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

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.

Published Mar 12, 2025 · Updated May 29, 2026

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Why patchRisk explained in plain English

Worst-case scenarioIf unpatched

A remote attacker, with a low-privilege account, can achieve partial data exposure, partial data tampering. Federal agencies are required to remediate by 2025-10-28 under CISA BOD 22-01.

How the attack worksNo clicks needed

This is a Cross-Site Scripting (XSS) (CWE-79) vulnerability in Synacor Zimbra Collaboration Suite (ZCS). 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. Exploitation requires remote network access, low attack complexity, a low-privilege authenticated account, and user interaction required.

Am I affected?Quick check

Probably yes if any of these apply:

●IT Security

●Running zimbra collaboration suite: 10.0.0 ≤ v < 10.0.13, 10.1.0 ≤ v < 10.1.5, 9.0.0

Real-world incidentsWhat we've seen

Active exploitation documented in the wild. Threat-research write-up: https://strikeready.com/blog/0day-ics-attack-in-the-wild/

How to patch

Get the fix

Apply the fixed package from your vendor. The advisory lists affected versions and the exact fixed build.

↗ Vendor advisory

Manual remediation steps

Apply the Vendor Patch

This vulnerability is in the CISA Known Exploited Vulnerabilities catalog — apply the vendor's security update as soon as possible.

CISA 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.