Threats / Synacor / CVE-2024-27443
CVE-2024-27443
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Synacor Zimbra Collaboration Suite (ZCS) vulnerability
Zimbra Collaboration Suite contains a cross-site scripting vulnerability in the CalendarInvite feature of the webmail classic interface. Attackers can inject malicious JavaScript via crafted calendar headers in email messages.
Verdict
Today item — known-exploited.
A reflected XSS flaw in calendar invitation handling allows remote attackers to execute arbitrary JavaScript in victim browsers without authentication, potentially compromising user sessions and sensitive webmail data.
CISA KEV Yes · 2025-05-194EPSS 0.23632 (verify live)5CVSS 6.1 (CISA)3CISA SSVC Exploitation: active · Automatable: no · Impact: total3Exploit Public PoC / detection · nuclei template14
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
5 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 2025-05-19).
Probability (EPSS)
EPSS 0.23632 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 6.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N) · Affected: Synacor, Zimbra Collaboration Suite (ZCS). Confirm exact fixed builds in the vendor advisory.CVSS — CISA-assigned via the ADP container (CVE.org), version 3.1; the CNA left no metrics, so CISA's own score is used ahead of NVD.
Weakness (CWE)
Mapped to CWE-79 Cross-site Scripting (XSS) — weakness family: Web / client.CWE assignment from the public NVD record (may be stale); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: no · Technical Impact: totalCISA'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 280 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.
Public exploit available
Public exploit tooling is catalogued for this CVE (a public proof-of-concept / detection template). Catalog entries: nuclei template. We link the catalog page(s) only — no payload or code is hosted or restated here.Link, never host — the catalog page is the source; basicsecurity.net does not redistribute payloads.
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 a calendar invitation email with JavaScript payload embedded in the header field.
Business
User receives email appearing legitimate but containing hidden malicious code.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I send the malicious email to target users of Zimbra webmail classic interface.
Business
Victims open the calendar invitation in their webmail client.
3
Lateral reach — past segmentation narrative 3
Attacker
I execute arbitrary JavaScript in the victim's browser session when they view the invitation.
Business
Attacker gains ability to steal session cookies, credentials, or perform actions as the authenticated user.
4
Data at risk — exfiltration narrative 4
Attacker
I harvest sensitive emails, contacts, or calendar data from the compromised account.
Business
Confidential business communications and personal information are exposed to unauthorized access.
5
Lights out — disruption & extortion narrative 5
Attacker
I maintain persistence by modifying account settings or forwarding rules.
Business
Ongoing unauthorized access to the mailbox enables data exfiltration and further lateral movement.
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