Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
Synacor / Zimbra Collaboration Suite (ZCS)
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.
- CVE identifier
- CVE-2025-27915
- Added to KEV
- Oct 7, 2025
- CISA catalog dueDate
- Oct 28, 2025
The dates above are catalog fields. The addition date does not establish when the vulnerability was discovered or first exploited. Confirm which requirements apply to your organisation separately.
Publisher'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.
This wording is attributed to the CISA snapshot. Verify current vendor guidance, affected versions and your deployment before applying changes.
Keep the signals separate.
- Ransomware use, as reported by CISA
- Unknown
- CVSS severity
- Not supplied by KEV
- EPSS probability
- 3.99%
EPSS score date: 2026-09-27T12:00:21Z. Percentile: 90.15%. Model version: v2026.06.15.
A dated estimate of exploitation probability over the following 30 days, not a severity score or a probability of a breach in your environment.
Inspect the EPSS source ↗“Unknown” ransomware use is an unknown source field, not proof that ransomware activity has not occurred.
Source notes
https://wiki.zimbra.com/wiki/Security_Center ; https://nvd.nist.gov/vuln/detail/CVE-2025-27915
Source & freshness
Source updated: Sep 27, 2026, 9:30 PM UTC
Successfully checked: Sep 27, 2026, 11:50 PM UTC
This snapshot is older than 36 hours. Check current publisher guidance.
Automated source record, not an editorial advisory or an independent discovery. No affected or fixed version is inferred, and no organisation-specific risk rating is generated.