---
id: "CVE-2025-66376"
kind: "vulnerability"
aliases: ["CVE-2025-66376"]
title: "synacor zimbra collaboration suite: Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading\u2026"
severity: "high"
cvss: 7.2
epss: null
exploited: true
status: "open"
category: "package"
products: [{"name": "Synacor Zimbra Collaboration Suite (ZCS)", "fixed": [], "affected": "", "ecosystem": "vendor"}, {"name": "synacor zimbra collaboration suite", "fixed": ["10.0.18"], "affected": ">= 10.0.0, < 10.0.18", "ecosystem": "vendor"}, {"name": "synacor zimbra collaboration suite", "fixed": ["10.1.13"], "affected": ">= 10.1.0, < 10.1.13", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["email", "exploited", "package", "self-hosted", "zimbra"]
not_affected_if: ["managed-hosting", "non-classic-ui"]
published_at: "2026-01-05T15:15:44+00:00"
first_seen_at: "2026-10-23T19:11:51+00:00"
updated_at: "2026-10-23T23:44:51+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2025-66376", "first_seen_at": "2026-10-23T19:11:51+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-66376", "first_seen_at": "2026-10-23T23:44:51+00:00"}]
---

# synacor zimbra collaboration suite: Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading…

Zimbra Collaboration Suite's Classic UI has a cross-site scripting flaw: CSS @import directives inside email HTML are abused to run script. An attacker who gets a crafted email in front of a user can run script in that user's Zimbra session, which can expose mail and session data.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Synacor Zimbra Collaboration Suite (ZCS) affected ? fixed ?
- vendor:synacor zimbra collaboration suite affected >= 10.0.0, < 10.0.18 fixed 10.0.18
- vendor:synacor zimbra collaboration suite affected >= 10.1.0, < 10.1.13 fixed 10.1.13

## Fix

Apply the vendor mitigation per Synacor/Zimbra instructions, or discontinue use of the product if mitigations are unavailable (CISA KEV required action). Exact fixed version: unknown.

## Timeline

- 2026-10-23T19:11:51+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2025-66376
- 2026-10-23T23:44:51+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2025-66376

## From the sources

Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability in the Classic UI where attackers could abuse Cascading Style Sheets (CSS) @import directives in email HTML.

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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_