CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability

Medium severity, CVSS 5.4. Actively exploited: in CISA KEV since 2025-10-07. EPSS: 4.1% chance of exploitation in the next 30 days.

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.

Affected products

• Synacor Zimbra Collaboration Suite: from 10.0.0, before 10.0.13 (fixed in 10.0.13); from 10.1.0, before 10.1.5 (fixed in 10.1.5); version 9.0.0 only

Published 2025-03-12. Last modified 2026-06-17.