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CVE-2025-66376

Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting

7.2 HIGHActively exploited (CISA KEV)EPSS 17.3%
  1. Published5 Jan 2026
  2. Added to CISA KEV18 Mar 2026
  3. Remediation due1 Apr 2026
Actively exploited (CISA KEV)

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.

Added to KEV: 18 Mar 2026Federal remediation due: 1 Apr 2026

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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeChanged
ConfidentialityLow
IntegrityLow
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

Affected products

Synacor Zimbra Collaboration Suite≥ 10.0.0 < 10.0.18 · ≥ 10.1.0 < 10.1.13

Remediation

Fixed in
Synacor Zimbra Collaboration Suite≥ 10.0.18

References

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Frequently asked

Is CVE-2025-66376 being actively exploited?

Yes. CVE-2025-66376 is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, added 18 Mar 2026.

How severe is CVE-2025-66376?

CVE-2025-66376 has a CVSS 3.1 base score of 7.2, rated high. EPSS estimates a 17.3% probability of exploitation.

Which products are affected by CVE-2025-66376?

Affected products include Synacor Zimbra Collaboration Suite.

How do I fix CVE-2025-66376?

Upgrade affected software to a fixed version, for example Synacor Zimbra Collaboration Suite 10.0.18 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2025-66376 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. Because it is on the CISA KEV list, Secuno flags any affected asset as top-priority automatically. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.