CVE Intelligence
CISA KEV Medium Cross-site scripting

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

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.

Published Jan 5, 2026 · Updated Oct 13, 2026 · Sources: CISA Known Exploited Vulnerabilities, NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP, GitHub Security Advisories · CNA: mitre

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

CISA KEV · Mar 18, 2026
How the score was calculated
  • Confirmed exploitation (CISA KEV)+35
  • EPSS 20.2% (30-day exploitation probability)+15
  • Technical severity (medium)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation
Confirmed
CISA KEV, listed Mar 18, 2026
EPSS FIRST
20.23%
ahead of 97% of scored CVEs · Oct 14, 2026
Affects
Synacor
Zimbra Collaboration Suite and 1 more
CVSS base
6.1
Medium · 4 source(s)
Prioritization reference: 7.2 provisional

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation
Active
none · proof-of-concept · active
Automatable
No
can an attacker script the kill-chain steps
Technical impact
Partial
partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.

CISA required action 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.

US federal deadline: Apr 1, 2026 — passed 197 day(s) ago

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

  1. Identify exposed assets running the affected vendor/product/version combinations (Synacor Zimbra Collaboration Suite, Zimbra Collaboration).
  2. Treat as emergency remediation: the CVE is in CISA KEV (confirmed exploitation).
  3. The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
  4. No fix documented in the sources: reduce exposure and monitor the vendor advisory.
  5. Exploitation requires user interaction: tighten email/web filtering and warn affected users.
  6. EPSS ahead of 97% of scored CVEs: prioritize it in the patch queue.
  7. Search available logs for exploit probes matching the vulnerability class (cross-site scripting), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Synacor Zimbra Collaboration Suite ≥ 10.0.0 and < 10.0.18≥ 10.1.0 and < 10.1.13 Vulnerable NIST National Vulnerability Database
Zimbra Collaboration ≥ 10.0 and < 10.0.18≥ 10.1 and < 10.1.13 Vulnerable CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
7.2 CVSS 3.1 High 3.9 2.7 cve@mitre.org via NIST National Vulnerability Database
7.2 CVSS 3.1 High — — GitHub, Inc. via GitHub Security Advisories
7.2 CVSS 3.1 High — — mitre via CVE Program (cve.org) e CISA ADP
6.1 CVSS 3.1 Medium 2.8 2.7 nvd@nist.gov via NIST National Vulnerability Database
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N (score shown at the top: nvd@nist.gov)
Scoring entities (NIST, CNA, ADP) score independently; each score is shown with its origin. For prioritization we use 7.2 (cve@mitre.org via NIST National Vulnerability Database) as a conservative provisional reference — not a definitive official score.

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-79 · Cross-site Scripting (XSS)

Other CVEs with this weakness →

CISA KEV details CISA

CISA lists a CVE once it has evidence of exploitation against real targets.

Listed
Mar 18, 2026
Federal deadline
Apr 1, 2026
passed 197 day(s) ago
Ransomware use
Unknown

CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a reference date.

EPSS over time FIRST

Oct 12, 2026 → Oct 14, 2026 · 3 reading(s) · max 20.23%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

  • Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
  • Monitor CSP violation reports and script payloads in reflected parameters.
  • Hunt retroactively in logs from before Mar 18, 2026 (KEV listing): exploitation may have started earlier.
web/proxy/WAF logsnetwork IDS/IPSapplication logs

Timeline

What happened to this CVE, newest first.

  • 2026
  • Affected products added · zimbra/collaboration
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • SSVC assessment updated
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • Record published
    Oct 13, 2026
  • Affected products added · synacor/zimbra collaboration suite
    Oct 13, 2026 · NIST National Vulnerability Database
  • CVSS changed · 6.1
    Oct 13, 2026 · NIST National Vulnerability Database
  • Source status changed · → Analyzed
    Oct 13, 2026 · NIST National Vulnerability Database
  • Added to CISA KEV (detected)
    Oct 13, 2026 · CISA Known Exploited Vulnerabilities
  • Record created in this database
    Oct 13, 2026 · CISA Known Exploited Vulnerabilities
  • Added to CISA KEV catalog
    Mar 17, 2026 · CISA
  • CVE published
    Jan 5, 2026 · mitre

Source activity

Repeated readings are counted rather than listed.

  • 2× References added · Oct 13, 2026 – Oct 13, 2026
  • 1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

8 on the record

Elsewhere on this site

Synacor
every CVE for this vendor
2026-03
what else CISA listed that month
Cross-site scripting
same class
CWE-79
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-73573 In Zimbra Collaboration (ZCS) before 10.1.17, a path traversal vulnerability exists in… Aug 13, 2026
  • CVE-2026-73575 Cross-Site Request Forgery (CSRF) in Synacor Zimbra Collaboration Suite Aug 13, 2026
  • CVE-2026-73574 Incorrect Resource Transfer Between Spheres in Synacor Zimbra Collaboration Suite Aug 13, 2026
  • CVE-2026-73576 In Zimbra Collaboration (ZCS) before 10.1.17, weak cryptographic key generation… Aug 13, 2026
  • CVE-2026-73570 Zimbra Collaboration Suite (ZCS) OS Command Injection Vulnerability KEV Aug 13, 2026
  • CVE-2026-73571 Incorrect Authorization in Synacor Zimbra Collaboration Suite Aug 13, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

  • No fixed version, patch or mitigation documented.
  • No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
  • No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
  • No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2025-66376 is "Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability", rated Medium. It affects Zimbra Collaboration Suite (Synacor); versions are in the scope table. It is in CISA KEV: there is evidence of real-world exploitation. CVSS 6.1 describes technical severity. EPSS 20.23% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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