# CVE-2025-66376: Synacor Zimbra Collaboration Suite stored XSS in Classic UI

Source: https://junglewise.ai/threats/cve-2025-66376-synacor-zimbra-collaboration-suite-stored-xss-in-classic-ui  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-01-05. Updated: 2026-05-15  
License: CC BY 4.0 with a link back

CVE-2025-66376 · Severity: critical · CVSS 7.2 · Exploited in the wild · Published 2026-01-05

Zimbra Collaboration Suite is a widely used enterprise email and collaboration platform. A security flaw in its classic web interface allows attackers to send malicious emails that can execute unauthorized code in a user's browser. This could lead to the theft of login credentials, unauthorized access to sensitive emails, or the hijacking of user sessions.

Technologies: [Synacor Zimbra Collaboration Suite](https://junglewise.ai/threats/technologies/zimbra-collaboration-suite), [Synacor Zimbra Collaboration Suite (ZCS)](https://junglewise.ai/threats/technologies/zimbra-collaboration-suite-zcs). Vendors: [Synacor](https://junglewise.ai/threats/vendors/synacor).

## Quick answers

**Is CVE-2025-66376 exploited in the wild?** Yes. CVE-2025-66376 is known to be exploited in the wild, first recorded on 18 March 2026. Treat it as urgent.

**How severe is CVE-2025-66376?** Critical severity, with a CVSS score of 7.2, published 5 January 2026.

**Which products and versions are affected?** Synacor Zimbra Collaboration Suite 10.0.0 before 10.0.18, 10.1.0 before 10.1.13.

**Is there a fix for CVE-2025-66376?** A patch was published on 2026-03-18: Updated versions 10.0.18 and 10.1.13 confirmed as fixes Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2025-66376 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2025-66376?** 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 Classic UI of Zimbra Collaboration Suite (ZCS) versions 10.x. The flaw is rooted in the improper sanitization of Cascading Style Sheets (CSS) '@import' directives within HTML-formatted email messages. An unauthenticated remote attacker can exploit this by sending a specially crafted email to a target user. When the victim views the email in the Classic UI, the malicious CSS directive can trigger the execution of arbitrary JavaScript in the context of the user's session. This vulnerability has been observed being exploited in the wild. Patches are available in versions 10.0.18 and 10.1.13.

## Affected products

- Synacor Zimbra Collaboration Suite 10.0.0 before 10.0.18, 10.1.0 before 10.1.13

## Timeline

- 2026-01-05: disclosed: Initial CVE publication
- 2026-03-18: kev added: CISA added the vulnerability to the Known Exploited Vulnerabilities (KEV) catalog
- 2026-03-18: patched: Updated versions 10.0.18 and 10.1.13 confirmed as fixes

## Related threats

- [Zimbra Collaboration Suite OS command injection in SMTP](https://junglewise.ai/threats/cve-2026-73570-zimbra-collaboration-suite-os-command-injection-in-smtp)
- [Synacor Zimbra Collaboration Suite XSS in Classic UI](https://junglewise.ai/threats/cve-2025-48700-synacor-zimbra-collaboration-suite-xss-in-classic-ui)
- [Synacor Zimbra Collaboration Suite SSRF in WebEx zimlet](https://junglewise.ai/threats/cve-2020-7796-synacor-zimbra-collaboration-suite-ssrf-in-webex-zimlet)
- [Synacor Zimbra Collaboration Suite file inclusion in RestFilter servlet](https://junglewise.ai/threats/cve-2025-68645-synacor-zimbra-collaboration-suite-file-inclusion-in-restfilter)
- [Synacor Zimbra Collaboration Suite XSS in Classic Web Client](https://junglewise.ai/threats/cve-2025-27915-synacor-zimbra-collaboration-suite-xss-in-classic-web-client)