Exploited CVEs / Synacor / CVE-2025-66376
CVE-2025-66376
Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability. Exploitation in the wild is confirmed by CISA, not predicted. Federal agencies were required to remediate by 2026-04-01 (that deadline has passed); treat that date as the outer bound for your own environment. The fix is in the vendor advisory: Zimbra Security Center, linked below.
Actively exploited Synacor Zimbra Collaboration Suite (ZCS) vulnerability, Medium severity, CVSS 6.1. Added to the CISA KEV catalog 2026-03-18.
Risk summary
Updated July 30, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.
Senserva AI Opinion for CVE-2025-66376
This Zimbra Classic UI flaw is exploitable by simply sending a crafted email: CSS @import directives in the message HTML execute a cross-site scripting payload when the recipient views it. Rated Medium Severity at CVSS 6.1 with a 12% EPSS score, it was confirmed exploited on 2026-03-18, consistent with the espionage-style targeting Zimbra webmail keeps attracting. No clicks or attachments are required from the victim.
Do this: Patch Zimbra now and audit mailboxes for new forwarding rules or filters created since the emails started arriving.
AI-generated from public data, 2026-07-07. Verify against the vendor advisory before acting.
Senserva AI Opinion and rich prompt for CVE-2025-66376
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for CVE-2025-66376
Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.
CISA required action
Federal (BOD 22-01) remediation due date: 2026-04-01 (past due).
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Official fix for CVE-2025-66376
The authoritative fix and affected-version list are published by the vendor. Find the Zimbra Collaboration Suite (ZCS) advisory on Zimbra Security Center.
Common questions about CVE-2025-66376
Is CVE-2025-66376 actively exploited?
Yes. CVE-2025-66376 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2026-03-18, which means exploitation in the wild has been confirmed, not predicted. Its EPSS 30-day exploitation probability is 22%.
What is the CISA deadline and required action for CVE-2025-66376?
CISA set the federal (BOD 22-01) remediation due date at 2026-04-01, which has passed; treat any unremediated system as overdue. The verbatim 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.
What fixes CVE-2025-66376?
The authoritative fix is in the vendor advisory: see Zimbra Security Center for the patched Zimbra Collaboration Suite (ZCS) versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.
Can I ask my own AI about CVE-2025-66376?
Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2025-66376's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (5 AM and 3 PM US Central).
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Authoritative references
Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.
