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

Exploited in the wild High 7.2 Published 5 Jan 2026 Updated 19 Mar 2026

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

Exploited in the wild: Since 18 Mar 2026
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

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

Which versions are affected by CVE-2025-66376?

It affects 1 product.

Products include 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.

Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.

Is CVE-2025-66376 being exploited?

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

All Synacor Zimbra Collaboration Suite vulnerabilities

Sources

Also known as GHSA-h7wg-85fj-3c6g.