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Synacor, Zimbra Collaboration Suite (ZCS)
Description
Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability that 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 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.
CVSS severity
Medium (5.4/10)CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
Sourced from NVD, reflects how severe this vulnerability is if exploited, not whether it has been. Every advisory on this page is already a CISA-confirmed active exploitation case regardless of its CVSS score.
EPSS, exploit probability
Medium4.1%
estimated chance of real-world exploitation in the next 30 days, higher than 90.6% of every CVE FIRST.org scores
Refreshed 10/28/2026, via FIRST.org's EPSS model, not CVSS, this measures likelihood of exploitation, not how severe it would be.
Public exploit references (via NVD)
Required action
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
CISA-recommended due date: 2025-10-28
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