CVE-2025-66376

MEDIUM

6.1

CVSS Score

Published: January 5, 2026 at 12:00 AM UTC

Updated: October 7, 2026 at 10:10 AM UTC

3 products affected across 2 vendorsCollaborationCollaborationZimbra Collaboration Suite

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Severity

MEDIUM6.1

Exploitation

CISA KEV

Exploit prediction

EPSS 20%

SSVC

Active

Actively Exploited

CISA Known Exploited Vulnerabilities (KEV) Catalog

EPSS Score20.2%

EPSS (Exploit Prediction Scoring System) estimates a 20.2% chance this CVE is exploited in the wild within the next 30 days — higher than 97% of all CVEs. It complements CVSS: a high EPSS on a mid-severity CVE can still be urgent.

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.

Required Action

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Ransomware Use

Unknown

Added to KEV

Mar 18, 2026

Remediation Due

Apr 1, 2026

208 days overdue

Notes

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

https://nvd.nist.gov/vuln/detail/CVE-2025-66376

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.

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Collaboration · Synacor

95%

10.0.0 ≤ v < 10.0.1810.1.0 ≤ v < 10.1.13

Affected range stops below 10.0.18, so it looks safe — our inference, not a vendor statement

Affected range stops below 10.1.13, so it looks safe — our inference, not a vendor statement

Collaboration · Zimbra

95%

10.0 ≤ v < 10.0.1810.1 ≤ v < 10.1.13

Affected range stops below 10.0.18, so it looks safe — our inference, not a vendor statement

Affected range stops below 10.1.13, so it looks safe — our inference, not a vendor statement

Zimbra Collaboration Suite · Zimbra

60%

10.0 ≤ v < 10.0.1810.1 ≤ v < 10.1.1310.0 10.1

Affected range stops below 10.0.18, so it looks safe — our inference, not a vendor statement

Affected range stops below 10.1.13, so it looks safe — our inference, not a vendor statement

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

References (6)

Vendor Advisory

Source: cverecord

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

Release NotesVendor Advisory

Source: cverecord

https://wiki.zimbra.com/wiki/Security_Center

Product

Source: cverecord

https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy

Release Notes

Source: cverecord

https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13#Security_Fixes

Release Notes

Source: cverecord

https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18#Security_Fixes

US Government Resource

Source: 134c704f-9b21-4f2e-91b3-4a467353bcc0

https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376

SSVC — Action Prioritization

SSVCActiveNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

6.1MEDIUM[email protected]

Attack Vector: NETWORK
Complexity: LOW
Privileges: NONE
User Interaction: REQUIRED
Scope: CHANGED
Confidentiality: LOW
Integrity: LOW
Availability: NONE

Vector?

Quick Info

StatusANALYZED

CWE

CWE-79

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