CVE-2025-66376: Zimbra Collaboration XSS Vulnerability

CVE-2025-66376 Overview

A stored Cross-Site Scripting (XSS) vulnerability has been identified in Zimbra Collaboration (ZCS) affecting the Classic UI component. The vulnerability allows attackers to inject malicious scripts through specially crafted HTML email messages that leverage Cascading Style Sheets (CSS) @import directives. When a user views the malicious email in the Classic UI, the injected script executes in the context of their browser session, potentially leading to session hijacking, credential theft, or further attacks against the organization.

Critical Impact

Attackers can execute arbitrary JavaScript in victim browsers by sending malicious HTML emails, potentially compromising user sessions and sensitive data within the Zimbra environment.

Affected Products

• Zimbra Collaboration (ZCS) 10 before version 10.0.18
• Zimbra Collaboration (ZCS) 10.1 before version 10.1.13

Discovery Timeline

• 2026-01-05 - CVE-2025-66376 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-66376

Vulnerability Analysis

This vulnerability falls under CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The root issue lies in insufficient sanitization of CSS content within HTML email messages processed by Zimbra's Classic UI.

The vulnerability specifically targets the CSS @import directive handling mechanism. When an attacker crafts an HTML email containing malicious CSS @import statements, the Classic UI fails to properly sanitize or block these directives. This allows the attacker to reference external resources or inject inline scripts that execute when the email is rendered in a victim's browser.

The network-based attack vector means exploitation requires no authentication from the attacker's perspective—they simply need to send a malicious email to a target user. The vulnerability has a changed scope, meaning successful exploitation can impact resources beyond the vulnerable component, potentially affecting the broader user session and any connected services.

Root Cause

The vulnerability stems from inadequate input validation and sanitization of CSS content within HTML email messages. The Classic UI component does not properly filter or escape CSS @import directives embedded in email messages, allowing attackers to bypass security controls and inject executable content. This represents a failure in the email parsing and rendering pipeline where CSS content should be stripped or sanitized before display.

Attack Vector

The attack is network-based and can be executed remotely without requiring any prior authentication. An attacker crafts a malicious HTML email containing CSS @import directives designed to load external malicious stylesheets or execute JavaScript. When a legitimate user opens this email using Zimbra's Classic UI, the malicious CSS is processed and the injected script executes within the user's browser context.

The attack flow involves:

1. Attacker composes an HTML email with embedded CSS @import directives pointing to attacker-controlled resources or containing inline malicious payloads
2. Email is delivered to target user's Zimbra mailbox
3. Victim opens the email using the Classic UI interface
4. Zimbra renders the email without properly sanitizing the CSS content
5. Malicious JavaScript executes in the victim's authenticated browser session
6. Attacker gains access to session tokens, can perform actions as the victim, or exfiltrate sensitive data

Detection Methods for CVE-2025-66376

Indicators of Compromise

• Emails containing unusual CSS @import directives pointing to external domains
• Outbound connections from user browsers to unknown or suspicious external URLs during email viewing
• Unusual JavaScript execution patterns in web server logs associated with the Classic UI
• User reports of unexpected behavior when viewing certain emails

Detection Strategies

• Implement email content scanning to detect CSS @import directives in HTML emails
• Monitor web application firewall logs for requests containing XSS patterns targeting the Classic UI endpoints
• Review Zimbra server logs for anomalous email rendering activities
• Deploy browser-based XSS detection mechanisms to identify script injection attempts

Monitoring Recommendations

• Enable detailed logging for the Classic UI component to track email rendering events
• Configure alerting for outbound connections initiated during email viewing sessions
• Monitor for unusual session activity patterns that may indicate compromised accounts
• Implement Content Security Policy (CSP) headers where possible to detect policy violations

How to Mitigate CVE-2025-66376

Immediate Actions Required

• Upgrade Zimbra Collaboration (ZCS) 10 installations to version 10.0.18 or later
• Upgrade Zimbra Collaboration (ZCS) 10.1 installations to version 10.1.13 or later
• Consider temporarily restricting access to the Classic UI and directing users to the Modern UI if available
• Implement additional email filtering rules to block suspicious CSS content

Patch Information

Zimbra has released security patches addressing this vulnerability in versions 10.0.18 and 10.1.13. Organizations should immediately apply these updates to protect against exploitation. Detailed patch information is available through the Zimbra 10.0.18 Security Fixes and Zimbra 10.1.13 Security Fixes documentation. Additional security resources can be found at the Zimbra Security Center.

Workarounds

• If immediate patching is not possible, restrict access to the Classic UI interface and require users to use alternative interfaces
• Implement web application firewall rules to filter emails containing CSS @import directives
• Configure email gateway filtering to strip or quarantine HTML emails with suspicious CSS content
• Enable additional Content Security Policy headers on the Zimbra web interface to mitigate script execution

# Example: Review current Zimbra version before patching
zmcontrol -v

# After patching, verify the update was applied successfully
zmcontrol -v
# Expected output should show version 10.0.18 or 10.1.13 or later