Threat Intelligence / CVE / CVE-2025-66376

CVE-2025-66376 — Synacor Zimbra Collaboration Suite

CISA KEV

CVSS v3.1 7.2 (HIGH) · EPSS 10.0% (higher than 93.0% of all scored CVEs) · Priority 9/10 · Published 2026-01-05

As of 2026-03-18, CVE-2025-66376 is a HIGH-severity vulnerability in Synacor Zimbra Collaboration Suite, CVSS v3.1 7.2, EPSS 10.0% (93.0th percentile). It is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-03-18), with a US federal remediation deadline of 2026-04-01. Threadlinqs Intelligence links 1 tracked threat campaign to CVE-2025-66376, most recently “Zimbra Collaboration Suite Stored XSS via CSS @import Active Exploitation (CVE-2025-66376) — Operation GhostMail”.

Last updated: 2026-03-18

What is CVE-2025-66376?

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.

The record classifies CVE-2025-66376 under weakness class CWE-79. Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 269 days ago.

Severity and exploitation probability

CVSS v3.1 base score: 7.2 — HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
EPSS (FIRST): 10.0% probability of exploitation in the next 30 days, higher than 93.0% of all scored CVEs
CISA KEV: Listed since 2026-03-18, federal remediation deadline 2026-04-01
Threadlinqs priority: 9/10 — CISA KEV-listed, which Threadlinqs floors at 9
Published: 2026-01-05, last modified 2026-03-18

Is CVE-2025-66376 being exploited?

CISA added CVE-2025-66376 to the Known Exploited Vulnerabilities catalog on 2026-03-18, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2026-04-01 under BOD 22-01. It currently carries a trending score of 38 in the Threadlinqs vulnerability feed.

Affected products and versions

Synacor: Zimbra Collaboration Suite

How to fix CVE-2025-66376

Vendor advisory: https://wiki.zimbra.com/wiki/Security_Center. Because CVE-2025-66376 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2026-04-01. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.

Threat activity tracking CVE-2025-66376

1 tracked threat in the Threadlinqs corpus references CVE-2025-66376, either in the campaign’s CVE list or as an indicator on the campaign record.

Zimbra Collaboration Suite Stored XSS via CSS @import Active Exploitation (CVE-2025-66376) — Operation GhostMail — CRITICAL · 2026-03-21

Sources

Seeded from threat-derived and not yet processed by the Threadlinqs enrichment pipeline, so blank CVSS, EPSS or KEV fields above mean NOT MEASURED rather than measured-absent.

Full detection coverage & IOCs for threats exploiting CVE-2025-66376 are available via the Threadlinqs MCP server (Purple tier). View plans →

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