CVE-2025-27915: in the CISA KEV catalogue

Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability. CISA added CVE-2025-27915 to the Known Exploited Vulnerabilities catalogue on 2025-10-07 with a default due date of 2025-10-28. Known ransomware campaign use: Unknown. Forensic triage flag: No.

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. (CISA description)

Vendor / product

Synacor Zimbra Collaboration Suite (ZCS)

CWE

CWE-79

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.

EPSS

4.1%, percentile 90 (scored 2026-11-03)

EPSS puts about 4.1 in 100 odds (4.1%) on exploitation activity being observed in the next 30 days. Its score is higher than or equal to that of 90% of scored CVEs. This is a model estimate about exploitation anywhere, not about your environment.

Your own deadline under CISA BOD 26-04 depends on exposure, automatability and technical impact: work it out with the BOD 26-04 calculator.