CVE-2025-27915: Synacor Zimbra Collaboration Suite XSS in Classic Web Client

CVE-2025-27915 · Severity: critical · CVSS 5.4 · Exploited in the wild · Published 2025-10-07

Zimbra Collaboration Suite is a widely used enterprise email and collaboration platform. A security vulnerability in its web interface allows attackers to execute malicious code when a user views a specially crafted calendar (ICS) file. This could allow an attacker to hijack user sessions, steal sensitive emails, or redirect incoming messages to an external address without the user's knowledge.

Technologies: Synacor Zimbra Collaboration Suite, Synacor Zimbra Collaboration Suite (ZCS). Vendors: Synacor.

Quick answers

Is CVE-2025-27915 exploited in the wild?
Yes. CVE-2025-27915 is known to be exploited in the wild, first recorded on 7 October 2025. Treat it as urgent.
How severe is CVE-2025-27915?
Critical severity, with a CVSS score of 5.4, published 7 October 2025.
Which products and versions are affected?
Synacor Zimbra Collaboration Suite 9.0, 10.0, 10.1.
Is there a fix for CVE-2025-27915?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2025-27915 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2025-27915?
Check whether you run Synacor Zimbra Collaboration Suite or Synacor Zimbra Collaboration Suite (ZCS) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A stored cross-site scripting (XSS) vulnerability exists in the Zimbra Collaboration Suite (ZCS) Classic Web Client. The root cause is insufficient sanitization of HTML content within uploaded or received ICS (calendar) files. Specifically, malicious JavaScript can be embedded using an 'ontoggle' event handler within a <details> tag. When a victim views an email containing the malicious ICS entry, the script executes in the context of their active session. This allows an attacker to perform actions on behalf of the user, such as modifying email filters to exfiltrate data. This vulnerability has been observed being exploited in the wild.

Affected products

  • Synacor Zimbra Collaboration Suite (ZCS) 9.0, 10.0, 10.1

Timeline

  • 2025-10-07: disclosed
  • 2025-10-07: kev added: Added to CISA Known Exploited Vulnerabilities catalog.

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