CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
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.
| Vendor | Synacor |
|---|---|
| Product | Zimbra Collaboration Suite (ZCS) |
| Date added to KEV | 2025-10-07 |
| Due date (BOD 22-01) | 2025-10-28 |
| Known ransomware campaign use | Unknown |
| CWE | CWE-79 |
| 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. |
What to do
- Identify every instance of Synacor Zimbra Collaboration Suite (ZCS) in your environment (asset inventory, EDR, vulnerability scanner).
- Apply CISA's 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.
- Federal civilian agencies must act by 2025-10-28 under BOD 22-01; other organizations can use the same date as a target.
- Check the vendor advisory and NVD entry linked below for affected versions, then record the fix in your change log.
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Sources
- NVD entry for CVE-2025-27915 (severity scores, if any, live there)
- CISA KEV catalog entry
- https://wiki.zimbra.com/wiki/Security_Center
- https://nvd.nist.gov/vuln/detail/CVE-2025-27915