What is the CVSS score of CVE-2025-66376?

CVE-2025-66376 has a CVSS 3.1 base score of 6.1 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N. EPSS exploitation probability: 0.0%.

Is CVE-2025-66376 being actively exploited?

Yes, CVE-2025-66376 is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, confirming active in-the-wild exploitation. Immediate patching is required.

How to fix or mitigate CVE-2025-66376?

Within 24 hours: Identify all affected systems and apply vendor patches immediately. Verify anti-CSRF tokens and content security policies are enforced.

Zimbra Collaboration Suite CVE-2025-66376

MEDIUM

Cross-site Scripting (XSS) (CWE-79)

2026-01-05 cve@mitre.org

XSS Zimbra Collaboration Suite Synacor

6.1

CVSS 3.1 · NVD

View

Vulnerability Assessment

Weakness	CWE CWE-79 OWASP Top 10:2025 A05:2025 - Injection ASVS 5.0 V1 - Encoding and Sanitization Output encoding, context-aware escaping MITRE ATT&CK 2 techniques across 2 tactics: Initial Access, Execution The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up. Sign in to read
Impact	CVSS 7.2 (High). KEV-listed — actively exploited. Zimbra is a high-value target (used by government agencies, enterprises). Stored XSS persists in mailbox — affects all viewers. Classic UI is still widely deployed. No PoC publicly confirmed but exploitation observed.
Exploit ScenarioDerived	An attacker sends a crafted HTML email to a Zimbra user containing CSS @import directives that load external malicious content. When the recipient (or any user with access to the mailbox, including administrators) views the email in the Classic UI, the injected JavaScript executes in their session context. The attacker can steal session cookies, read other emails, send emails as the victim, or escalate to full account compromise. If an admin views the email, the attacker may gain administrative control of the entire Zimbra instance.

This Month Plan within 30 days. Why: MEDIUM severity (CVSS 4.0-6.9).

Exploited in the wild (CISA KEV) No patch listed yet 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 12, 2026

Zimbra Collaboration Suite (ZCS) 10.x contains a stored XSS vulnerability in the Classic UI that allows attackers to execute arbitrary JavaScript through CSS @import directives in HTML emails. KEV-listed, this vulnerability (CVE-2025-66376) enables session hijacking and account takeover when administrators or users view malicious emails, making it a high-value target for email-based espionage campaigns.

Earlier versions of this analysis (1)

v1 Mar 12, 2026 - 21:54 CVSS 7.2

Zimbra Collaboration Suite Classic UI fails to properly sanitize CSS @import directives in HTML emails (CWE-79), allowing stored cross-site scripting that executes when any user views the malicious email.

RemediationAI

Upgrade to ZCS 10.0.18+ or 10.1.13+ immediately. Switch users to the Modern UI which is not affected. Implement email content filtering to strip CSS @import directives. Monitor for suspicious email content patterns.

Affects your stack

Find out whether the stacks and servers you run are on an affected version.

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

Technical ContextAI · show details

The vulnerability exploits CSS @import directives within HTML email content. While Zimbra sanitizes many XSS vectors in email rendering, the Classic UI fails to block CSS @import statements that can load external stylesheets or inject JavaScript through CSS expression evaluation. Because the malicious payload is stored in the email itself, it executes every time any user views it, making it particularly effective for targeting organizations using Zimbra's Classic web interface.

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

NVD PRIMARY 6.1 MEDIUM

AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Lifecycle Timeline

Severity Changed

Sep 30, 2026 - 23:23 NVD

HIGH MEDIUM

CVSS changed

Sep 30, 2026 - 23:23 NVD

7.2 (HIGH) 6.1 (MEDIUM)

Added to CISA KEV

Mar 18, 2026 - 20:13 cisa

CISA KEV

CVE Published

Jan 05, 2026 - 15:15 nvd

HIGH 7.2

DescriptionNVD

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.

Executive ViewAI

Why It Matters

CVE-2025-66376 presents moderate security risk, a cross-site scripting vulnerability rated CVSS 7.2. Attackers could inject malicious scripts to steal user sessions, credentials, or perform actions on behalf of authenticated users. This vulnerability is actively exploited in the wild (KEV-listed) and requires immediate remediation. Immediate patching is required per CISA KEV mandate.

Business Impact: Severe

Compensating Controls

Apply vendor patch. If immediate patching is not feasible: (1) Implement Content-Security-Policy headers; (2) Enable HttpOnly and SameSite cookie attributes; (3) Deploy WAF XSS/CSRF rules.

Threat Intelligence Overview

Threat Score

4.7/10

Composite threat intelligence

EPSS Score

0.0%

Top 86% exploitable

Threat Intel

1 source

Data Quality

6 sources

Confidence: High

NVD

CVE.org

CISA KEV

EPSS CISA

VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue

Severity MEDIUM

CISA KEV Yes

Public POC No

EPSS 0.0%

Patch available No

Active threat Yes

Deciding rule

MEDIUM severity (CVSS 4.0-6.9)

What every action level means

Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.

Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.

This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.

This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.

Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived

Immediate

Exploitation· derived

Active

System Exposure· derived

Open

Automatable· derived

Human Impact

Very High

Priority Score

Low Medium High Critical

KEV: +50

EPSS: 0

CVSS: +30

PoC: 0

Weaponized: 0

Threat intel: +10

Gov advisory: 0

Attack Technique

Cross-Site Scripting MEDIUM

Cross-Site Scripting occurs when an application accepts untrusted data and sends it to a web browser without proper validation or encoding.

Affected Products (CPE)NVD

cpe:2.3:a:synacor:zimbra_collaboration_suite:*:*:*:*:*:*:*:*

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Threat IntelligenceMISP

CISA Advisories

AA26-204A

Sources: CISA (public domain)

References

Advisories & patches

ADVISORY https://vuldb.com/?id.339552 REF https://wiki.zimbra.com/wiki/Security_Center REF https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18#Security_Fixes REF https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13#Security_Fixes REF https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy REF https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories REF https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376

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GitHub PoC

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