch Information

Refer to the INCIBE Security Notice for advisory details covering multiple IceWarp Mail Server vulnerabilities, including CVE-2025-40630. Coordinate with IceWarp support to obtain the fixed build for version 11.4.0 deployments.

Workarounds

Configure a reverse proxy in front of IceWarp to normalize URL paths and strip consecutive forward slashes before forwarding requests
Add server-side validation that restricts redirect targets to an allowlist of trusted internal hostnames
Enforce browser security headers such as Content-Security-Policy with a restrictive frame-ancestors and form-action directive to limit redirect abuse

bash

# Example nginx normalization rule to reject double-slash paths
location / {
    if ($request_uri ~* "^//") {
        return 400;
    }
    if ($request_uri ~* "%2e%2e") {
        return 400;
    }
    proxy_pass http://icewarp_backend;
}