Vulnerability

CVE-2025-66376

High· 7.2KEVCVSS7.2EPSS11%

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

Threat signal

87/100

High

Severity

high

CVSS

7.2

EPSS

11%

Active

KEV

CISA KEV — actively exploited

73d overdue

AffectedSynacor — Zimbra Collaboration Suite (ZCS)

Added to KEV

2026-03-18

Federal patch due (BOD 22-01)

2026-04-01

Required action (CISA verbatim)

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

Vendor advisory + references

CISA description: “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.”

CVSS

v3.1Open in FIRST calculator →

7.2/ 10HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

Attack VectorNetworkExploitable remotely over the internet — no local access needed.
Attack ComplexityLowNo special conditions — exploit works reliably.
Privileges RequiredNoneUnauthenticated — no credentials needed.
User InteractionNoneFully automated — no victim action needed.
ScopeChangedImpact extends beyond the vulnerable component (e.g. sandbox escape).
Confidentiality ImpactLowSome data disclosure; attacker doesn't fully control what.
Integrity ImpactLowSome data modification; attacker doesn't fully control extent.
Availability ImpactNoneNo availability impact.

EPSS

Exploit prediction · 30-day horizonFIRST.org →

10.9%

probability of exploitation in next 30 days

p94

higher than 94% of all CVEs

3-day trend↑ 0pp

Above the FIRST 'patch on a priority schedule' threshold.

Vendor VEX

No VEX statements published for CVE-2025-66376. Vendors publish VEX (Vulnerability Exploitability eXchange) to assert per-product whether a CVE is actually exploitable in their distribution.

SSVCAct

Active exploitation with total impact on essential/support systems

exploitation: active
automatable: no
impact: total
mission: support

Lifecycle

2026-03-18Added to CISA KEVransomware: Unknown
2026-03-18Published
2026-04-01Federal patch due (BOD 22-01)
2026-06-12Last modified1 source

No source-event history recorded yet. New revisions will appear here as ingestors re-fetch this CVE.