CVE-2024-27443: Synacor Zimbra Collaboration Suite (ZCS)

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

Yes, if you run Synacor Zimbra Collaboration Suite (ZCS). CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2025-05-19, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2025-06-09; treat that as the outer limit, not the target.
EPSS 24.6%, percentile 98: FIRST’s model estimates a 24.6% chance of exploitation activity in the next 30 days, higher than 98% of all scored CVEs (very high). EPSS is a probability, not a verdict.
Needs a user to click or open something. Phishing makes that realistic, but it is not wormable.

What it is

An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and 10.0. A Cross-Site Scripting (XSS) vulnerability exists in the CalendarInvite feature of the Zimbra webmail classic user interface, because of improper input validation in the handling of the calendar header. An attacker can exploit this via an email message containing a crafted calendar header with an embedded XSS payload. When a victim views this message in the Zimbra webmail classic interface, the payload is executed in the context of the victim's session, potentially leading to execution of arbitrary JavaScript code.

CISA’s summary

Zimbra Collaboration contains a cross-site scripting (XSS) vulnerability in the CalendarInvite feature of the Zimbra webmail classic user interface. An attacker can exploit this vulnerability via an email message containing a crafted calendar header, leading to the execution of arbitrary JavaScript code.

Weakness type: CWE-79.

Product: Synacor Zimbra Collaboration Suite (ZCS) · Assigned by: mitre · Published: 2024-08-12

Affected and fixed versions

Vendor	Product	Affected versions (per CVE record)
n/a	n/a	see vendor advisory

Severity scores

Scored by	CVSS	Score	Vector
CISA-ADP	3.1	6.1 Medium	`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`

Attack vector: network (remotely reachable)
Attack complexity: low
Privileges required: none (no login)
User interaction: required (a user must click/open something)
Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

CISA KEV: added 2025-05-19; federal due date 2025-06-09. Known ransomware use: Unknown.
CISA required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
CISA SSVC assessment: exploitation active, automatable no, technical impact total.
FIRST EPSS (model of 2026-10-21): 24.61% probability of exploitation activity in the next 30 days; percentile 98.

Timeline

2024-08-12 — CVE record published by mitre
2025-05-19 — Added to CISA’s Known Exploited Vulnerabilities catalog
2025-06-09 — CISA remediation deadline for US federal agencies
2025-10-21 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Other references from the CVE record and advisory

welivesecurity.com welivesecurity.com

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