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Known exploited · Synacor

CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability

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.

Added October 7, 2025Due October 28, 2025CISA KEV catalog 2026.10.01

Vendor
Synacor
Product
Zimbra Collaboration Suite (ZCS)
Added to KEV
October 7, 2025
CISA due date
October 28, 2025
Ransomware use
Unknown

What CISA requires

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Forensic triage

No

What to do now

  1. Find every instance of Zimbra Collaboration Suite (ZCS) you run, including the forgotten ones, and check its version against the vendor advisory.
  2. Apply the vendor's fix or mitigation, before October 28, 2025 if you are bound by CISA's deadline, sooner if it faces the internet.
  3. Assume it was tried against you already: look for signs of exploitation from the date it was added, not from the day you patched.
  4. Until every instance is patched, block the attack at the edge: a WAF in blocking mode stops cross-site scripting payloads.

References

CVE-2025-27915 in the National Vulnerability Database

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The Dome's WAF runs the OWASP Core Rule Set in blocking mode and stops cross-site scripting payloads before they reach your application.

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Source: CISA Known Exploited Vulnerabilities Catalog (public domain), catalog version 2026.10.01.