VulnTideExploited vulnerabilities, by vendor. Data as of 28 Oct 2026.
Known exploitedIn CISA's KEV catalog since 18 March 2026Ransomware use: unknown
Synacor Zimbra Collaboration Suite (ZCS)

CVE-2025-66376: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 18 Mar 2026, 72 days after the CVE record was published.

How likely is exploitation in the next 30 days?
20.2%

EPSS score, percentile 97.5 of all scored records

How severe?
7.2 High

CVSS v3.1 base score, given by the CNA

What should I do?

CISA deadline 1 Apr 2026. Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

The CVE record links no vendor advisory, patch or mitigation.

Which versions are affected?

Affected

  • from 10.0 to before 10.0.18
  • from 10.1 to before 10.1.13

Versions by product

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

What to do

Required action, set by CISA

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 due date for this action: 1 April 2026.

Advisories and patches

The CVE record links no vendor advisory, patch or mitigation.

Linked from CISA's catalog entry

Affected products and versions

As listed in the CVE record.

Zimbra Collaboration
StatusVersions
Affectedfrom 10.0 to before 10.0.18
Affectedfrom 10.1 to before 10.1.13

Versions not listed are: unaffected.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this vulnerability

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.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

  • CISA added CVE-2025-66376 to its Known Exploited Vulnerabilities (KEV) catalog on 18 Mar 2026.
  • CISA's ransomware flag for this entry is "Unknown": no evidence of ransomware use in the catalog, which is not proof of none.
  • The due date CISA sets for the required action is 1 Apr 2026.
  • The CVE record was published on 5 Jan 2026, 72 days before it was added to KEV.
  • The CVE record carries a CVSS v3.1 base score of 7.2 (High), given by its CNA.
  • FIRST's EPSS model puts the probability of exploitation in the next 30 days at 20.2% (percentile 97.5), as of 28 Oct 2026.
  • CISA's SSVC assessment in the CVE record: exploitation active, automatable no, technical impact partial.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.

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.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

Weakness

  • CWE-79: Improper Neutralization of Input During Web Page Generation (XSS or 'Cross-site Scripting')

CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.

Timeline

DateEvent
5 Jan 2026CVE record published
18 Mar 2026Added to CISA KEV
19 Mar 2026CVE record last updated
1 Apr 2026CISA due date for the required action

References

More exploited Synacor vulnerabilities

  • CVE-2026-73570: Zimbra Collaboration Suite (ZCS) OS Command Injection Vulnerability (added 21 Aug 2026)
  • CVE-2025-48700: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability (added 20 Apr 2026)
  • CVE-2020-7796: Synacor Zimbra Collaboration Suite (ZCS) Server-Side Request Forgery Vulnerability (added 17 Feb 2026)
  • CVE-2025-68645: Synacor Zimbra Collaboration Suite (ZCS) PHP Remote File Inclusion Vulnerability (added 22 Jan 2026)
  • CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability (added 7 Oct 2025)
  • CVE-2019-9621: Synacor Zimbra Collaboration Suite (ZCS) Server-Side Request Forgery (SSRF) Vulnerability (added 7 Jul 2025)