Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
Synacor · Zimbra Collaboration Suite (ZCS)
What it is
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.
Why we say that
- CISA's federal remediation deadline (2025-10-28) has passed
- Listed in the CISA Known Exploited Vulnerabilities catalogue
Every line above comes from a named public source. Where the evidence is thin, we say so rather than inventing confidence — see methodology.
What to do
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
What it costs when this goes wrong
This is where UK enforcement meets the vulnerability record. Where an unfixed vulnerability leads to a breach, the ICO treats it as a security failure under UK GDPR. Recent cases:
- The Electoral Commission Reprimand issued to the Electoral Commission in respect of Articles 5(1)(f) and 32(1)(b). Between 24 August 2021 and 27 October 20…
- Gain Capital UK Limited Gain Capital UK have been issued a Reprimand in respect of Articles 32 (2) and 32 (1) (b). An unauthorised third party leveraged a…
- Chartered Institute for Securities & Investment An unauthorised third party exploited a known vulnerability in the Sitefinity software to leverage a bruteforce attack to upload a…