BlackTreeCVE Intelligence
← Back to the CVE catalogue
Full vulnerability report · 2026
CVE-2025-66376High confidence

Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability

Synacor · Zimbra Collaboration Suite (ZCS)

7.2HighCVSS 3.1
Recommended action
Protect now

CISA confirms exploitation in the wild and lists 2026-04-01 as the remediation due date.

Fix not verified
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:Critical, raised one band.upgradedsince 18 Mar 2026

Evidence used

  • CISA confirms exploitation in the wild.
  • The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
  • EPSS is 20.23% for the current model date.

Compensating controls

  • Restrict the affected network interface to trusted sources where business-safe.
  • Increase monitoring for the attack path and post-exploitation behaviour described in the report.

Verification

  1. Confirm that the asset runs Synacor Zimbra Collaboration Suite (ZCS) and falls inside the recorded affected range.
  2. Recheck the vendor advisory before scheduling a change because no verified fixed version is currently retained.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 3 daysRemediation target: Within 90 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2026-0850

CISA KEV mirrored by ENISA

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.

EUVD state
Present in the current official mapping
Known exploitation
Recorded by ENISA since 18 Mar 2026. Evidence sources: cisa_kev.
ENISA score
7.2 · CVSS 3.1
Advisory evidence
1 linked advisory record
Recommended actionProtect now

CISA confirms exploitation in the wild and lists 2026-04-01 as the remediation due date.

Fix not verified
01

What, why and how

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.

What

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.

Why

Attacker-controlled content can reach a browser as executable script without sufficient output encoding or sanitisation.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

What

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.

Why

Attacker-controlled content can reach a browser as executable script without sufficient output encoding or sanitisation.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Confirmed in the wild

CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2026-03-18.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path
a network path → Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') → cause the confidentiality, integrity or availability impact described by the vendor
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
None
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Changed: exploitation can affect a different security authority
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-79 ↗

CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'). The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.SChangedScope: The attack can affect a component governed by a different security authority.CLowConfidentiality impact: A successful attack can cause a limited loss.ILowIntegrity impact: A successful attack can cause a limited loss.ANoneAvailability impact: No direct loss is represented by this metric.
Post-exploitation / living off the land
No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
NetworkUnauthenticatedCWE-79CISA KEV
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

ENISA EUVD · EUVD-2026-0850Known-exploited evidence recorded

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.

Official EUVD record ↗
BSI · German · WID-SEC-2025-2911Synacor Zimbra: Mehrere Schwachstellen

Ein Angreifer kann mehrere Schwachstellen in Synacor Zimbra ausnutzen, um einen Cross-Site Scripting Angriff durchzuführen, um Sicherheitsvorkehrungen zu umgehen, und um beliebigen Programmcode auszuführen.

Official advisory ↗
Cyber Security Agency of Singapore · English · CSA-SB-20260107Security Bulletin 07 Jan 2026

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 07 Jan 2026, published on 7 January 2026. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
CERT-FR · French · CERTFR-2025-AVI-0970Multiples vulnérabilités dans Synacor Zimbra Collaboration

De multiples vulnérabilités ont été découvertes dans Synacor Zimbra Collaboration. Certaines d'entre elles permettent à un attaquant de provoquer une atteinte à la confidentialité des données, une injection de code indirecte à distance (XSS) et une injection de requêtes illégitimes par rebond (CSRF).

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2026-008047Synacor Inc.のZimbra Collaboration Suiteにおけるクロスサイトスクリプティングの脆弱性

Zimbra Collaboration（ZCS）10のバージョン10.0.18より前および10.1のバージョン10.1.13より前のバージョンでは、HTML電子メールメッセージ内のカスケーディングスタイルシート（CSS）の@importディレクティブを介してClassic UIに保存型のクロスサイトスクリプティング（XSS）が発生する可能性があります。

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-6731美 CISA 발표 주요 Exploit 정보공유(Update. 2026-03-18)

Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Fix not verified
Affected
Collaboration: 10.0 < 10.0.18, 10.1 < 10.1.13
Fixed
No fixed version is explicitly recorded in the structured CVE data.
Action
No verified patch reference is present in the current structured sources. Check the vendor advisory before making a change.
Workaround
No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04

Evidence and provenance

Published 5 Jan 2026 · Last source change 19 Mar 2026, 03:55 UTC · CWE-79 · Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2026-0850
Product sourceCISA KEV
Remediation sourceCVE/CNA references
CWE sourceCNA
NVD statusNVD enriched

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Affected versionsThe structured affected or fixed version information changed.
    Before
    10.0 < 10.0.18; 10.1 < 10.1.13 · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    After
    Collaboration: 10.0 < 10.0.18, 10.1 < 10.1.13 · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    CNA ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Added to CISA KEV
ENISA EUVD · EUVD-2026-0850↗CISA KEV record↗CVE authority or vendor reference↗Vendor advisory↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2025-66376 · cve.blacktree.nl