# CVE-2025-66376: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting

> Attackers can execute arbitrary scripts in victims' browsers via malicious CSS in HTML emails, potentially leading to account takeover.

- **Severity:** High 7.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Yes, since 18 Mar 2026 (CISA KEV)
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 5 Jan 2026
- **Updated:** 19 Mar 2026
- **Also known as:** GHSA-h7wg-85fj-3c6g

## What is CVE-2025-66376?

CVE-2025-66376 is a high-severity cross-site scripting vulnerability in Synacor Zimbra Collaboration Suite, published on 5 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2025-66376?

It affects 1 product.


Products: Synacor Zimbra Collaboration Suite.

## How do I fix CVE-2025-66376?

No fix has been published for CVE-2025-66376 yet. Limit exposure of the affected software until one is available. US federal agencies must fix it by 1 Apr 2026.

## Is CVE-2025-66376 being exploited?

Yes. Attackers have exploited CVE-2025-66376 in the wild since at least 18 Mar 2026.

## Other vulnerabilities in Synacor Zimbra Collaboration Suite

- [CVE-2024-45519](https://insights.sbomify.com/CVE-2024-45519/): Synacor Zimbra Collaboration Suite (ZCS) Command Execution (critical 10.0, exploited in the wild)
- [CVE-2022-41352](https://insights.sbomify.com/CVE-2022-41352/): Synacor Zimbra Collaboration Suite (ZCS) Arbitrary File Upload (critical 9.8, exploited in the wild)
- [CVE-2022-37042](https://insights.sbomify.com/CVE-2022-37042/): Synacor Zimbra Collaboration Suite (ZCS) Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2022-27924](https://insights.sbomify.com/CVE-2022-27924/): Synacor Zimbra Collaboration Suite (ZCS) Command Injection (critical 9.8, exploited in the wild)
- [CVE-2020-7796](https://insights.sbomify.com/CVE-2020-7796/): Synacor Zimbra Collaboration Suite (ZCS) Server-Side Request Forgery (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-66376 and fix it if it is affected.

What it is: CVE-2025-66376 is a high-severity cross-site scripting vulnerability in Synacor Zimbra Collaboration Suite, published on 5 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: Yes. Attackers have exploited CVE-2025-66376 in the wild since at least 18 Mar 2026.

Affected product: Synacor Zimbra Collaboration Suite. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-66376/ (Markdown: https://insights.sbomify.com/CVE-2025-66376/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-66376
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-66376
- OSV: https://osv.dev/vulnerability/CVE-2025-66376

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-66376/
Summaries are generated and may contain mistakes; the official description above is the record.