Threats / Synacor / CVE-2025-66376
CVE-2025-66376
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Synacor Zimbra Collaboration Suite (ZCS) vulnerability
Synacor Zimbra Collaboration Suite contains a cross-site scripting vulnerability in the Classic UI where CSS @import directives in email HTML enable attacker injection.
Verdict
Today item — known-exploited.
A reflected or stored XSS flaw allows attackers to inject malicious scripts via CSS @import directives in email messages, potentially compromising user sessions, stealing credentials, or performing actions on behalf of victims within the Zimbra interface.
CISA KEV Yes · 2026-03-185EPSS 0.21621 (verify live)6CVSS 7.23CISA SSVC Exploitation: active · Automatable: no · Impact: partial4
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
6 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-03-18).
Probability (EPSS)
EPSS 0.21621 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 7.2 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N) · Affected: Synacor, Zimbra Collaboration Suite (ZCS). Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-79 Cross-site Scripting (XSS) — weakness family: Web / client.CWE — CNA-assigned (CVE.org); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: no · Technical Impact: partialCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 72 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft an email containing HTML with malicious CSS @import directives that execute JavaScript when the message is rendered in the Classic UI.
Business
User email accounts are compromised, enabling unauthorized access to sensitive communications and calendar data.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I inject scripts that steal session tokens or authentication cookies from the victim's browser session.
Business
Attackers gain persistent access to the messaging platform, potentially exfiltrating confidential business communications.
3
Lateral reach — past segmentation narrative 3
Attacker
I execute actions within the victim's Zimbra session, such as forwarding emails, modifying contacts, or accessing shared resources.
Business
Organizational data integrity is compromised and sensitive information is exposed to unauthorized parties.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05