Synacor · Zimbra Collaboration Suite (ZCS)
CVE-2025-27915
Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
What it is
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.
CISA: 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.
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.
Deadline for US federal agencies: October 28, 2025
More exploited flaws from Synacor
| Added | Vulnerability | Product | Severity | Attack chance |
|---|---|---|---|---|
| Aug 21, 2026 | OS Command Injection | Zimbra Collaboration Suite (ZCS) | High 8.9 | 72% |
| Apr 20, 2026 | Cross-site Scripting | Zimbra Collaboration Suite (ZCS) | Medium 6.1 | 1.7% |
| Mar 18, 2026 | Cross-Site Scripting | Zimbra Collaboration Suite (ZCS) | High 7.2 | 20% |
| Jul 7, 2025 | Server-Side Request Forgery (SSRF) | Zimbra Collaboration Suite (ZCS) | High 7.5 | 81% |
| May 19, 2025 | Cross-Site Scripting (XSS) | Zimbra Collaboration Suite (ZCS) | Medium 6.1 | 25% |
| Feb 25, 2025 | Cross-Site Scripting (XSS) | Zimbra Collaboration Suite (ZCS) | Critical 9.0 | 77% |
| Oct 3, 2024 | Command Execution | Zimbra Collaboration Suite (ZCS) | Critical 10.0 | 99.9% |
| Jul 27, 2023 | Cross-Site Scripting (XSS) | Zimbra Collaboration Suite (ZCS) | Medium 6.1 | 49% |
Sources: Known Exploited Vulnerabilities (CISA, public domain), CVE records (cve.org) and EPSS (FIRST). The chance of attacks is the EPSS estimate that someone will try to exploit the flaw in the next 30 days.