CVE-2025-66376: Zimbra Collaboration Suite XSS Vulnerability

CVE-2025-66376 Overview

A stored cross-site scripting (XSS) vulnerability exists in Zimbra Collaboration Suite (ZCS) versions 10 before 10.0.18 and 10.1 before 10.1.13. The vulnerability allows attackers to inject malicious JavaScript through Cascading Style Sheets (CSS) @import directives embedded within HTML email messages when viewed through the Classic UI interface. This attack vector enables persistent script execution in the context of authenticated user sessions.

Critical Impact

This vulnerability is actively exploited in the wild and has been added to the CISA Known Exploited Vulnerabilities (KEV) catalog. Attackers can steal session cookies, capture credentials, and perform actions on behalf of compromised users within the Zimbra webmail interface.

Affected Products

• Synacor Zimbra Collaboration Suite versions 10.0.x before 10.0.18
• Synacor Zimbra Collaboration Suite versions 10.1.x before 10.1.13
• Zimbra Classic UI webmail interface

Discovery Timeline

• 2026-01-05 - CVE-2025-66376 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2025-66376

Vulnerability Analysis

The vulnerability resides in the Zimbra Classic UI's handling of HTML email content, specifically in how CSS stylesheets are processed. When an email containing malicious CSS @import directives is rendered, the Classic UI fails to properly sanitize these directives, allowing attackers to inject and execute arbitrary JavaScript code in the victim's browser session. This is a stored XSS vulnerability (CWE-79), meaning the malicious payload persists in the email message and executes each time a user views the compromised email.

The attack is particularly dangerous because it requires no privileges from the attacker—only the ability to send an email to the target. User interaction is required (the victim must open the malicious email), but the exploitation occurs transparently without any visual indication to the user.

Root Cause

The root cause is insufficient input validation and output encoding when processing CSS @import directives within HTML email content. The Classic UI's email rendering engine does not adequately filter or escape potentially dangerous CSS constructs before inserting them into the DOM. This allows CSS-based injection vectors to bypass existing XSS protections that may be in place for more common attack vectors like <script> tags or inline event handlers.

Attack Vector

The attack vector is network-based, requiring an attacker to craft and send a specially formatted HTML email to a target Zimbra user. The malicious email contains CSS @import directives that reference attacker-controlled resources or embed JavaScript execution payloads. When the victim opens the email in the Classic UI, the browser processes the CSS, triggering the XSS payload.

The vulnerability can be exploited to:

• Steal session tokens and authentication cookies
• Capture user credentials through fake login forms
• Read sensitive email content
• Send emails on behalf of the compromised user
• Redirect users to malicious websites

Due to the nature of stored XSS, the attack payload persists and can affect multiple users if the email is forwarded or viewed by additional recipients.

Detection Methods for CVE-2025-66376

Indicators of Compromise

• Emails containing unusual CSS @import directives pointing to external domains
• HTTP requests from Zimbra web servers to unexpected external URLs during email rendering
• JavaScript execution errors in browser console when viewing specific emails
• Unusual cookie or session token exfiltration attempts in network traffic
• Reports from users about unexpected behavior when viewing certain emails

Detection Strategies

• Implement email content scanning rules to detect CSS @import directives with suspicious URLs or JavaScript-like content
• Monitor web application firewall logs for XSS attack signatures in email-related requests
• Deploy browser-based security tools that detect and block DOM-based XSS execution
• Review Zimbra audit logs for anomalous email viewing patterns or session activity
• Configure network monitoring to detect data exfiltration attempts from the webmail interface

Monitoring Recommendations

• Enable detailed logging for the Zimbra Classic UI component
• Monitor for sudden changes in user session behavior or geographic login locations
• Set up alerts for emails containing CSS directives from untrusted sources
• Review web server access logs for requests to suspicious external resources during email rendering

How to Mitigate CVE-2025-66376

Immediate Actions Required

• Upgrade Zimbra Collaboration Suite to version 10.0.18 or later (for 10.0.x installations)
• Upgrade Zimbra Collaboration Suite to version 10.1.13 or later (for 10.1.x installations)
• Consider temporarily disabling the Classic UI and directing users to the Modern UI if immediate patching is not possible
• Implement email content filtering to block messages with suspicious CSS constructs
• Review email quarantine and spam filters to catch potential exploit attempts

Patch Information

Synacor has released security patches addressing this vulnerability in Zimbra Collaboration Suite versions 10.0.18 and 10.1.13. Organizations should prioritize applying these updates immediately given the active exploitation of this vulnerability. Detailed patch information is available in the Zimbra 10.0.18 Security Fixes and Zimbra 10.1.13 Security Fixes release notes. Additional security guidance can be found at the Zimbra Security Center.

Workarounds

• Disable the Classic UI and enforce use of the Modern UI for all users until patching is complete
• Implement Content Security Policy (CSP) headers to restrict CSS and JavaScript execution sources
• Deploy web application firewall rules to filter incoming emails with CSS @import directives
• Enable additional email content scanning at the gateway level to detect and quarantine suspicious HTML emails
• Educate users to report any suspicious emails or unexpected behavior when viewing messages

# Configuration example - Enable CSP headers in Zimbra (nginx proxy)
# Add to /opt/zimbra/conf/nginx/includes/nginx.conf.web.https.default
add_header Content-Security-Policy "default-src 'self'; style-src 'self' 'unsafe-inline'; script-src 'self'" always;

# Restart the proxy service after making changes
su - zimbra -c "zmproxyctl restart"