Actively exploited

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

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.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment

CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 18 Mar 2026.
Rated High severity (CVSS 7.2, scored by Vendor).
This product type is often exposed to the internet, which is where attackers scan for it first.
It has been in the catalogue for 233 days. Systems still unpatched now have almost certainly been scanned for it.

The Vulnerability

What It Is

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.

Affected Versions

Product	Versions
Zimbra Collaboration	10.0 to before 10.0.18 10.1 to before 10.1.13

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

1
Find it
Search your asset inventory and external attack surface for Synacor Zimbra Collaboration Suite (ZCS). Include cloud images, appliances and anything a supplier runs for you.
Attack Surface Management
2
Fix it
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Vulnerability Management
3
Hunt for compromise
If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.
Incident Response
4
Prove it is fixed
Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.
Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated high, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 1 Apr 2026 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited Synacor Flaws

All 19 exploited Synacor vulnerabilities

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FOR VENDORS

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

Should You Worry?

What It Is

Affected Versions

What To Do Now

Find it

Fix it

Hunt for compromise

Prove it is fixed

Vendor Advisories

The Compliance Clock

More Exploited Synacor Flaws

Not Sure If You Are Exposed?

Sources