Security Digest · Vulnerability record · Known exploited
CVE-2025-66376: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability
Exploited in the wild (CISA KEV). 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.
CISA KEV · added 2026-03-18 CVSS 7.2 High EPSS 20.2% · p97
Should you act?
Exploited: patch now if you run it
CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- Yes, if you run Synacor Zimbra Collaboration Suite (ZCS). CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-03-18, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-04-01; treat that as the outer limit, not the target.
- EPSS 20.2%, percentile 97: FIRST’s model estimates a 20.2% chance of exploitation activity in the next 30 days, higher than 97% of all scored CVEs (very high). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
What it is
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.
CISA’s summary
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.
Weakness type: CWE-79.
Product: Synacor Zimbra Collaboration Suite (ZCS) · Assigned by: mitre · Published: 2026-01-05
Affected and fixed versions
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| Zimbra | Collaboration | 10.0 < 10.0.18, 10.1 < 10.1.13 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| mitre | 3.1 | 7.2 High | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N |
- Attack vector: network (remotely reachable)
- Attack complexity: low
- Privileges required: none (no login)
- User interaction: none
- Scope: changed (can affect other components)
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- CISA KEV: added 2026-03-18; federal due date 2026-04-01. Known ransomware use: Unknown.
- CISA 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.
- CISA SSVC assessment: exploitation active, automatable no, technical impact partial.
- FIRST EPSS (model of 2026-10-21): 20.23% probability of exploitation activity in the next 30 days; percentile 97.
Timeline
- — CVE record published by mitre
- — Added to CISA’s Known Exploited Vulnerabilities catalog
- — CVE record last updated
- — CISA remediation deadline for US federal agencies
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vendor and project advisories, patches
- Listed in CISA KEV notes (wiki.zimbra.com) wiki.zimbra.com
Vulnerability records
- CVE.org record CVE-2025-66376 cve.org
- NVD entry CVE-2025-66376 nvd.nist.gov
Government
Other references from the CVE record and advisory
- wiki.zimbra.com wiki.zimbra.com
- wiki.zimbra.com wiki.zimbra.com
- wiki.zimbra.com wiki.zimbra.com
- wiki.zimbra.com wiki.zimbra.com
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