Zimbra Collaboration Suite (ZCS) ICS File Parsing Stored XSS Vulnerability (CVE-2025-27915) Security Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTZimbra Collaboration Suite (ZCS) versions 9.0, 10.0, and 10.1 utilizing the Classic Web Client
Viewing an email containing a malicious ICS calendar file triggers arbitrary JavaScript execution within the victim's session, enabling unauthorized email redirection and data exfiltration
Apply official Zimbra security patches (9.0.0 Patch 44, 10.0.13, 10.1.5 or later) immediately, and restrict the use of the Classic Web Client until patched
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Refer to the official Zimbra release notes and upgrade your installation to the patched versions (9.0.0 Patch 44, 10.0.13, or 10.1.5).
# For Ubuntu/Debian-based Zimbra installations
apt-get update && apt-get install --only-upgrade zimbra-patch -y
# For RedHat/CentOS-based Zimbra installations
yum clean all && yum update zimbra-patch -y
# Restart Zimbra services as the zimbra user
su - zimbra -c 'zmcontrol restart'2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, enforce the Modern Web Client or configure a Web Application Firewall (WAF) to block requests containing malicious event handlers.
# Example Nginx configuration to block requests containing the ontoggle payload
server {
listen 443 ssl;
server_name mail.yourdomain.com;
location / {
if ($request_uri ~* 'ontoggle=') {
return 403 'Malicious Request Blocked';
}
proxy_pass http://zimbra_backend;
}
}3. Log Audit & IoC Verification Commands
Inspect Zimbra logs for evidence of malicious ICS file processing and audit user accounts for unauthorized mail forwarding rules.
# Search Zimbra mailbox logs for the 'ontoggle' event handler
grep -i 'ontoggle' /opt/zimbra/log/mailbox.log
# Audit all user accounts for external email forwarding rules
su - zimbra -c "zmprov -l gaa | while read user; do echo 'User: '\$user; zmprov ga \$user zimbraMailDeliveryAddress zimbraMailForwardingAddress; done"Block Exploits Instantly with Managed Virtual Patching Rules
Can't patch production immediately? Deploy managed WAF rules at the edge to intercept RCE, SQLi, and deserialization payloads before they hit your origin server.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2025-27915) exists in the Classic Web Client of Zimbra Collaboration Suite (ZCS) due to insufficient sanitization of HTML content embedded within iCalendar (ICS) files. When the application parses and renders the ICS file structure, it fails to properly filter malicious HTML tags and attributes, allowing arbitrary script execution in the context of the user's browser session.
2. Exploitation Mechanics & Attack Scenario
An attacker crafts a malicious ICS file containing an HTML tag embedded with an 'ontoggle' event handler and a malicious JavaScript payload. This file is sent as an email attachment or inline calendar invite to the target user. When the victim opens or previews the email in the Classic Web Client, the 'ontoggle' event is triggered, executing the script. The script runs silently in the background, leveraging the victim's active session to modify mail filters (e.g., forwarding all incoming mail to an attacker-controlled address) or exfiltrate sensitive mailbox data.
3. Infrastructure & Business Impact
As this vulnerability is actively exploited in the wild and listed in the CISA KEV catalog, it poses an immediate threat to organizations running unpatched Zimbra instances. Successful exploitation leads to complete compromise of the affected user's mailbox, unauthorized data exfiltration, and potential compliance violations. Furthermore, compromised accounts can be leveraged to launch internal spear-phishing campaigns, escalating privileges across the corporate network.
Official Vendor Advisories & References
An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and 10.0 and 10.1. A stored cross-site scripting (XSS) vulnerability exists in the Classic Web Client due to insufficient sanitization of HTML content in ICS files. When a user views an e-mail message containing a malicious ICS entry, its embedded JavaScript executes via an ontoggle event inside a <details> tag. This allows an attacker to run arbitrary JavaScript within the victim's session, potentially leading to unauthorized actions such as setting e-mail filters to redirect messages to an attacker-controlled address. As a result, an attacker can perform unauthorized actions on the victim's account, including e-mail redirection and data exfiltration.
Frequently Asked SecOps Questions (CVE-2025-27915)
FAQQ1. How critical is CVE-2025-27915 and is it actively exploited in the wild?
CVE-2025-27915 is rated CVSS 5.4 (MEDIUM) with a 30-day EPSS exploitation probability of 4.1%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2025-27915?
Primary impacted vendors include zimbra across zimbra collaboration suite. Scope summary: Zimbra Collaboration Suite (ZCS) versions 9.0, 10.0, and 10.1 without the latest security patches
Q3. What temporary mitigations or workarounds exist for CVE-2025-27915 before patching?
[Mitigation] Apply official Zimbra security patches (9.0.0 Patch 44, 10.0.13, 10.1.5 or later) immediately, and restrict the use of the Classic Web Client until patched Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2025-27915
Instantly format this advisory into an executive CISO summary or engineering patch ticket.
🔗 Related Advisories in Similar Stack (zimbra)
View Category →Zimbra Collaboration Suite (ZCS) Remote Code Execution Vulnerability Advisory (CVE-2026-73570, CISA KEV Listed)
[Affected Scope] Zimbra Collaboration (ZCS) versions prior to 10.1.20, specifically when the optional zimbra-snmp package is installed and SNMP notifications are enabled.
Microsoft Exchange Server Cross-Site Scripting (XSS) and Spoofing Vulnerability Advisory (CVE-2026-42897)
[Affected Scope] All supported on-premises Microsoft Exchange Server environments.
CVE-2026-120114 Technical Vulnerability Analysis & Patch Advisory (CVSS 7.2)
[Affected Scope] Systems running unpatched versions impacted by CVE-2026-120114
WordPress WooCommerce PDF Invoices & Packing Slips Stored XSS Vulnerability (CVE-2026-107750) Security Advisory
[Affected Scope] WordPress WooCommerce PDF Invoices & Packing Slips plugin versions up to, and including, 5.16.4
Subscribe to Daily Vulnerability Briefings & KEV Alerts
Concise morning briefings for SecOps, DevSecOps, and SRE teams covering newly exploited CVEs and mitigation priorities.